Welcome To the Department of microbiology Tansian University

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DNA</h1>
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From Wikipedia, the free encyclopedia</div>
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For a non-technical introduction to the topic, see <a href="http://en.wikipedia.org/wiki/Introduction_to_genetics" title="Introduction to genetics">Introduction to genetics</a>. For other uses, see <a class="mw-disambig" href="http://en.wikipedia.org/wiki/DNA_%28disambiguation%29" title="DNA (disambiguation)">DNA (disambiguation)</a>.</div>
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<a class="image" href="http://en.wikipedia.org/wiki/File:DNA_Structure%2BKey%2BLabelled.pn_NoBB.png"><img alt="" class="thumbimage" data-file-height="3000" data-file-width="3075" height="332" src="http://upload.wikimedia.org/wikipedia/commons/thumb/4/4c/DNA_Structure%2BKey%2BLabelled.pn_NoBB.png/340px-DNA_Structure%2BKey%2BLabelled.pn_NoBB.png" width="340" /></a>
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The structure of the DNA <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Double_helix" title="Double helix">double helix</a>. The <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Atoms" title="Atoms">atoms</a> in the structure are colour-coded by <a href="http://en.wikipedia.org/wiki/Chemical_element" title="Chemical element">element</a> and the detailed structure of two base pairs are shown in the bottom right.</div>
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<a class="image" href="http://en.wikipedia.org/wiki/File:ADN_animation.gif"><img alt="" class="thumbimage" data-file-height="313" data-file-width="181" height="313" src="http://upload.wikimedia.org/wikipedia/commons/8/81/ADN_animation.gif" width="181" /></a>
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The structure of part of a DNA <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Double_helix" title="Double helix">double helix</a></div>
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Deoxyribonucleic acid (<a href="http://upload.wikimedia.org/wikipedia/commons/f/f2/En-us-Deoxyribonucleic_acid.ogg" title="Listen"><img alt="Listen" data-file-height="11" data-file-width="11" height="11" src="http://upload.wikimedia.org/wikipedia/commons/thumb/3/3b/Speakerlink-new.svg/11px-Speakerlink-new.svg.png" width="11" /></a><a href="http://en.wikipedia.org/wiki/File:En-us-Deoxyribonucleic_acid.ogg" title="File:En-us-Deoxyribonucleic acid.ogg">i</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English" title="Help:IPA for English">/</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">d</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">i</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">ˌ</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">ɒ</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">k</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">s</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">i</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">ˌ</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">r</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">aɪ</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">b</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">ɵ</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">.</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">nj</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">uː</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">ˌ</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">k</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">l</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">eɪ</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">.</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">ɨ</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">k</a> <a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">ˈ</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">æ</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">s</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">ɪ</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English#Key" title="Help:IPA for English">d</a><a href="http://en.wikipedia.org/wiki/Help:IPA_for_English" title="Help:IPA for English">/</a>; DNA) is a <a href="http://en.wikipedia.org/wiki/Molecule" title="Molecule">molecule</a> that encodes the <a href="http://en.wikipedia.org/wiki/Genetics" title="Genetics">genetic</a> instructions used in the development and functioning of all known living <a href="http://en.wikipedia.org/wiki/Organism" title="Organism">organisms</a> and many <a href="http://en.wikipedia.org/wiki/Virus" title="Virus">viruses</a>. DNA is a <a href="http://en.wikipedia.org/wiki/Nucleic_acid" title="Nucleic acid">nucleic acid</a>; alongside <a href="http://en.wikipedia.org/wiki/Protein" title="Protein">proteins</a> and <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Carbohydrates" title="Carbohydrates">carbohydrates</a>, nucleic acids compose the three major <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Macromolecules" title="Macromolecules">macromolecules</a> essential for all known forms of <a href="http://en.wikipedia.org/wiki/Life" title="Life">life</a>. Most DNA molecules consist of two <a href="http://en.wikipedia.org/wiki/Biopolymer" title="Biopolymer">biopolymer</a> strands coiled around each other to form a <a href="http://en.wikipedia.org/wiki/Nucleic_acid_double_helix" title="Nucleic acid double helix">double helix</a>. The two DNA strands are known as <a href="http://en.wikipedia.org/wiki/Polynucleotide" title="Polynucleotide">polynucleotides</a> since they are composed of <a href="http://en.wikipedia.org/wiki/Monomer" title="Monomer">simpler units</a> called <a href="http://en.wikipedia.org/wiki/Nucleotide" title="Nucleotide">nucleotides</a>. Each nucleotide is composed of a <a href="http://en.wikipedia.org/wiki/Nitrogenous_base" title="Nitrogenous base">nitrogen-containing</a> <a href="http://en.wikipedia.org/wiki/Nucleobase" title="Nucleobase">nucleobase</a>—either <a href="http://en.wikipedia.org/wiki/Guanine" title="Guanine">guanine</a> (G), <a href="http://en.wikipedia.org/wiki/Adenine" title="Adenine">adenine</a> (A), <a href="http://en.wikipedia.org/wiki/Thymine" title="Thymine">thymine</a> (T), or <a href="http://en.wikipedia.org/wiki/Cytosine" title="Cytosine">cytosine</a> (C)—as well as a <a href="http://en.wikipedia.org/wiki/Monosaccharide" title="Monosaccharide">monosaccharide</a> sugar called <a href="http://en.wikipedia.org/wiki/Deoxyribose" title="Deoxyribose">deoxyribose</a> and a <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Phosphate_group" title="Phosphate group">phosphate group</a>. The nucleotides are joined to one another in a chain by <a href="http://en.wikipedia.org/wiki/Covalent_bond" title="Covalent bond">covalent bonds</a> between the sugar of one nucleotide and the phosphate of the next, resulting in an alternating <a href="http://en.wikipedia.org/wiki/Backbone_chain" title="Backbone chain">sugar-phosphate backbone</a>. According to <a href="http://en.wikipedia.org/wiki/Base_pair" title="Base pair">base pairing</a> rules (A with T and C with G), <a href="http://en.wikipedia.org/wiki/Hydrogen_bond" title="Hydrogen bond">hydrogen bonds</a> bind the nitrogenous bases of the two separate polynucleotide strands to make double-stranded DNA.

DNA is well-suited for biological <a href="http://en.wikipedia.org/wiki/Information" title="Information">information</a>
 storage. The DNA backbone is resistant to cleavage, and both strands of
 the double-stranded structure store the same biological information. 
Biological information is replicated as the two strands are separated. A
 significant portion of DNA (more than 98% for humans) is <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Non-coding_DNA" title="Non-coding DNA">non-coding</a>, meaning that these sections do not serve as patterns for protein sequences.

The two strands of DNA run in opposite directions to each other and are therefore <a href="http://en.wikipedia.org/wiki/Antiparallel_%28biochemistry%29" title="Antiparallel (biochemistry)">anti-parallel</a>. Attached to each sugar is one of four types of nucleobases (informally, bases). It is the <a href="http://en.wikipedia.org/wiki/Nucleic_acid_sequence" title="Nucleic acid sequence">sequence</a> of these four nucleobases along the backbone that encodes biological information. Under the <a href="http://en.wikipedia.org/wiki/Genetic_code" title="Genetic code">genetic code</a>, <a href="http://en.wikipedia.org/wiki/RNA" title="RNA">RNA</a> strands are translated to specify the sequence of <a href="http://en.wikipedia.org/wiki/Amino_acid" title="Amino acid">amino acids</a> within proteins. These RNA strands are initially created using DNA strands as a template in a process called <a href="http://en.wikipedia.org/wiki/Transcription_%28genetics%29" title="Transcription (genetics)">transcription</a>.

Within cells, DNA is organized into long structures called <a href="http://en.wikipedia.org/wiki/Chromosome" title="Chromosome">chromosomes</a>. During <a href="http://en.wikipedia.org/wiki/Cell_division" title="Cell division">cell division</a> these chromosomes are duplicated in the process of <a href="http://en.wikipedia.org/wiki/DNA_replication" title="DNA replication">DNA replication</a>, providing each cell its own complete set of chromosomes. <a href="http://en.wikipedia.org/wiki/Eukaryote" title="Eukaryote">Eukaryotic organisms</a> (<a href="http://en.wikipedia.org/wiki/Animal" title="Animal">animals</a>, <a href="http://en.wikipedia.org/wiki/Plant" title="Plant">plants</a>, <a href="http://en.wikipedia.org/wiki/Fungus" title="Fungus">fungi</a>, and <a href="http://en.wikipedia.org/wiki/Protist" title="Protist">protists</a>) store most of their DNA inside the <a href="http://en.wikipedia.org/wiki/Cell_nucleus" title="Cell nucleus">cell nucleus</a> and some of their DNA in <a href="http://en.wikipedia.org/wiki/Organelle" title="Organelle">organelles</a>, such as <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Mitochondria" title="Mitochondria">mitochondria</a> or <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Chloroplasts" title="Chloroplasts">chloroplasts</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-1">[1]</a> In contrast, <a href="http://en.wikipedia.org/wiki/Prokaryote" title="Prokaryote">prokaryotes</a> (<a href="http://en.wikipedia.org/wiki/Bacteria" title="Bacteria">bacteria</a> and <a href="http://en.wikipedia.org/wiki/Archaea" title="Archaea">archaea</a>) store their DNA only in the <a href="http://en.wikipedia.org/wiki/Cytoplasm" title="Cytoplasm">cytoplasm</a>. Within the chromosomes, <a href="http://en.wikipedia.org/wiki/Chromatin" title="Chromatin">chromatin</a> proteins such as <a href="http://en.wikipedia.org/wiki/Histone" title="Histone">histones</a>
 compact and organize DNA. These compact structures guide the 
interactions between DNA and other proteins, helping control which parts
 of the DNA are transcribed.

Scientists use DNA as a molecular tool to explore physical laws and theories, such as the <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Ergodic_theorem" title="Ergodic theorem">ergodic theorem</a> and the theory of <a href="http://en.wikipedia.org/wiki/Elasticity_%28physics%29" title="Elasticity (physics)">elasticity</a>.
 The unique material properties of DNA have made it an attractive 
molecule for material scientists and engineers interested in micro- and 
nano-fabrication. Among notable advances in this field are <a href="http://en.wikipedia.org/wiki/DNA_origami" title="DNA origami">DNA origami</a> and DNA-based hybrid materials.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-2">[2]</a>

The obsolete synonym "desoxyribonucleic acid" may occasionally be encountered, for example, in pre-1953 genetics.

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Contents</h2>
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<ul>
<li class="toclevel-1 tocsection-1"><a href="http://en.wikipedia.org/wiki/DNA#Properties">1 Properties</a>
<ul>
<li class="toclevel-2 tocsection-2"><a href="http://en.wikipedia.org/wiki/DNA#Nucleobase_classification">1.1 Nucleobase classification</a></li>
<li class="toclevel-2 tocsection-3"><a href="http://en.wikipedia.org/wiki/DNA#Grooves">1.2 Grooves</a></li>
<li class="toclevel-2 tocsection-4"><a href="http://en.wikipedia.org/wiki/DNA#Base_pairing">1.3 Base pairing</a></li>
<li class="toclevel-2 tocsection-5"><a href="http://en.wikipedia.org/wiki/DNA#Sense_and_antisense">1.4 Sense and antisense</a></li>
<li class="toclevel-2 tocsection-6"><a href="http://en.wikipedia.org/wiki/DNA#Supercoiling">1.5 Supercoiling</a></li>
<li class="toclevel-2 tocsection-7"><a href="http://en.wikipedia.org/wiki/DNA#Alternate_DNA_structures">1.6 Alternate DNA structures</a></li>
<li class="toclevel-2 tocsection-8"><a href="http://en.wikipedia.org/wiki/DNA#Alternative_DNA_chemistry">1.7 Alternative DNA chemistry</a></li>
<li class="toclevel-2 tocsection-9"><a href="http://en.wikipedia.org/wiki/DNA#Quadruplex_structures">1.8 Quadruplex structures</a></li>
<li class="toclevel-2 tocsection-10"><a href="http://en.wikipedia.org/wiki/DNA#Branched_DNA">1.9 Branched DNA</a></li>
</ul>
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<li class="toclevel-1 tocsection-11"><a href="http://en.wikipedia.org/wiki/DNA#DNA_as_an_anionic_polymer">2 DNA as an anionic polymer</a></li>
<li class="toclevel-1 tocsection-12"><a href="http://en.wikipedia.org/wiki/DNA#Chemical_modifications_and_altered_DNA_packaging">3 Chemical modifications and altered DNA packaging</a>
<ul>
<li class="toclevel-2 tocsection-13"><a href="http://en.wikipedia.org/wiki/DNA#Base_modifications_and_DNA_packaging">3.1 Base modifications and DNA packaging</a></li>
<li class="toclevel-2 tocsection-14"><a href="http://en.wikipedia.org/wiki/DNA#Damage">3.2 Damage</a></li>
</ul>
</li>
<li class="toclevel-1 tocsection-15"><a href="http://en.wikipedia.org/wiki/DNA#Biological_functions">4 Biological functions</a>
<ul>
<li class="toclevel-2 tocsection-16"><a href="http://en.wikipedia.org/wiki/DNA#Genes_and_genomes">4.1 Genes and genomes</a></li>
<li class="toclevel-2 tocsection-17"><a href="http://en.wikipedia.org/wiki/DNA#Transcription_and_translation">4.2 Transcription and translation</a></li>
<li class="toclevel-2 tocsection-18"><a href="http://en.wikipedia.org/wiki/DNA#Replication">4.3 Replication</a></li>
</ul>
</li>
<li class="toclevel-1 tocsection-19"><a href="http://en.wikipedia.org/wiki/DNA#Interactions_with_proteins">5 Interactions with proteins</a>
<ul>
<li class="toclevel-2 tocsection-20"><a href="http://en.wikipedia.org/wiki/DNA#DNA-binding_proteins">5.1 DNA-binding proteins</a></li>
<li class="toclevel-2 tocsection-21"><a href="http://en.wikipedia.org/wiki/DNA#DNA-modifying_enzymes">5.2 DNA-modifying enzymes</a>
<ul>
<li class="toclevel-3 tocsection-22"><a href="http://en.wikipedia.org/wiki/DNA#Nucleases_and_ligases">5.2.1 Nucleases and ligases</a></li>
<li class="toclevel-3 tocsection-23"><a href="http://en.wikipedia.org/wiki/DNA#Topoisomerases_and_helicases">5.2.2 Topoisomerases and helicases</a></li>
<li class="toclevel-3 tocsection-24"><a href="http://en.wikipedia.org/wiki/DNA#Polymerases">5.2.3 Polymerases</a></li>
</ul>
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</ul>
</li>
<li class="toclevel-1 tocsection-25"><a href="http://en.wikipedia.org/wiki/DNA#Genetic_recombination">6 Genetic recombination</a></li>
<li class="toclevel-1 tocsection-26"><a href="http://en.wikipedia.org/wiki/DNA#Evolution">7 Evolution</a></li>
<li class="toclevel-1 tocsection-27"><a href="http://en.wikipedia.org/wiki/DNA#Uses_in_technology">8 Uses in technology</a>
<ul>
<li class="toclevel-2 tocsection-28"><a href="http://en.wikipedia.org/wiki/DNA#Genetic_engineering">8.1 Genetic engineering</a></li>
<li class="toclevel-2 tocsection-29"><a href="http://en.wikipedia.org/wiki/DNA#Forensics">8.2 Forensics</a></li>
<li class="toclevel-2 tocsection-30"><a href="http://en.wikipedia.org/wiki/DNA#Bioinformatics">8.3 Bioinformatics</a></li>
<li class="toclevel-2 tocsection-31"><a href="http://en.wikipedia.org/wiki/DNA#DNA_nanotechnology">8.4 DNA nanotechnology</a></li>
<li class="toclevel-2 tocsection-32"><a href="http://en.wikipedia.org/wiki/DNA#History_and_anthropology">8.5 History and anthropology</a></li>
<li class="toclevel-2 tocsection-33"><a href="http://en.wikipedia.org/wiki/DNA#Information_storage">8.6 Information storage</a></li>
</ul>
</li>
<li class="toclevel-1 tocsection-34"><a href="http://en.wikipedia.org/wiki/DNA#History_of_DNA_research">9 History of DNA research</a></li>
<li class="toclevel-1 tocsection-35"><a href="http://en.wikipedia.org/wiki/DNA#See_also">10 See also</a></li>
<li class="toclevel-1 tocsection-36"><a href="http://en.wikipedia.org/wiki/DNA#References">11 References</a></li>
<li class="toclevel-1 tocsection-37"><a href="http://en.wikipedia.org/wiki/DNA#Further_reading">12 Further reading</a></li>
<li class="toclevel-1 tocsection-38"><a href="http://en.wikipedia.org/wiki/DNA#External_links">13 External links</a></li>
</ul>
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<h2>
Properties</h2>
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<a class="image" href="http://en.wikipedia.org/wiki/File:DNA_chemical_structure.svg"><img alt="" class="thumbimage" data-file-height="1750" data-file-width="1500" height="350" src="http://upload.wikimedia.org/wikipedia/commons/thumb/e/e4/DNA_chemical_structure.svg/300px-DNA_chemical_structure.svg.png" width="300" /></a>
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Chemical structure of DNA; <a href="http://en.wikipedia.org/wiki/Hydrogen_bond" title="Hydrogen bond">hydrogen bonds</a> shown as dotted lines</div>
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DNA is a long <a href="http://en.wikipedia.org/wiki/Polymer" title="Polymer">polymer</a> made from repeating units called <a href="http://en.wikipedia.org/wiki/Nucleotide" title="Nucleotide">nucleotides</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-3">[3]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-Alberts-4">[4]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-Butler-5">[5]</a> DNA was first identified and isolated by <a href="http://en.wikipedia.org/wiki/Friedrich_Miescher" title="Friedrich Miescher">Friedrich Miescher</a> in 1871, and the double helix structure of DNA was first discovered by <a href="http://en.wikipedia.org/wiki/James_Watson" title="James Watson">James Watson</a> and <a href="http://en.wikipedia.org/wiki/Francis_Crick" title="Francis Crick">Francis Crick</a>, using experimental data collected by <a href="http://en.wikipedia.org/wiki/Rosalind_Franklin" title="Rosalind Franklin">Rosalind Franklin</a> and <a href="http://en.wikipedia.org/wiki/Maurice_Wilkins" title="Maurice Wilkins">Maurice Wilkins</a>. The structure of DNA of all species comprises two helical chains each coiled round the same axis, and each with a pitch of 34 <a class="mw-redirect" href="http://en.wikipedia.org/wiki/%C3%85ngstr%C3%B6m" title="Ångström">ångströms</a> (3.4 <a href="http://en.wikipedia.org/wiki/Nanometre" title="Nanometre">nanometres</a>) and a radius of 10 ångströms (1.0 nanometres).<a href="http://en.wikipedia.org/wiki/DNA#cite_note-FWPUB-6">[6]</a>
 According to another study, when measured in a particular solution, the
 DNA chain measured 22 to 26 ångströms wide (2.2 to 2.6 nanometres), and
 one nucleotide unit measured 3.3 Å (0.33 nm) long.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-7">[7]</a>
 Although each individual repeating unit is very small, DNA polymers can
 be very large molecules containing millions of nucleotides. For 
instance, the largest human <a href="http://en.wikipedia.org/wiki/Chromosome" title="Chromosome">chromosome</a>, chromosome <a href="http://en.wikipedia.org/wiki/Chromosome_1_%28human%29" title="Chromosome 1 (human)">number 1</a>, consists of approximately 220 million <a href="http://en.wikipedia.org/wiki/Base_pair" title="Base pair">base pairs</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-8">[8]</a> and is 85 mm long.

In living organisms DNA does not usually exist as a single molecule, 
but instead as a pair of molecules that are held tightly together.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-autogenerated2-9">[9]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-berg-10">[10]</a> These two long strands entwine like vines, in the shape of a <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Double_helix" title="Double helix">double helix</a>.
 The nucleotide repeats contain both the segment of the backbone of the 
molecule, which holds the chain together, and a nucleobase, which 
interacts with the other DNA strand in the helix. A nucleobase linked to
 a sugar is called a <a href="http://en.wikipedia.org/wiki/Nucleoside" title="Nucleoside">nucleoside</a> and a base linked to a sugar and one or more phosphate groups is called a <a href="http://en.wikipedia.org/wiki/Nucleotide" title="Nucleotide">nucleotide</a>. A polymer comprising multiple linked nucleotides (as in DNA) is called a <a href="http://en.wikipedia.org/wiki/Polynucleotide" title="Polynucleotide">polynucleotide</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-IUPAC-11">[11]</a>

The backbone of the DNA strand is made from alternating <a href="http://en.wikipedia.org/wiki/Phosphate" title="Phosphate">phosphate</a> and <a href="http://en.wikipedia.org/wiki/Carbohydrate" title="Carbohydrate">sugar</a> residues.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Ghosh-12">[12]</a> The sugar in DNA is <a href="http://en.wikipedia.org/wiki/Deoxyribose" title="Deoxyribose">2-deoxyribose</a>, which is a <a href="http://en.wikipedia.org/wiki/Pentose" title="Pentose">pentose</a> (five-<a href="http://en.wikipedia.org/wiki/Carbon" title="Carbon">carbon</a>) sugar. The sugars are joined together by phosphate groups that form <a href="http://en.wikipedia.org/wiki/Phosphodiester_bond" title="Phosphodiester bond">phosphodiester bonds</a> between the third and fifth carbon <a href="http://en.wikipedia.org/wiki/Atom" title="Atom">atoms</a> of adjacent sugar rings. These asymmetric <a href="http://en.wikipedia.org/wiki/Covalent_bond" title="Covalent bond">bonds</a>
 mean a strand of DNA has a direction. In a double helix the direction 
of the nucleotides in one strand is opposite to their direction in the 
other strand: the strands are antiparallel. The asymmetric ends of DNA strands are called the <a href="http://en.wikipedia.org/wiki/Directionality_%28molecular_biology%29" title="Directionality (molecular biology)">5′</a> (five prime) and <a href="http://en.wikipedia.org/wiki/Directionality_%28molecular_biology%29" title="Directionality (molecular biology)">3′</a> (three prime)
 ends, with the 5′ end having a terminal phosphate group and the 3′ end a
 terminal hydroxyl group. One major difference between DNA and <a href="http://en.wikipedia.org/wiki/RNA" title="RNA">RNA</a> is the sugar, with the 2-deoxyribose in DNA being replaced by the alternative pentose sugar <a href="http://en.wikipedia.org/wiki/Ribose" title="Ribose">ribose</a> in RNA.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-berg-10">[10]</a>

A section of DNA. The bases lie horizontally between the two spiraling strands.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-13">[13]</a> (<a href="http://en.wikipedia.org/wiki/File:DNA_orbit_animated.gif" title="File:DNA orbit animated.gif">animated version</a>).</div>
</div>
</div>
The DNA double helix is stabilized primarily by two forces: <a href="http://en.wikipedia.org/wiki/Hydrogen_bond" title="Hydrogen bond">hydrogen bonds</a> between nucleotides and <a href="http://en.wikipedia.org/wiki/Stacking_%28chemistry%29" title="Stacking (chemistry)">base-stacking</a> interactions among <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Aromatic" title="Aromatic">aromatic</a> nucleobases.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Yakovchuk2006-14">[14]</a> In the aqueous environment of the cell, the conjugated <a href="http://en.wikipedia.org/wiki/Pi_bond" title="Pi bond">π bonds</a> of nucleotide bases align perpendicular to the axis of the DNA molecule, minimizing their interaction with the <a href="http://en.wikipedia.org/wiki/Solvation_shell" title="Solvation shell">solvation shell</a> and therefore, the <a href="http://en.wikipedia.org/wiki/Gibbs_free_energy" title="Gibbs free energy">Gibbs free energy</a>. The four bases found in DNA are <a href="http://en.wikipedia.org/wiki/Adenine" title="Adenine">adenine</a> (abbreviated A), <a href="http://en.wikipedia.org/wiki/Cytosine" title="Cytosine">cytosine</a> (C), <a href="http://en.wikipedia.org/wiki/Guanine" title="Guanine">guanine</a> (G) and <a href="http://en.wikipedia.org/wiki/Thymine" title="Thymine">thymine</a> (T). These four bases are attached to the sugar/phosphate to form the complete nucleotide, as shown for <a href="http://en.wikipedia.org/wiki/Adenosine_monophosphate" title="Adenosine monophosphate">adenosine monophosphate</a>.

<h3>
Nucleobase classification</h3>
The nucleobases are classified into two types: the <a href="http://en.wikipedia.org/wiki/Purine" title="Purine">purines</a>, A and G, being fused five- and six-membered <a href="http://en.wikipedia.org/wiki/Heterocyclic_compound" title="Heterocyclic compound">heterocyclic compounds</a>, and the <a href="http://en.wikipedia.org/wiki/Pyrimidine" title="Pyrimidine">pyrimidines</a>, the six-membered rings C and T.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-berg-10">[10]</a> A fifth pyrimidine nucleobase, <a href="http://en.wikipedia.org/wiki/Uracil" title="Uracil">uracil</a> (U), usually takes the place of thymine in RNA and differs from thymine by lacking a <a href="http://en.wikipedia.org/wiki/Methyl_group" title="Methyl group">methyl group</a> on its ring. In addition to RNA and DNA a large number of artificial <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Nucleic_acid_analogues" title="Nucleic acid analogues">nucleic acid analogues</a> have also been created to study the properties of nucleic acids, or for use in biotechnology.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-15">[15]</a>

Uracil is not usually found in DNA, occurring only as a breakdown product of cytosine. However, in a number of bacteriophages – Bacillus subtilis bacteriophages PBS1 and PBS2 and Yersinia bacteriophage piR1-37 – thymine has been replaced by uracil.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Kiljunen2005-16">[16]</a> Another phage - Staphylococcal phage S6 - has been identified with a genome where thymine has been replaced by uracil.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Uchiyama2014-17">[17]</a>

<a href="http://en.wikipedia.org/wiki/Base_J" title="Base J">Base J</a> (beta-d-glucopyranosyloxymethyluracil), a modified form of uracil, is also found in a number of organisms: the flagellates <a class="new" href="http://en.wikipedia.org/w/index.php?title=Diplonema_%28protozoa%29&action=edit&redlink=1" title="Diplonema (protozoa) (page does not exist)">Diplonema</a> and <a href="http://en.wikipedia.org/wiki/Euglena" title="Euglena">Euglena</a>, and all the <a href="http://en.wikipedia.org/wiki/Kinetoplastida" title="Kinetoplastida">kinetoplastid</a> genera<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Simpson1998-18">[18]</a>
 Biosynthesis of J occurs in two steps: in the first step a specific 
thymidine in DNA is converted into hydroxymethyldeoxyuridine; in the 
second HOMedU is glycosylated to form J.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Borst2008-19">[19]</a> Proteins that bind specifically to this base have been identified.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Cross1999-20">[20]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-DiPaolo2005-21">[21]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-Vainio2009-22">[22]</a> These proteins appear to be distant relatives of the Tet1 oncogene that is involved in the pathogenesis of <a href="http://en.wikipedia.org/wiki/Acute_myeloid_leukemia" title="Acute myeloid leukemia">acute myeloid leukemia</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Iyer2009-23">[23]</a> J appears to act as a termination signal for <a href="http://en.wikipedia.org/wiki/RNA_polymerase_II" title="RNA polymerase II">RNA polymerase II</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-van_Luenen2012-24">[24]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-Hazelbaker2012-25">[25]</a>

Major and minor grooves of DNA. Minor groove is a binding site for the dye <a href="http://en.wikipedia.org/wiki/Hoechst_stain" title="Hoechst stain">Hoechst 33258</a>.</div>
</div>
</div>
<h3>
Grooves</h3>
Twin helical strands form the DNA backbone. Another double helix may 
be found tracing the spaces, or grooves, between the strands. These 
voids are adjacent to the base pairs and may provide a <a href="http://en.wikipedia.org/wiki/Binding_site" title="Binding site">binding site</a>.
 As the strands are not symmetrically located with respect to each 
other, the grooves are unequally sized. One groove, the major groove, is
 22 Å wide and the other, the minor groove, is 12 Å wide.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-26">[26]</a>
 The width of the major groove means that the edges of the bases are 
more accessible in the major groove than in the minor groove. As a 
result, proteins such as <a href="http://en.wikipedia.org/wiki/Transcription_factor" title="Transcription factor">transcription factors</a>
 that can bind to specific sequences in double-stranded DNA usually make
 contact with the sides of the bases exposed in the major groove.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Pabo1984-27">[27]</a> This situation varies in unusual conformations of DNA within the cell (see below),
 but the major and minor grooves are always named to reflect the 
differences in size that would be seen if the DNA is twisted back into 
the ordinary B form.

<h3>
Base pairing</h3>
<div class="hatnote boilerplate further">
Further information: <a href="http://en.wikipedia.org/wiki/Base_pair" title="Base pair">Base pair</a></div>
In a DNA double helix, each type of nucleobase on one strand bonds 
with just one type of nucleobase on the other strand. This is called 
complementary <a href="http://en.wikipedia.org/wiki/Base_pair" title="Base pair">base pairing</a>. Here, purines form <a href="http://en.wikipedia.org/wiki/Hydrogen_bond" title="Hydrogen bond">hydrogen bonds</a>
 to pyrimidines, with adenine bonding only to thymine in two hydrogen 
bonds, and cytosine bonding only to guanine in three hydrogen bonds. 
This arrangement of two nucleotides binding together across the double 
helix is called a base pair. As hydrogen bonds are not <a href="http://en.wikipedia.org/wiki/Covalent_bond" title="Covalent bond">covalent</a>,
 they can be broken and rejoined relatively easily. The two strands of 
DNA in a double helix can therefore be pulled apart like a zipper, 
either by a mechanical force or high <a href="http://en.wikipedia.org/wiki/Temperature" title="Temperature">temperature</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-28">[28]</a>
 As a result of this complementarity, all the information in the 
double-stranded sequence of a DNA helix is duplicated on each strand, 
which is vital in DNA replication. Indeed, this reversible and specific 
interaction between complementary base pairs is critical for all the 
functions of DNA in living organisms.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Alberts-4">[4]</a>

As noted above, most DNA molecules are actually two polymer strands, 
bound together in a helical fashion by noncovalent bonds; this double 
stranded structure (dsDNA) is maintained largely by the 
intrastrand base stacking interactions, which are strongest for G,C 
stacks. The two strands can come apart – a process known as melting – to
 form two single-stranded DNA molecules (ssDNA) molecules. 
Melting occurs at high temperature, low salt and high pH (low pH also 
melts DNA, but since DNA is unstable due to acid depurination, low pH is
 rarely used).

The stability of the dsDNA form depends not only on the GC-content (%
 G,C basepairs) but also on sequence (since stacking is sequence 
specific) and also length (longer molecules are more stable). The 
stability can be measured in various ways; a common way is the "melting 
temperature", which is the temperature at which 50% of the ds molecules 
are converted to ss molecules; melting temperature is dependent on ionic
 strength and the concentration of DNA. As a result, it is both the 
percentage of GC base pairs and the overall length of a DNA double helix
 that determines the strength of the association between the two strands
 of DNA. Long DNA helices with a high GC-content have 
stronger-interacting strands, while short helices with high AT content 
have weaker-interacting strands.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-29">[29]</a> In biology, parts of the DNA double helix that need to separate easily, such as the TATAAT <a href="http://en.wikipedia.org/wiki/Pribnow_box" title="Pribnow box">Pribnow box</a> in some <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Promoter_%28biology%29" title="Promoter (biology)">promoters</a>, tend to have a high AT content, making the strands easier to pull apart.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-30">[30]</a>

In the laboratory, the strength of this interaction can be measured 
by finding the temperature necessary to break the hydrogen bonds, their <a class="mw-redirect" href="http://en.wikipedia.org/wiki/DNA_melting" title="DNA melting">melting temperature</a> (also called Tm
 value). When all the base pairs in a DNA double helix melt, the strands
 separate and exist in solution as two entirely independent molecules. 
These single-stranded DNA molecules (ssDNA) have no single common shape, but some conformations are more stable than others.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-31">[31]</a>

<h3>
Sense and antisense</h3>
<div class="hatnote boilerplate further">
Further information: <a href="http://en.wikipedia.org/wiki/Sense_%28molecular_biology%29" title="Sense (molecular biology)">Sense (molecular biology)</a></div>
A DNA sequence is called "sense" if its sequence is the same as that of a <a href="http://en.wikipedia.org/wiki/Messenger_RNA" title="Messenger RNA">messenger RNA</a> copy that is translated into protein.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-32">[32]</a>
 The sequence on the opposite strand is called the "antisense" sequence.
 Both sense and antisense sequences can exist on different parts of the 
same strand of DNA (i.e. both strands can contain both sense and 
antisense sequences). In both prokaryotes and eukaryotes, antisense RNA 
sequences are produced, but the functions of these RNAs are not entirely
 clear.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-33">[33]</a> One proposal is that antisense RNAs are involved in regulating <a href="http://en.wikipedia.org/wiki/Gene_expression" title="Gene expression">gene expression</a> through RNA-RNA base pairing.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-34">[34]</a>

A few DNA sequences in prokaryotes and eukaryotes, and more in <a href="http://en.wikipedia.org/wiki/Plasmid" title="Plasmid">plasmids</a> and <a href="http://en.wikipedia.org/wiki/Virus" title="Virus">viruses</a>, blur the distinction between sense and antisense strands by having <a href="http://en.wikipedia.org/wiki/Overlapping_gene" title="Overlapping gene">overlapping genes</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-35">[35]</a>
 In these cases, some DNA sequences do double duty, encoding one protein
 when read along one strand, and a second protein when read in the 
opposite direction along the other strand. In <a href="http://en.wikipedia.org/wiki/Bacteria" title="Bacteria">bacteria</a>, this overlap may be involved in the regulation of gene transcription,<a href="http://en.wikipedia.org/wiki/DNA#cite_note-36">[36]</a> while in viruses, overlapping genes increase the amount of information that can be encoded within the small viral genome.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-37">[37]</a>

<h3>
Supercoiling</h3>
<div class="hatnote boilerplate further">
Further information: <a href="http://en.wikipedia.org/wiki/DNA_supercoil" title="DNA supercoil">DNA supercoil</a></div>
DNA can be twisted like a rope in a process called <a href="http://en.wikipedia.org/wiki/DNA_supercoil" title="DNA supercoil">DNA supercoiling</a>.
 With DNA in its "relaxed" state, a strand usually circles the axis of 
the double helix once every 10.4 base pairs, but if the DNA is twisted 
the strands become more tightly or more loosely wound.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-38">[38]</a>
 If the DNA is twisted in the direction of the helix, this is positive 
supercoiling, and the bases are held more tightly together. If they are 
twisted in the opposite direction, this is negative supercoiling, and 
the bases come apart more easily. In nature, most DNA has slight 
negative supercoiling that is introduced by <a href="http://en.wikipedia.org/wiki/Enzyme" title="Enzyme">enzymes</a> called <a href="http://en.wikipedia.org/wiki/Topoisomerase" title="Topoisomerase">topoisomerases</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Champoux-39">[39]</a> These enzymes are also needed to relieve the twisting stresses introduced into DNA strands during processes such as <a href="http://en.wikipedia.org/wiki/Transcription_%28genetics%29" title="Transcription (genetics)">transcription</a> and <a href="http://en.wikipedia.org/wiki/DNA_replication" title="DNA replication">DNA replication</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Wang-40">[40]</a>

From left to right, the structures of A, B and Z DNA</div>
</div>
</div>
<h3>
Alternate DNA structures</h3>
<div class="hatnote boilerplate further">
Further information: <a href="http://en.wikipedia.org/wiki/Molecular_Structure_of_Nucleic_Acids:_A_Structure_for_Deoxyribose_Nucleic_Acid" title="Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid">Molecular Structure of Nucleic Acids: A Structure for Deoxyribose Nucleic Acid</a>, <a href="http://en.wikipedia.org/wiki/Molecular_models_of_DNA" title="Molecular models of DNA">Molecular models of DNA</a>, and <a class="mw-redirect" href="http://en.wikipedia.org/wiki/DNA_structure" title="DNA structure">DNA structure</a></div>
DNA exists in many possible <a href="http://en.wikipedia.org/wiki/Conformational_isomerism" title="Conformational isomerism">conformations</a> that include <a href="http://en.wikipedia.org/wiki/A-DNA" title="A-DNA">A-DNA</a>, B-DNA, and <a href="http://en.wikipedia.org/wiki/Z-DNA" title="Z-DNA">Z-DNA</a> forms, although, only B-DNA and Z-DNA have been directly observed in functional organisms.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Ghosh-12">[12]</a>
 The conformation that DNA adopts depends on the hydration level, DNA 
sequence, the amount and direction of supercoiling, chemical 
modifications of the bases, the type and concentration of metal <a href="http://en.wikipedia.org/wiki/Ion" title="Ion">ions</a>, as well as the presence of <a href="http://en.wikipedia.org/wiki/Polyamine" title="Polyamine">polyamines</a> in solution.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-41">[41]</a>

The first published reports of A-DNA <a href="http://en.wikipedia.org/wiki/X-ray_scattering_techniques" title="X-ray scattering techniques">X-ray diffraction patterns</a>—and also B-DNA—used analyses based on <a href="http://en.wikipedia.org/wiki/Patterson_function" title="Patterson function">Patterson transforms</a> that provided only a limited amount of structural information for oriented fibers of DNA.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-42">[42]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-NatFranGos-43">[43]</a> An alternate analysis was then proposed by Wilkins et al., in 1953, for the in vivo B-DNA X-ray diffraction/scattering patterns of highly hydrated DNA fibers in terms of squares of <a href="http://en.wikipedia.org/wiki/Bessel_function" title="Bessel function">Bessel functions</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-NatWilk-44">[44]</a> In the same journal, <a href="http://en.wikipedia.org/wiki/James_Watson" title="James Watson">James Watson</a> and <a href="http://en.wikipedia.org/wiki/Francis_Crick" title="Francis Crick">Francis Crick</a> presented their <a href="http://en.wikipedia.org/wiki/Molecular_models_of_DNA" title="Molecular models of DNA">molecular modeling</a> analysis of the DNA X-ray diffraction patterns to suggest that the structure was a double-helix.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-FWPUB-6">[6]</a>

Although the "B-DNA form" is most common under the conditions found in cells,<a href="http://en.wikipedia.org/wiki/DNA#cite_note-45">[45]</a> it is not a well-defined conformation but a family of related DNA conformations<a href="http://en.wikipedia.org/wiki/DNA#cite_note-46">[46]</a>
 that occur at the high hydration levels present in living cells. Their 
corresponding X-ray diffraction and scattering patterns are 
characteristic of molecular <a href="http://en.wikipedia.org/wiki/Paracrystalline" title="Paracrystalline">paracrystals</a> with a significant degree of disorder.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-47">[47]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-48">[48]</a>

Compared to B-DNA, the A-DNA form is a wider right-handed spiral, 
with a shallow, wide minor groove and a narrower, deeper major groove. 
The A form occurs under non-physiological conditions in partially 
dehydrated samples of DNA, while in the cell it may be produced in 
hybrid pairings of DNA and RNA strands, as well as in enzyme-DNA 
complexes.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-49">[49]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-50">[50]</a> Segments of DNA where the bases have been chemically modified by <a href="http://en.wikipedia.org/wiki/Methylation" title="Methylation">methylation</a> may undergo a larger change in conformation and adopt the <a href="http://en.wikipedia.org/wiki/Z-DNA" title="Z-DNA">Z form</a>. Here, the strands turn about the helical axis in a <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Left-handed" title="Left-handed">left-handed</a> spiral, the opposite of the more common B form.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-51">[51]</a>
 These unusual structures can be recognized by specific Z-DNA binding 
proteins and may be involved in the regulation of transcription.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-52">[52]</a>

<h3>
Alternative DNA chemistry</h3>
For a number of years <a href="http://en.wikipedia.org/wiki/Astrobiology" title="Astrobiology">exobiologists</a> have proposed the existence of a <a href="http://en.wikipedia.org/wiki/Shadow_biosphere" title="Shadow biosphere">shadow biosphere</a>,
 a postulated microbial biosphere of Earth that uses radically different
 biochemical and molecular processes than currently known life. One of 
the proposals was the existence of lifeforms that use <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Arsenic_DNA" title="Arsenic DNA">arsenic instead of phosphorus in DNA</a>. A report in 2010 of the possibility in the <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Bacterium" title="Bacterium">bacterium</a> <a href="http://en.wikipedia.org/wiki/GFAJ-1" title="GFAJ-1">GFAJ-1</a>, was announced,<a href="http://en.wikipedia.org/wiki/DNA#cite_note-arsenic_extremophile-53">[53]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-arsenic_extremophile-53">[53]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-Space-54">[54]</a> though the research was disputed,<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Space-54">[54]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-55">[55]</a>
 and evidence suggests the bacterium actively prevents the incorporation
 of arsenic into the DNA backbone and other biomolecules.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Nature-56">[56]</a>

<h3>
Quadruplex structures</h3>
<div class="hatnote boilerplate further">
Further information: <a href="http://en.wikipedia.org/wiki/G-quadruplex" title="G-quadruplex">G-quadruplex</a></div>
At the ends of the linear chromosomes are specialized regions of DNA called <a href="http://en.wikipedia.org/wiki/Telomere" title="Telomere">telomeres</a>. The main function of these regions is to allow the cell to replicate chromosome ends using the enzyme <a href="http://en.wikipedia.org/wiki/Telomerase" title="Telomerase">telomerase</a>, as the enzymes that normally replicate DNA cannot copy the extreme 3′ ends of chromosomes.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Greider-57">[57]</a> These specialized chromosome caps also help protect the DNA ends, and stop the <a href="http://en.wikipedia.org/wiki/DNA_repair" title="DNA repair">DNA repair</a> systems in the cell from treating them as damage to be corrected.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Nugent-58">[58]</a> In <a href="http://en.wikipedia.org/wiki/List_of_distinct_cell_types_in_the_adult_human_body" title="List of distinct cell types in the adult human body">human cells</a>, telomeres are usually lengths of single-stranded DNA containing several thousand repeats of a simple TTAGGG sequence.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-59">[59]</a>

DNA quadruplex formed by <a href="http://en.wikipedia.org/wiki/Telomere" title="Telomere">telomere</a> repeats. The looped conformation of the DNA backbone is very different from the typical DNA helix.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-60">[60]</a></div>
</div>
</div>
These guanine-rich sequences may stabilize chromosome ends by forming
 structures of stacked sets of four-base units, rather than the usual 
base pairs found in other DNA molecules. Here, four guanine bases form a
 flat plate and these flat four-base units then stack on top of each 
other, to form a stable <a href="http://en.wikipedia.org/wiki/G-quadruplex" title="G-quadruplex">G-quadruplex</a> structure.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Burge-61">[61]</a> These structures are stabilized by hydrogen bonding between the edges of the bases and <a href="http://en.wikipedia.org/wiki/Chelation" title="Chelation">chelation</a> of a metal ion in the centre of each four-base unit.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-62">[62]</a>
 Other structures can also be formed, with the central set of four bases
 coming from either a single strand folded around the bases, or several 
different parallel strands, each contributing one base to the central 
structure.

In addition to these stacked structures, telomeres also form large 
loop structures called telomere loops, or T-loops. Here, the 
single-stranded DNA curls around in a long circle stabilized by 
telomere-binding proteins.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-63">[63]</a>
 At the very end of the T-loop, the single-stranded telomere DNA is held
 onto a region of double-stranded DNA by the telomere strand disrupting 
the double-helical DNA and base pairing to one of the two strands. This <a href="http://en.wikipedia.org/wiki/Triple-stranded_DNA" title="Triple-stranded DNA">triple-stranded</a> structure is called a displacement loop or <a href="http://en.wikipedia.org/wiki/D-loop" title="D-loop">D-loop</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Burge-61">[61]</a>

<div class="thumb tright" style="background: #f9f9f9; border: 1px solid #ccc; margin: 0.5em;">
<table border="0" cellpadding="2" cellspacing="0" style="border: 1px solid #ccc; font-size: 85%; margin: 0.3em; width: 200px;">
<tbody>
<tr>
<td><a class="image" href="http://en.wikipedia.org/wiki/File:Branch-dna-single.svg"><img alt="Branch-dna-single.svg" data-file-height="150" data-file-width="200" height="71" src="http://upload.wikimedia.org/wikipedia/commons/thumb/5/54/Branch-dna-single.svg/95px-Branch-dna-single.svg.png" width="95" /></a></td>
<td><a class="image" href="http://en.wikipedia.org/wiki/File:Branch-DNA-multiple.svg"><img alt="Branch-DNA-multiple.svg" data-file-height="150" data-file-width="200" height="71" src="http://upload.wikimedia.org/wikipedia/commons/thumb/0/08/Branch-DNA-multiple.svg/95px-Branch-DNA-multiple.svg.png" width="95" /></a></td>
</tr>
<tr>
<td align="center">Single branch</td>
<td align="center">Multiple branches</td>
</tr>
</tbody></table>
<div style="border: none; font-size: 90%; width: 200px;">
<div class="thumbcaption">
<a class="mw-redirect" href="http://en.wikipedia.org/wiki/Branched_DNA" title="Branched DNA">Branched DNA</a> can form networks containing multiple branches.</div>
</div>
</div>
<h3>
Branched DNA</h3>
<div class="hatnote boilerplate further">
Further information: <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Branched_DNA" title="Branched DNA">Branched DNA</a> and <a href="http://en.wikipedia.org/wiki/DNA_nanotechnology" title="DNA nanotechnology">DNA nanotechnology</a></div>
In DNA <a class="mw-redirect" href="http://en.wikipedia.org/wiki/DNA_end#Frayed_ends" title="DNA end">fraying</a>
 occurs when non-complementary regions exist at the end of an otherwise 
complementary double-strand of DNA. However, branched DNA can occur if a
 third strand of DNA is introduced and contains adjoining regions able 
to hybridize with the frayed regions of the pre-existing double-strand. 
Although the simplest example of branched DNA involves only three 
strands of DNA, complexes involving additional strands and multiple 
branches are also possible.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-64">[64]</a> Branched DNA can be used in <a href="http://en.wikipedia.org/wiki/Nanotechnology" title="Nanotechnology">nanotechnology</a> to construct geometric shapes, see the section on <a href="http://en.wikipedia.org/wiki/DNA#Uses_in_technology" title="DNA">uses in technology</a> below.

<h2>
DNA as an anionic polymer</h2>
To the polymer chemist, DNA is a stiff, highly charged polymer, 
similar to poly styrene sulfonate. However, unlike most other polymers, 
DNA is availalbe in precise lengths, eg the DNA from bacterophage lambda
 is 48,502 basepairs long. For polymer chemists, the availablilty of 
monodisperse lengths makes DNA an interesting model.

<h2>
Chemical modifications and altered DNA packaging</h2>
<div class="thumb tright" style="background: #f9f9f9; border: 1px solid #ccc; margin: 0.5em;">
<table border="0" cellpadding="2" cellspacing="0" style="border: 1px solid #ccc; font-size: 85%; margin: 0.3em; width: 300px;">
<tbody>
<tr>
<td><a class="image" href="http://en.wikipedia.org/wiki/File:Cytosin.svg"><img alt="Cytosin.svg" data-file-height="153" data-file-width="112" height="102" src="http://upload.wikimedia.org/wikipedia/commons/thumb/d/dd/Cytosin.svg/75px-Cytosin.svg.png" width="75" /></a></td>
<td><a class="image" href="http://en.wikipedia.org/wiki/File:5-Methylcytosine.svg"><img alt="5-Methylcytosine.svg" data-file-height="543" data-file-width="620" height="83" src="http://upload.wikimedia.org/wikipedia/commons/thumb/3/3a/5-Methylcytosine.svg/95px-5-Methylcytosine.svg.png" width="95" /></a></td>
<td><a class="image" href="http://en.wikipedia.org/wiki/File:Thymin.svg"><img alt="Thymin.svg" data-file-height="154" data-file-width="180" height="83" src="http://upload.wikimedia.org/wikipedia/commons/thumb/1/15/Thymin.svg/97px-Thymin.svg.png" width="97" /></a></td>
</tr>
<tr>
<td align="center"><a href="http://en.wikipedia.org/wiki/Cytosine" title="Cytosine">cytosine</a></td>
<td align="center"><a href="http://en.wikipedia.org/wiki/5-Methylcytosine" title="5-Methylcytosine">5-methylcytosine</a></td>
<td align="center"><a href="http://en.wikipedia.org/wiki/Thymine" title="Thymine">thymine</a></td>
</tr>
</tbody></table>
<div style="border: none; font-size: 90%; width: 300px;">
<div class="thumbcaption">
Structure of cytosine with and without the 5-methyl group. <a href="http://en.wikipedia.org/wiki/Deamination" title="Deamination">Deamination</a> converts 5-methylcytosine into thymine.</div>
</div>
</div>
<h3>
Base modifications and DNA packaging</h3>
<div class="hatnote boilerplate further">
Further information: <a href="http://en.wikipedia.org/wiki/DNA_methylation" title="DNA methylation">DNA methylation</a>, <a href="http://en.wikipedia.org/wiki/Chromatin_remodeling" title="Chromatin remodeling">Chromatin remodeling</a></div>
The expression of genes is influenced by how the DNA is packaged in chromosomes, in a structure called <a href="http://en.wikipedia.org/wiki/Chromatin" title="Chromatin">chromatin</a>.
 Base modifications can be involved in packaging, with regions that have
 low or no gene expression usually containing high levels of <a href="http://en.wikipedia.org/wiki/Methylation" title="Methylation">methylation</a> of <a href="http://en.wikipedia.org/wiki/Cytosine" title="Cytosine">cytosine</a> bases. DNA packaging and its influence on gene expression can also occur by covalent modifications of the <a href="http://en.wikipedia.org/wiki/Histone" title="Histone">histone</a>
 protein core around which DNA is wrapped in the chromatin structure or 
else by remodeling carried out by chromatin remodeling complexes (see <a href="http://en.wikipedia.org/wiki/Chromatin_remodeling" title="Chromatin remodeling">Chromatin remodeling</a>). There is, further, <a href="http://en.wikipedia.org/wiki/Crosstalk_%28biology%29" title="Crosstalk (biology)">crosstalk</a> between DNA methylation and histone modification, so they can coordinately affect chromatin and gene expression.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-65">[65]</a>

For one example, cytosine methylation, produces <a href="http://en.wikipedia.org/wiki/5-Methylcytosine" title="5-Methylcytosine">5-methylcytosine</a>, which is important for <a href="http://en.wikipedia.org/wiki/X-inactivation" title="X-inactivation">X-chromosome inactivation</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-66">[66]</a> The average level of methylation varies between organisms – the worm <a href="http://en.wikipedia.org/wiki/Caenorhabditis_elegans" title="Caenorhabditis elegans">Caenorhabditis elegans</a> lacks cytosine methylation, while <a href="http://en.wikipedia.org/wiki/Vertebrate" title="Vertebrate">vertebrates</a> have higher levels, with up to 1% of their DNA containing 5-methylcytosine.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-67">[67]</a> Despite the importance of 5-methylcytosine, it can <a href="http://en.wikipedia.org/wiki/Deamination" title="Deamination">deaminate</a> to leave a thymine base, so methylated cytosines are particularly prone to <a href="http://en.wikipedia.org/wiki/Mutation" title="Mutation">mutations</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-68">[68]</a> Other base modifications include adenine methylation in bacteria, the presence of <a class="mw-redirect" href="http://en.wikipedia.org/wiki/5-hydroxymethylcytosine" title="5-hydroxymethylcytosine">5-hydroxymethylcytosine</a> in the <a href="http://en.wikipedia.org/wiki/Brain" title="Brain">brain</a>,<a href="http://en.wikipedia.org/wiki/DNA#cite_note-69">[69]</a> and the <a href="http://en.wikipedia.org/wiki/Glycosylation" title="Glycosylation">glycosylation</a> of uracil to produce the "J-base" in <a href="http://en.wikipedia.org/wiki/Kinetoplastida" title="Kinetoplastida">kinetoplastids</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-70">[70]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-71">[71]</a>

<h3>
Damage</h3>
<div class="hatnote boilerplate further">
Further information: <a href="http://en.wikipedia.org/wiki/DNA_damage_%28naturally_occurring%29" title="DNA damage (naturally occurring)">DNA damage (naturally occurring)</a>, <a href="http://en.wikipedia.org/wiki/Mutation" title="Mutation">Mutation</a>, <a href="http://en.wikipedia.org/wiki/DNA_damage_theory_of_aging" title="DNA damage theory of aging">DNA damage theory of aging</a></div>
<div class="thumb tright">
<div class="thumbinner" style="width: 222px;">
<a class="image" href="http://en.wikipedia.org/wiki/File:Benzopyrene_DNA_adduct_1JDG.png"><img alt="" class="thumbimage" data-file-height="1566" data-file-width="1131" height="305" src="http://upload.wikimedia.org/wikipedia/commons/thumb/d/d8/Benzopyrene_DNA_adduct_1JDG.png/220px-Benzopyrene_DNA_adduct_1JDG.png" width="220" /></a>
<div class="thumbcaption">

A <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Covalent" title="Covalent">covalent</a> <a href="http://en.wikipedia.org/wiki/Adduct" title="Adduct">adduct</a> between a <a href="http://en.wikipedia.org/wiki/Cytochrome_P450,_family_1,_member_A1" title="Cytochrome P450, family 1, member A1">metabolically activated</a> form of <a href="http://en.wikipedia.org/wiki/Benzo%28a%29pyrene" title="Benzo(a)pyrene">benzo[a]pyrene</a>, the major <a href="http://en.wikipedia.org/wiki/Mutagen" title="Mutagen">mutagen</a> in <a href="http://en.wikipedia.org/wiki/Tobacco_smoking" title="Tobacco smoking">tobacco smoke</a>, and DNA<a href="http://en.wikipedia.org/wiki/DNA#cite_note-72">[72]</a></div>
</div>
</div>
DNA can be damaged by many sorts of <a href="http://en.wikipedia.org/wiki/Mutagen" title="Mutagen">mutagens</a>, which change the DNA sequence. Mutagens include <a href="http://en.wikipedia.org/wiki/Oxidizing_agent" title="Oxidizing agent">oxidizing agents</a>, <a href="http://en.wikipedia.org/wiki/Alkylation" title="Alkylation">alkylating agents</a> and also high-energy <a href="http://en.wikipedia.org/wiki/Electromagnetic_radiation" title="Electromagnetic radiation">electromagnetic radiation</a> such as <a href="http://en.wikipedia.org/wiki/Ultraviolet" title="Ultraviolet">ultraviolet</a> light and <a href="http://en.wikipedia.org/wiki/X-ray" title="X-ray">X-rays</a>. The type of DNA damage produced depends on the type of mutagen. For example, UV light can damage DNA by producing <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Thymine_dimer" title="Thymine dimer">thymine dimers</a>, which are cross-links between pyrimidine bases.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-73">[73]</a> On the other hand, oxidants such as <a href="http://en.wikipedia.org/wiki/Radical_%28chemistry%29" title="Radical (chemistry)">free radicals</a> or <a href="http://en.wikipedia.org/wiki/Hydrogen_peroxide" title="Hydrogen peroxide">hydrogen peroxide</a> produce multiple forms of damage, including base modifications, particularly of guanosine, and double-strand breaks.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-74">[74]</a> A typical human cell contains about 150,000 bases that have suffered oxidative damage.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-75">[75]</a> Of these oxidative lesions, the most dangerous are double-strand breaks, as these are difficult to repair and can produce <a href="http://en.wikipedia.org/wiki/Point_mutation" title="Point mutation">point mutations</a>, <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Genetic_insertion" title="Genetic insertion">insertions</a> and <a href="http://en.wikipedia.org/wiki/Deletion_%28genetics%29" title="Deletion (genetics)">deletions</a> from the DNA sequence, as well as <a href="http://en.wikipedia.org/wiki/Chromosomal_translocation" title="Chromosomal translocation">chromosomal translocations</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-76">[76]</a> These mutations can cause <a href="http://en.wikipedia.org/wiki/Cancer" title="Cancer">cancer</a>.
 Because of inherent limitations in the DNA repair mechanisms, if humans
 lived long enough, they would all eventually develop cancer.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Weinberg-77">[77]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-78">[78]</a> DNA damages that are <a href="http://en.wikipedia.org/wiki/DNA_damage_%28naturally_occurring%29" title="DNA damage (naturally occurring)">naturally occurring</a>,
 due to normal cellular processes that produce reactive oxygen species, 
the hydrolytic activities of cellular water, etc., also occur 
frequently. Although most of these damages are repaired, in any cell 
some DNA damage may remain despite the action of repair processes. These
 remaining DNA damages accumulate with age in mammalian postmitotic 
tissues. This accumulation appears to be an important underlying cause 
of aging.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-79">[79]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-80">[80]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-81">[81]</a>

Many mutagens fit into the space between two adjacent base pairs, this is called <a href="http://en.wikipedia.org/wiki/Intercalation_%28biochemistry%29" title="Intercalation (biochemistry)">intercalation</a>. Most intercalators are <a href="http://en.wikipedia.org/wiki/Aromaticity" title="Aromaticity">aromatic</a> and planar molecules; examples include <a href="http://en.wikipedia.org/wiki/Ethidium_bromide" title="Ethidium bromide">ethidium bromide</a>, <a href="http://en.wikipedia.org/wiki/Acridine" title="Acridine">acridines</a>, <a href="http://en.wikipedia.org/wiki/Daunorubicin" title="Daunorubicin">daunomycin</a>, and <a href="http://en.wikipedia.org/wiki/Doxorubicin" title="Doxorubicin">doxorubicin</a>.
 For an intercalator to fit between base pairs, the bases must separate,
 distorting the DNA strands by unwinding of the double helix. This 
inhibits both transcription and DNA replication, causing toxicity and 
mutations.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-82">[82]</a> As a result, DNA intercalators may be <a href="http://en.wikipedia.org/wiki/Carcinogen" title="Carcinogen">carcinogens</a>, and in the case of thalidomide, a <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Teratogen" title="Teratogen">teratogen</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-83">[83]</a> Others such as <a href="http://en.wikipedia.org/wiki/Benzo%28a%29pyrene" title="Benzo(a)pyrene">benzo[a]pyrene diol epoxide</a> and <a href="http://en.wikipedia.org/wiki/Aflatoxin" title="Aflatoxin">aflatoxin</a> form DNA adducts that induce errors in replication.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-84">[84]</a> Nevertheless, due to their ability to inhibit DNA transcription and replication, other similar toxins are also used in <a href="http://en.wikipedia.org/wiki/Chemotherapy" title="Chemotherapy">chemotherapy</a> to inhibit rapidly growing <a href="http://en.wikipedia.org/wiki/Cancer" title="Cancer">cancer</a> cells.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-85">[85]</a>

<h2>
Biological functions</h2>
<div class="thumb tright">
<div class="thumbinner" style="width: 322px;">
<a class="image" href="http://en.wikipedia.org/wiki/File:Eukaryote_DNA-en.svg"><img alt="" class="thumbimage" data-file-height="734" data-file-width="1189" height="198" src="http://upload.wikimedia.org/wikipedia/commons/thumb/e/e2/Eukaryote_DNA-en.svg/320px-Eukaryote_DNA-en.svg.png" width="320" /></a>
<div class="thumbcaption">

Location of eukaryote <a href="http://en.wikipedia.org/wiki/Nuclear_DNA" title="Nuclear DNA">nuclear DNA</a> within the chromosomes.</div>
</div>
</div>
DNA usually occurs as linear <a href="http://en.wikipedia.org/wiki/Chromosome" title="Chromosome">chromosomes</a> in <a href="http://en.wikipedia.org/wiki/Eukaryote" title="Eukaryote">eukaryotes</a>, and circular chromosomes in <a href="http://en.wikipedia.org/wiki/Prokaryote" title="Prokaryote">prokaryotes</a>. The set of chromosomes in a cell makes up its <a href="http://en.wikipedia.org/wiki/Genome" title="Genome">genome</a>; the <a href="http://en.wikipedia.org/wiki/Human_genome" title="Human genome">human genome</a> has approximately 3 billion base pairs of DNA arranged into 46 chromosomes.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-86">[86]</a> The information carried by DNA is held in the <a class="mw-redirect" href="http://en.wikipedia.org/wiki/DNA_sequence" title="DNA sequence">sequence</a> of pieces of DNA called <a href="http://en.wikipedia.org/wiki/Gene" title="Gene">genes</a>. <a href="http://en.wikipedia.org/wiki/Transmission_%28genetics%29" title="Transmission (genetics)">Transmission</a>
 of genetic information in genes is achieved via complementary base 
pairing. For example, in transcription, when a cell uses the information
 in a gene, the DNA sequence is copied into a complementary RNA sequence
 through the attraction between the DNA and the correct RNA nucleotides.
 Usually, this RNA copy is then used to make a matching <a href="http://en.wikipedia.org/wiki/Peptide_sequence" title="Peptide sequence">protein sequence</a> in a process called <a href="http://en.wikipedia.org/wiki/Translation_%28biology%29" title="Translation (biology)">translation</a>,
 which depends on the same interaction between RNA nucleotides. In 
alternative fashion, a cell may simply copy its genetic information in a
 process called DNA replication. The details of these functions are 
covered in other articles; here we focus on the interactions between DNA
 and other molecules that mediate the function of the genome.

<h3>
Genes and genomes</h3>
<div class="hatnote boilerplate further">
Further information: <a href="http://en.wikipedia.org/wiki/Cell_nucleus" title="Cell nucleus">Cell nucleus</a>, <a href="http://en.wikipedia.org/wiki/Chromatin" title="Chromatin">Chromatin</a>, <a href="http://en.wikipedia.org/wiki/Chromosome" title="Chromosome">Chromosome</a>, <a href="http://en.wikipedia.org/wiki/Gene" title="Gene">Gene</a>, <a href="http://en.wikipedia.org/wiki/Noncoding_DNA" title="Noncoding DNA">Noncoding DNA</a></div>
Genomic DNA is tightly and orderly packed in the process called <a href="http://en.wikipedia.org/wiki/DNA_condensation" title="DNA condensation">DNA condensation</a> to fit the small available volumes of the cell. In eukaryotes, DNA is located in the <a href="http://en.wikipedia.org/wiki/Cell_nucleus" title="Cell nucleus">cell nucleus</a>, as well as small amounts in <a href="http://en.wikipedia.org/wiki/Mitochondrion" title="Mitochondrion">mitochondria</a> and <a href="http://en.wikipedia.org/wiki/Chloroplast" title="Chloroplast">chloroplasts</a>. In prokaryotes, the DNA is held within an irregularly shaped body in the cytoplasm called the <a href="http://en.wikipedia.org/wiki/Nucleoid" title="Nucleoid">nucleoid</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-87">[87]</a>
 The genetic information in a genome is held within genes, and the 
complete set of this information in an organism is called its <a href="http://en.wikipedia.org/wiki/Genotype" title="Genotype">genotype</a>. A gene is a unit of <a href="http://en.wikipedia.org/wiki/Heredity" title="Heredity">heredity</a> and is a region of DNA that influences a particular characteristic in an organism. Genes contain an <a href="http://en.wikipedia.org/wiki/Open_reading_frame" title="Open reading frame">open reading frame</a> that can be transcribed, as well as <a href="http://en.wikipedia.org/wiki/Regulatory_sequence" title="Regulatory sequence">regulatory sequences</a> such as <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Promoter_%28biology%29" title="Promoter (biology)">promoters</a> and <a href="http://en.wikipedia.org/wiki/Enhancer_%28genetics%29" title="Enhancer (genetics)">enhancers</a>, which control the transcription of the open reading frame.

In many <a href="http://en.wikipedia.org/wiki/Species" title="Species">species</a>, only a small fraction of the total sequence of the <a href="http://en.wikipedia.org/wiki/Genome" title="Genome">genome</a> encodes protein. For example, only about 1.5% of the human genome consists of protein-coding <a href="http://en.wikipedia.org/wiki/Exon" title="Exon">exons</a>, with over 50% of human DNA consisting of non-coding <a href="http://en.wikipedia.org/wiki/Repeated_sequence_%28DNA%29" title="Repeated sequence (DNA)">repetitive sequences</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-88">[88]</a> The reasons for the presence of so much <a href="http://en.wikipedia.org/wiki/Noncoding_DNA" title="Noncoding DNA">noncoding DNA</a> in eukaryotic genomes and the extraordinary differences in <a href="http://en.wikipedia.org/wiki/Genome_size" title="Genome size">genome size</a>, or <a href="http://en.wikipedia.org/wiki/C-value" title="C-value">C-value</a>, among species represent a long-standing puzzle known as the "<a href="http://en.wikipedia.org/wiki/C-value_enigma" title="C-value enigma">C-value enigma</a>".<a href="http://en.wikipedia.org/wiki/DNA#cite_note-89">[89]</a> However, some DNA sequences that do not code protein may still encode functional <a href="http://en.wikipedia.org/wiki/Non-coding_RNA" title="Non-coding RNA">non-coding RNA</a> molecules, which are involved in the <a href="http://en.wikipedia.org/wiki/Regulation_of_gene_expression" title="Regulation of gene expression">regulation of gene expression</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-90">[90]</a>

<a href="http://en.wikipedia.org/wiki/T7_RNA_polymerase" title="T7 RNA polymerase">T7 RNA polymerase</a> (blue) producing a mRNA (green) from a DNA template (orange).<a href="http://en.wikipedia.org/wiki/DNA#cite_note-91">[91]</a></div>
</div>
</div>
Some noncoding DNA sequences play structural roles in chromosomes. <a href="http://en.wikipedia.org/wiki/Telomere" title="Telomere">Telomeres</a> and <a href="http://en.wikipedia.org/wiki/Centromere" title="Centromere">centromeres</a> typically contain few genes, but are important for the function and stability of chromosomes.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Nugent-58">[58]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-92">[92]</a> An abundant form of noncoding DNA in humans are <a href="http://en.wikipedia.org/wiki/Pseudogene" title="Pseudogene">pseudogenes</a>, which are copies of genes that have been disabled by mutation.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-93">[93]</a> These sequences are usually just molecular <a href="http://en.wikipedia.org/wiki/Fossil" title="Fossil">fossils</a>, although they can occasionally serve as raw genetic material for the creation of new genes through the process of <a href="http://en.wikipedia.org/wiki/Gene_duplication" title="Gene duplication">gene duplication</a> and <a href="http://en.wikipedia.org/wiki/Divergent_evolution" title="Divergent evolution">divergence</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-94">[94]</a>

<h3>
Transcription and translation</h3>
<div class="hatnote boilerplate further">
Further information: <a href="http://en.wikipedia.org/wiki/Genetic_code" title="Genetic code">Genetic code</a>, <a href="http://en.wikipedia.org/wiki/Transcription_%28genetics%29" title="Transcription (genetics)">Transcription (genetics)</a>, <a href="http://en.wikipedia.org/wiki/Protein_biosynthesis" title="Protein biosynthesis">Protein biosynthesis</a></div>
A gene is a sequence of DNA that contains genetic information and can influence the <a href="http://en.wikipedia.org/wiki/Phenotype" title="Phenotype">phenotype</a> of an organism. Within a gene, the sequence of bases along a DNA strand defines a <a href="http://en.wikipedia.org/wiki/Messenger_RNA" title="Messenger RNA">messenger RNA</a> sequence, which then defines one or more protein sequences. The relationship between the nucleotide sequences of genes and the <a href="http://en.wikipedia.org/wiki/Amino_acid" title="Amino acid">amino-acid</a> sequences of proteins is determined by the rules of <a href="http://en.wikipedia.org/wiki/Translation_%28biology%29" title="Translation (biology)">translation</a>, known collectively as the <a href="http://en.wikipedia.org/wiki/Genetic_code" title="Genetic code">genetic code</a>. The genetic code consists of three-letter 'words' called codons formed from a sequence of three nucleotides (e.g. ACT, CAG, TTT).

In transcription, the codons of a gene are copied into messenger RNA by <a href="http://en.wikipedia.org/wiki/RNA_polymerase" title="RNA polymerase">RNA polymerase</a>. This RNA copy is then decoded by a <a href="http://en.wikipedia.org/wiki/Ribosome" title="Ribosome">ribosome</a> that reads the RNA sequence by base-pairing the messenger RNA to <a href="http://en.wikipedia.org/wiki/Transfer_RNA" title="Transfer RNA">transfer RNA</a>, which carries amino acids. Since there are 4 bases in 3-letter combinations, there are 64 possible codons (43 combinations). These encode the twenty <a class="mw-redirect" href="http://en.wikipedia.org/wiki/List_of_standard_amino_acids" title="List of standard amino acids">standard amino acids</a>,
 giving most amino acids more than one possible codon. There are also 
three 'stop' or 'nonsense' codons signifying the end of the coding 
region; these are the TAA, TGA, and TAG codons.

DNA replication. The double helix is unwound by a <a href="http://en.wikipedia.org/wiki/Helicase" title="Helicase">helicase</a> and <a href="http://en.wikipedia.org/wiki/Topoisomerase" title="Topoisomerase">topoisomerase</a>. Next, one <a href="http://en.wikipedia.org/wiki/DNA_polymerase" title="DNA polymerase">DNA polymerase</a> produces the <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Replication_fork" title="Replication fork">leading strand</a> copy. Another DNA polymerase binds to the <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Replication_fork" title="Replication fork">lagging strand</a>. This enzyme makes discontinuous segments (called <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Okazaki_fragment" title="Okazaki fragment">Okazaki fragments</a>) before <a href="http://en.wikipedia.org/wiki/DNA_ligase" title="DNA ligase">DNA ligase</a> joins them together.</div>
</div>
</div>
<h3>
Replication</h3>
<div class="hatnote boilerplate further">
Further information: <a href="http://en.wikipedia.org/wiki/DNA_replication" title="DNA replication">DNA replication</a></div>
<a href="http://en.wikipedia.org/wiki/Cell_division" title="Cell division">Cell division</a>
 is essential for an organism to grow, but, when a cell divides, it must
 replicate the DNA in its genome so that the two daughter cells have the
 same genetic information as their parent. The double-stranded structure
 of DNA provides a simple mechanism for <a href="http://en.wikipedia.org/wiki/DNA_replication" title="DNA replication">DNA replication</a>. Here, the two strands are separated and then each strand's <a href="http://en.wikipedia.org/wiki/Complementary_DNA" title="Complementary DNA">complementary DNA</a> sequence is recreated by an <a href="http://en.wikipedia.org/wiki/Enzyme" title="Enzyme">enzyme</a> called <a href="http://en.wikipedia.org/wiki/DNA_polymerase" title="DNA polymerase">DNA polymerase</a>.
 This enzyme makes the complementary strand by finding the correct base 
through complementary base pairing, and bonding it onto the original 
strand. As DNA polymerases can only extend a DNA strand in a 5′ to 3′ 
direction, different mechanisms are used to copy the antiparallel 
strands of the double helix.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-95">[95]</a>
 In this way, the base on the old strand dictates which base appears on 
the new strand, and the cell ends up with a perfect copy of its DNA.

<h2>
Interactions with proteins</h2>
All the functions of DNA depend on interactions with proteins. These <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Protein_interactions" title="Protein interactions">protein interactions</a>
 can be non-specific, or the protein can bind specifically to a single 
DNA sequence. Enzymes can also bind to DNA and of these, the polymerases
 that copy the DNA base sequence in transcription and DNA replication 
are particularly important.

<h3>
DNA-binding proteins</h3>
<div class="hatnote boilerplate further">
Further information: <a href="http://en.wikipedia.org/wiki/DNA-binding_protein" title="DNA-binding protein">DNA-binding protein</a></div>
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<td><a class="image" href="http://en.wikipedia.org/wiki/File:Nucleosome1.png"><img alt="Nucleosome1.png" data-file-height="872" data-file-width="1328" height="171" src="http://upload.wikimedia.org/wikipedia/commons/thumb/b/be/Nucleosome1.png/260px-Nucleosome1.png" width="260" /></a></td>
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<td> </td>
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<div style="border: none; width: 260px;">
<div class="thumbcaption">
Interaction of DNA (shown in orange) with <a href="http://en.wikipedia.org/wiki/Histone" title="Histone">histones</a> (shown in blue). These proteins' basic amino acids bind to the acidic phosphate groups on DNA.</div>
</div>
</div>
Structural proteins that bind DNA are well-understood examples of 
non-specific DNA-protein interactions. Within chromosomes, DNA is held 
in complexes with structural proteins. These proteins organize the DNA 
into a compact structure called <a href="http://en.wikipedia.org/wiki/Chromatin" title="Chromatin">chromatin</a>. In eukaryotes this structure involves DNA binding to a complex of small basic proteins called <a href="http://en.wikipedia.org/wiki/Histone" title="Histone">histones</a>, while in prokaryotes multiple types of proteins are involved.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-96">[96]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-97">[97]</a> The histones form a disk-shaped complex called a <a href="http://en.wikipedia.org/wiki/Nucleosome" title="Nucleosome">nucleosome</a>,
 which contains two complete turns of double-stranded DNA wrapped around
 its surface. These non-specific interactions are formed through basic 
residues in the histones making <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Ionic_bond" title="Ionic bond">ionic bonds</a> to the acidic sugar-phosphate backbone of the DNA, and are therefore largely independent of the base sequence.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-98">[98]</a> Chemical modifications of these basic amino acid residues include <a href="http://en.wikipedia.org/wiki/Methylation" title="Methylation">methylation</a>, <a href="http://en.wikipedia.org/wiki/Phosphorylation" title="Phosphorylation">phosphorylation</a> and <a href="http://en.wikipedia.org/wiki/Acetylation" title="Acetylation">acetylation</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-99">[99]</a>
 These chemical changes alter the strength of the interaction between 
the DNA and the histones, making the DNA more or less accessible to <a href="http://en.wikipedia.org/wiki/Transcription_factor" title="Transcription factor">transcription factors</a> and changing the rate of transcription.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-100">[100]</a>
 Other non-specific DNA-binding proteins in chromatin include the 
high-mobility group proteins, which bind to bent or distorted DNA.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-101">[101]</a>
 These proteins are important in bending arrays of nucleosomes and 
arranging them into the larger structures that make up chromosomes.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-102">[102]</a>

A distinct group of DNA-binding proteins are the DNA-binding proteins
 that specifically bind single-stranded DNA. In humans, replication <a href="http://en.wikipedia.org/wiki/Protein_A" title="Protein A">protein A</a>
 is the best-understood member of this family and is used in processes 
where the double helix is separated, including DNA replication, 
recombination and DNA repair.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-103">[103]</a> These binding proteins seem to stabilize single-stranded DNA and protect it from forming <a href="http://en.wikipedia.org/wiki/Stem-loop" title="Stem-loop">stem-loops</a> or being degraded by <a href="http://en.wikipedia.org/wiki/Nuclease" title="Nuclease">nucleases</a>.

The lambda repressor <a href="http://en.wikipedia.org/wiki/Helix-turn-helix" title="Helix-turn-helix">helix-turn-helix</a> transcription factor bound to its DNA target<a href="http://en.wikipedia.org/wiki/DNA#cite_note-104">[104]</a></div>
</div>
</div>
In contrast, other proteins have evolved to bind to particular DNA 
sequences. The most intensively studied of these are the various <a href="http://en.wikipedia.org/wiki/Transcription_factor" title="Transcription factor">transcription factors</a>,
 which are proteins that regulate transcription. Each transcription 
factor binds to one particular set of DNA sequences and activates or 
inhibits the transcription of genes that have these sequences close to 
their promoters. The transcription factors do this in two ways. Firstly,
 they can bind the RNA polymerase responsible for transcription, either 
directly or through other mediator proteins; this locates the polymerase
 at the promoter and allows it to begin transcription.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-105">[105]</a> Alternatively, transcription factors can bind <a href="http://en.wikipedia.org/wiki/Enzyme" title="Enzyme">enzymes</a> that modify the histones at the promoter. This changes the accessibility of the DNA template to the polymerase.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-106">[106]</a>

As these DNA targets can occur throughout an organism's genome, 
changes in the activity of one type of transcription factor can affect 
thousands of genes.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-107">[107]</a> Consequently, these proteins are often the targets of the <a href="http://en.wikipedia.org/wiki/Signal_transduction" title="Signal transduction">signal transduction</a> processes that control responses to environmental changes or <a href="http://en.wikipedia.org/wiki/Cellular_differentiation" title="Cellular differentiation">cellular differentiation</a>
 and development. The specificity of these transcription factors' 
interactions with DNA come from the proteins making multiple contacts to
 the edges of the DNA bases, allowing them to "read" the DNA sequence. 
Most of these base-interactions are made in the major groove, where the 
bases are most accessible.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Pabo1984-27">[27]</a>

The <a href="http://en.wikipedia.org/wiki/Restriction_enzyme" title="Restriction enzyme">restriction enzyme</a> <a href="http://en.wikipedia.org/wiki/EcoRV" title="EcoRV">EcoRV</a> (green) in a complex with its substrate DNA<a href="http://en.wikipedia.org/wiki/DNA#cite_note-108">[108]</a></div>
</div>
</div>
<h3>
DNA-modifying enzymes</h3>
<h4>
Nucleases and ligases</h4>
<a href="http://en.wikipedia.org/wiki/Nuclease" title="Nuclease">Nucleases</a> are <a href="http://en.wikipedia.org/wiki/Enzyme" title="Enzyme">enzymes</a> that cut DNA strands by catalyzing the <a href="http://en.wikipedia.org/wiki/Hydrolysis" title="Hydrolysis">hydrolysis</a> of the <a href="http://en.wikipedia.org/wiki/Phosphodiester_bond" title="Phosphodiester bond">phosphodiester bonds</a>. Nucleases that hydrolyse nucleotides from the ends of DNA strands are called <a href="http://en.wikipedia.org/wiki/Exonuclease" title="Exonuclease">exonucleases</a>, while <a href="http://en.wikipedia.org/wiki/Endonuclease" title="Endonuclease">endonucleases</a> cut within strands. The most frequently used nucleases in <a href="http://en.wikipedia.org/wiki/Molecular_biology" title="Molecular biology">molecular biology</a> are the <a href="http://en.wikipedia.org/wiki/Restriction_enzyme" title="Restriction enzyme">restriction endonucleases</a>,
 which cut DNA at specific sequences. For instance, the EcoRV enzyme 
shown to the left recognizes the 6-base sequence 5′-GATATC-3′ and makes a
 cut at the vertical line. In nature, these enzymes protect <a href="http://en.wikipedia.org/wiki/Bacteria" title="Bacteria">bacteria</a> against <a href="http://en.wikipedia.org/wiki/Bacteriophage" title="Bacteriophage">phage</a> infection by digesting the phage DNA when it enters the bacterial cell, acting as part of the <a href="http://en.wikipedia.org/wiki/Restriction_modification_system" title="Restriction modification system">restriction modification system</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-109">[109]</a> In technology, these sequence-specific nucleases are used in <a href="http://en.wikipedia.org/wiki/Molecular_cloning" title="Molecular cloning">molecular cloning</a> and <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Genetic_fingerprinting" title="Genetic fingerprinting">DNA fingerprinting</a>.

Enzymes called <a href="http://en.wikipedia.org/wiki/DNA_ligase" title="DNA ligase">DNA ligases</a> can rejoin cut or broken DNA strands.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Doherty-110">[110]</a> Ligases are particularly important in <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Replication_fork" title="Replication fork">lagging strand</a> DNA replication, as they join together the short segments of DNA produced at the <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Replication_fork" title="Replication fork">replication fork</a> into a complete copy of the DNA template. They are also used in <a href="http://en.wikipedia.org/wiki/DNA_repair" title="DNA repair">DNA repair</a> and <a href="http://en.wikipedia.org/wiki/Genetic_recombination" title="Genetic recombination">genetic recombination</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Doherty-110">[110]</a>

<h4>
Topoisomerases and helicases</h4>
<a href="http://en.wikipedia.org/wiki/Topoisomerase" title="Topoisomerase">Topoisomerases</a> are enzymes with both nuclease and ligase activity. These proteins change the amount of <a href="http://en.wikipedia.org/wiki/DNA_supercoil" title="DNA supercoil">supercoiling</a>
 in DNA. Some of these enzymes work by cutting the DNA helix and 
allowing one section to rotate, thereby reducing its level of 
supercoiling; the enzyme then seals the DNA break.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Champoux-39">[39]</a>
 Other types of these enzymes are capable of cutting one DNA helix and 
then passing a second strand of DNA through this break, before rejoining
 the helix.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-111">[111]</a> Topoisomerases are required for many processes involving DNA, such as DNA replication and transcription.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Wang-40">[40]</a>

<a href="http://en.wikipedia.org/wiki/Helicase" title="Helicase">Helicases</a> are proteins that are a type of <a href="http://en.wikipedia.org/wiki/Molecular_motor" title="Molecular motor">molecular motor</a>. They use the chemical energy in <a href="http://en.wikipedia.org/wiki/Nucleoside_triphosphate" title="Nucleoside triphosphate">nucleoside triphosphates</a>, predominantly <a href="http://en.wikipedia.org/wiki/Adenosine_triphosphate" title="Adenosine triphosphate">ATP</a>, to break hydrogen bonds between bases and unwind the DNA double helix into single strands.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-112">[112]</a> These enzymes are essential for most processes where enzymes need to access the DNA bases.

<h4>
Polymerases</h4>
<a href="http://en.wikipedia.org/wiki/Polymerase" title="Polymerase">Polymerases</a> are <a href="http://en.wikipedia.org/wiki/Enzyme" title="Enzyme">enzymes</a> that synthesize polynucleotide chains from <a href="http://en.wikipedia.org/wiki/Nucleoside_triphosphate" title="Nucleoside triphosphate">nucleoside triphosphates</a>. The sequence of their products are created based on existing polynucleotide chains—which are called templates. These enzymes function by repeatedly adding a nucleotide to the 3′ <a href="http://en.wikipedia.org/wiki/Hydroxyl" title="Hydroxyl">hydroxyl group</a> at the end of the growing polynucleotide chain. As a consequence, all polymerases work in a 5′ to 3′ direction.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Joyce-113">[113]</a> In the <a href="http://en.wikipedia.org/wiki/Active_site" title="Active site">active site</a>
 of these enzymes, the incoming nucleoside triphosphate base-pairs to 
the template: this allows polymerases to accurately synthesize the 
complementary strand of their template. Polymerases are classified 
according to the type of template that they use.

In DNA replication, DNA-dependent <a href="http://en.wikipedia.org/wiki/DNA_polymerase" title="DNA polymerase">DNA polymerases</a>
 make copies of DNA polynucleotide chains. In order to preserve 
biological information, it is essential that the sequence of bases in 
each copy are precisely complementary to the sequence of bases in the 
template strand. Many DNA polymerases have a <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Proofreading_%28Biology%29" title="Proofreading (Biology)">proofreading</a>
 activity. Here, the polymerase recognizes the occasional mistakes in 
the synthesis reaction by the lack of base pairing between the 
mismatched nucleotides. If a mismatch is detected, a 3′ to 5′ <a href="http://en.wikipedia.org/wiki/Exonuclease" title="Exonuclease">exonuclease</a> activity is activated and the incorrect base removed.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-114">[114]</a> In most organisms, DNA polymerases function in a large complex called the <a href="http://en.wikipedia.org/wiki/Replisome" title="Replisome">replisome</a> that contains multiple accessory subunits, such as the <a href="http://en.wikipedia.org/wiki/DNA_clamp" title="DNA clamp">DNA clamp</a> or <a href="http://en.wikipedia.org/wiki/Helicase" title="Helicase">helicases</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-115">[115]</a>

RNA-dependent DNA polymerases are a specialized class of polymerases 
that copy the sequence of an RNA strand into DNA. They include <a href="http://en.wikipedia.org/wiki/Reverse_transcriptase" title="Reverse transcriptase">reverse transcriptase</a>, which is a <a href="http://en.wikipedia.org/wiki/Virus" title="Virus">viral</a> enzyme involved in the infection of cells by <a href="http://en.wikipedia.org/wiki/Retrovirus" title="Retrovirus">retroviruses</a>, and <a href="http://en.wikipedia.org/wiki/Telomerase" title="Telomerase">telomerase</a>, which is required for the replication of telomeres.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Greider-57">[57]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-116">[116]</a> Telomerase is an unusual polymerase because it contains its own RNA template as part of its structure.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Nugent-58">[58]</a>

Transcription is carried out by a DNA-dependent <a href="http://en.wikipedia.org/wiki/RNA_polymerase" title="RNA polymerase">RNA polymerase</a>
 that copies the sequence of a DNA strand into RNA. To begin 
transcribing a gene, the RNA polymerase binds to a sequence of DNA 
called a promoter and separates the DNA strands. It then copies the gene
 sequence into a <a href="http://en.wikipedia.org/wiki/Messenger_RNA" title="Messenger RNA">messenger RNA</a> transcript until it reaches a region of DNA called the <a href="http://en.wikipedia.org/wiki/Terminator_%28genetics%29" title="Terminator (genetics)">terminator</a>, where it halts and detaches from the DNA. As with human DNA-dependent DNA polymerases, <a href="http://en.wikipedia.org/wiki/RNA_polymerase_II" title="RNA polymerase II">RNA polymerase II</a>, the enzyme that transcribes most of the genes in the human genome, operates as part of a large <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Protein_complex" title="Protein complex">protein complex</a> with multiple regulatory and accessory subunits.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-117">[117]</a>

<h2>
Genetic recombination</h2>
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<table border="0" cellpadding="0" cellspacing="0" style="border: 1px solid #ccc; font-size: 85%; margin: 0.3em; width: 250px;">
<tbody>
<tr>
<td><a class="image" href="http://en.wikipedia.org/wiki/File:Holliday_Junction.svg"><img alt="Holliday Junction.svg" data-file-height="376" data-file-width="472" height="199" src="http://upload.wikimedia.org/wikipedia/commons/thumb/8/83/Holliday_Junction.svg/250px-Holliday_Junction.svg.png" width="250" /></a></td>
</tr>
<tr>
<td><a class="image" href="http://en.wikipedia.org/wiki/File:Holliday_junction_coloured.png"><img alt="Holliday junction coloured.png" data-file-height="1620" data-file-width="1620" height="250" src="http://upload.wikimedia.org/wikipedia/commons/thumb/9/92/Holliday_junction_coloured.png/250px-Holliday_junction_coloured.png" width="250" /></a></td>
</tr>
</tbody></table>
<div style="border: none; width: 250px;">
<div class="thumbcaption">
Structure of the <a href="http://en.wikipedia.org/wiki/Holliday_junction" title="Holliday junction">Holliday junction</a> intermediate in <a href="http://en.wikipedia.org/wiki/Genetic_recombination" title="Genetic recombination">genetic recombination</a>. The four separate DNA strands are coloured red, blue, green and yellow.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-118">[118]</a></div>
</div>
</div>
<div class="hatnote boilerplate further">
Further information: <a href="http://en.wikipedia.org/wiki/Genetic_recombination" title="Genetic recombination">Genetic recombination</a></div>
<div class="thumb tleft">
<div class="thumbinner" style="width: 252px;">
<a class="image" href="http://en.wikipedia.org/wiki/File:Chromosomal_Recombination.svg"><img alt="" class="thumbimage" data-file-height="334" data-file-width="517" height="162" src="http://upload.wikimedia.org/wikipedia/commons/thumb/b/b2/Chromosomal_Recombination.svg/250px-Chromosomal_Recombination.svg.png" width="250" /></a>
<div class="thumbcaption">

Recombination involves the breakage and rejoining of two chromosomes (M 
and F) to produce two re-arranged chromosomes (C1 and C2).</div>
</div>
</div>
A DNA helix usually does not interact with other segments of DNA, and
 in human cells the different chromosomes even occupy separate areas in 
the nucleus called "chromosome territories".<a href="http://en.wikipedia.org/wiki/DNA#cite_note-119">[119]</a>
 This physical separation of different chromosomes is important for the 
ability of DNA to function as a stable repository for information, as 
one of the few times chromosomes interact is during <a href="http://en.wikipedia.org/wiki/Chromosomal_crossover" title="Chromosomal crossover">chromosomal crossover</a> when they <a href="http://en.wikipedia.org/wiki/Genetic_recombination" title="Genetic recombination">recombine</a>. Chromosomal crossover is when two DNA helices break, swap a section and then rejoin.

Recombination allows chromosomes to exchange genetic information and 
produces new combinations of genes, which increases the efficiency of <a href="http://en.wikipedia.org/wiki/Natural_selection" title="Natural selection">natural selection</a> and can be important in the rapid evolution of new proteins.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-120">[120]</a> Genetic recombination can also be involved in DNA repair, particularly in the cell's response to double-strand breaks.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-121">[121]</a>

The most common form of chromosomal crossover is <a href="http://en.wikipedia.org/wiki/Homologous_recombination" title="Homologous recombination">homologous recombination</a>,
 where the two chromosomes involved share very similar sequences. 
Non-homologous recombination can be damaging to cells, as it can produce
 <a href="http://en.wikipedia.org/wiki/Chromosomal_translocation" title="Chromosomal translocation">chromosomal translocations</a> and genetic abnormalities. The recombination reaction is catalyzed by enzymes known as <a href="http://en.wikipedia.org/wiki/Recombinase" title="Recombinase">recombinases</a>, such as <a href="http://en.wikipedia.org/wiki/RAD51" title="RAD51">RAD51</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-122">[122]</a> The first step in recombination is a double-stranded break caused by either an <a href="http://en.wikipedia.org/wiki/Endonuclease" title="Endonuclease">endonuclease</a> or damage to the DNA.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-123">[123]</a> A series of steps catalyzed in part by the recombinase then leads to joining of the two helices by at least one <a href="http://en.wikipedia.org/wiki/Holliday_junction" title="Holliday junction">Holliday junction</a>,
 in which a segment of a single strand in each helix is annealed to the 
complementary strand in the other helix. The Holliday junction is a 
tetrahedral junction structure that can be moved along the pair of 
chromosomes, swapping one strand for another. The recombination reaction
 is then halted by cleavage of the junction and re-ligation of the 
released DNA.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-124">[124]</a>

<h2>
Evolution</h2>
<div class="hatnote boilerplate further">
Further information: <a class="mw-redirect" href="http://en.wikipedia.org/wiki/RNA_world_hypothesis" title="RNA world hypothesis">RNA world hypothesis</a></div>
DNA contains the genetic information that allows all modern living 
things to function, grow and reproduce. However, it is unclear how long 
in the 4-billion-year <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Timeline_of_evolution" title="Timeline of evolution">history of life</a>
 DNA has performed this function, as it has been proposed that the 
earliest forms of life may have used RNA as their genetic material.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-autogenerated1-125">[125]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-126">[126]</a> RNA may have acted as the central part of early <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Cell_metabolism" title="Cell metabolism">cell metabolism</a> as it can both transmit genetic information and carry out <a href="http://en.wikipedia.org/wiki/Catalysis" title="Catalysis">catalysis</a> as part of <a href="http://en.wikipedia.org/wiki/Ribozyme" title="Ribozyme">ribozymes</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-127">[127]</a> This ancient <a class="mw-redirect" href="http://en.wikipedia.org/wiki/RNA_world_hypothesis" title="RNA world hypothesis">RNA world</a> where nucleic acid would have been used for both catalysis and genetics may have influenced the <a href="http://en.wikipedia.org/wiki/Evolution" title="Evolution">evolution</a>
 of the current genetic code based on four nucleotide bases. This would 
occur, since the number of different bases in such an organism is a 
trade-off between a small number of bases increasing replication 
accuracy and a large number of bases increasing the catalytic efficiency
 of ribozymes.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-128">[128]</a>

However, there is no direct evidence of ancient genetic systems, as 
recovery of DNA from most fossils is impossible. This is because DNA 
survives in the environment for less than one million years, and slowly 
degrades into short fragments in solution.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-129">[129]</a>
 Claims for older DNA have been made, most notably a report of the 
isolation of a viable bacterium from a salt crystal 250 million years 
old,<a href="http://en.wikipedia.org/wiki/DNA#cite_note-130">[130]</a> but these claims are controversial.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-131">[131]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-132">[132]</a>

Building blocks of DNA (<a href="http://en.wikipedia.org/wiki/Adenine" title="Adenine">adenine</a>, <a href="http://en.wikipedia.org/wiki/Guanine" title="Guanine">guanine</a> and related <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Organic_molecules" title="Organic molecules">organic molecules</a>) may have been formed extraterrestrially in <a href="http://en.wikipedia.org/wiki/Outer_space" title="Outer space">outer space</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Callahan-133">[133]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-Steigerwald-134">[134]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-DNA-135">[135]</a> Complex DNA and <a href="http://en.wikipedia.org/wiki/RNA" title="RNA">RNA</a> <a href="http://en.wikipedia.org/wiki/Organic_compound" title="Organic compound">organic compounds</a> of <a href="http://en.wikipedia.org/wiki/Life" title="Life">life</a>, including <a href="http://en.wikipedia.org/wiki/Uracil" title="Uracil">uracil</a>, <a href="http://en.wikipedia.org/wiki/Cytosine" title="Cytosine">cytosine</a> and <a href="http://en.wikipedia.org/wiki/Thymine" title="Thymine">thymine</a>, have also been formed in the laboratory under conditions mimicking those found in <a href="http://en.wikipedia.org/wiki/Outer_space" title="Outer space">outer space</a>, using starting chemicals, such as <a href="http://en.wikipedia.org/wiki/Pyrimidine" title="Pyrimidine">pyrimidine</a>, found in <a href="http://en.wikipedia.org/wiki/Meteorite" title="Meteorite">meteorites</a>. Pyrimidine, like <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Polycyclic_aromatic_hydrocarbons" title="Polycyclic aromatic hydrocarbons">polycyclic aromatic hydrocarbons</a> (PAHs), the most carbon-rich chemical found in the <a href="http://en.wikipedia.org/wiki/Universe" title="Universe">universe</a>, may have been formed in <a href="http://en.wikipedia.org/wiki/Red_giant" title="Red giant">red giants</a> or in <a href="http://en.wikipedia.org/wiki/Cosmic_dust" title="Cosmic dust">interstellar dust</a> and gas clouds.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-NASA-20150303-136">[136]</a>

<h2>
Uses in technology</h2>
<div class="thumb tright">
<div class="thumbinner" style="width: 252px;">
<a class="image" href="http://en.wikipedia.org/wiki/File:DNA_DL90_%28geograph_2847164%29.jpg"><img alt="" class="thumbimage" data-file-height="1024" data-file-width="512" height="500" src="http://upload.wikimedia.org/wikipedia/commons/thumb/3/32/DNA_DL90_%28geograph_2847164%29.jpg/250px-DNA_DL90_%28geograph_2847164%29.jpg" width="250" /></a>
<div class="thumbcaption">

Sculpture of DNA, made out of shopping carts</div>
</div>
</div>
<h3>
Genetic engineering</h3>
<div class="hatnote boilerplate further">
Further information: <a href="http://en.wikipedia.org/wiki/Molecular_biology" title="Molecular biology">Molecular biology</a>, <a href="http://en.wikipedia.org/wiki/Nucleic_acid_methods" title="Nucleic acid methods">nucleic acid methods</a> and <a href="http://en.wikipedia.org/wiki/Genetic_engineering" title="Genetic engineering">genetic engineering</a></div>
Methods have been developed to purify DNA from organisms, such as <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Phenol-chloroform_extraction" title="Phenol-chloroform extraction">phenol-chloroform extraction</a>, and to manipulate it in the laboratory, such as <a href="http://en.wikipedia.org/wiki/Restriction_digest" title="Restriction digest">restriction digests</a> and the <a href="http://en.wikipedia.org/wiki/Polymerase_chain_reaction" title="Polymerase chain reaction">polymerase chain reaction</a>. Modern <a href="http://en.wikipedia.org/wiki/Biology" title="Biology">biology</a> and <a href="http://en.wikipedia.org/wiki/Biochemistry" title="Biochemistry">biochemistry</a> make intensive use of these techniques in recombinant DNA technology. <a href="http://en.wikipedia.org/wiki/Recombinant_DNA" title="Recombinant DNA">Recombinant DNA</a> is a man-made DNA sequence that has been assembled from other DNA sequences. They can be <a href="http://en.wikipedia.org/wiki/Transformation_%28genetics%29" title="Transformation (genetics)">transformed</a> into organisms in the form of <a href="http://en.wikipedia.org/wiki/Plasmid" title="Plasmid">plasmids</a> or in the appropriate format, by using a <a href="http://en.wikipedia.org/wiki/Viral_vector" title="Viral vector">viral vector</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-137">[137]</a> The <a href="http://en.wikipedia.org/wiki/Genetic_engineering" title="Genetic engineering">genetically modified</a> organisms produced can be used to produce products such as recombinant <a href="http://en.wikipedia.org/wiki/Protein" title="Protein">proteins</a>, used in <a href="http://en.wikipedia.org/wiki/Medical_research" title="Medical research">medical research</a>,<a href="http://en.wikipedia.org/wiki/DNA#cite_note-138">[138]</a> or be grown in <a href="http://en.wikipedia.org/wiki/Agriculture" title="Agriculture">agriculture</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-139">[139]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-140">[140]</a>

<h3>
Forensics</h3>
<div class="hatnote boilerplate further">
Further information: <a href="http://en.wikipedia.org/wiki/DNA_profiling" title="DNA profiling">DNA profiling</a></div>
<a href="http://en.wikipedia.org/wiki/Forensic_science" title="Forensic science">Forensic scientists</a> can use DNA in <a href="http://en.wikipedia.org/wiki/Blood" title="Blood">blood</a>, <a href="http://en.wikipedia.org/wiki/Semen" title="Semen">semen</a>, <a href="http://en.wikipedia.org/wiki/Skin" title="Skin">skin</a>, <a href="http://en.wikipedia.org/wiki/Saliva" title="Saliva">saliva</a> or <a href="http://en.wikipedia.org/wiki/Hair" title="Hair">hair</a> found at a <a href="http://en.wikipedia.org/wiki/Crime_scene" title="Crime scene">crime scene</a> to identify a matching DNA of an individual, such as a perpetrator. This process is formally termed <a href="http://en.wikipedia.org/wiki/DNA_profiling" title="DNA profiling">DNA profiling</a>, but may also be called "<a class="mw-redirect" href="http://en.wikipedia.org/wiki/Genetic_fingerprinting" title="Genetic fingerprinting">genetic fingerprinting</a>". In DNA profiling, the lengths of variable sections of repetitive DNA, such as <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Short_tandem_repeat" title="Short tandem repeat">short tandem repeats</a> and <a href="http://en.wikipedia.org/wiki/Minisatellite" title="Minisatellite">minisatellites</a>, are compared between people. This method is usually an extremely reliable technique for identifying a matching DNA.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-141">[141]</a> However, identification can be complicated if the scene is contaminated with DNA from several people.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-142">[142]</a> DNA profiling was developed in 1984 by British geneticist Sir <a href="http://en.wikipedia.org/wiki/Alec_Jeffreys" title="Alec Jeffreys">Alec Jeffreys</a>,<a href="http://en.wikipedia.org/wiki/DNA#cite_note-143">[143]</a> and first used in forensic science to convict Colin Pitchfork in the 1988 <a href="http://en.wikipedia.org/wiki/Colin_Pitchfork" title="Colin Pitchfork">Enderby murders</a> case.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-144">[144]</a>

The development of forensic science, and the ability to now obtain 
genetic matching on minute samples of blood, skin, saliva or hair has 
led to a re-examination of a number of cases. Evidence can now be 
uncovered that was not scientifically possible at the time of the 
original examination. Combined with the removal of the <a href="http://en.wikipedia.org/wiki/Double_jeopardy" title="Double jeopardy">double jeopardy</a>
 law in some places, this can allow cases to be reopened where previous 
trials have failed to produce sufficient evidence to convince a jury. 
People charged with serious crimes may be required to provide a sample 
of DNA for matching purposes. The most obvious defence to DNA matches 
obtained forensically is to claim that cross-contamination of evidence 
has taken place. This has resulted in meticulous strict handling 
procedures with new cases of serious crime. DNA profiling is also used 
to identify victims of mass casualty incidents.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-145">[145]</a>
 As well as positively identifying bodies or body parts in serious 
accidents, DNA profiling is being successfully used to identify 
individual victims in mass war graves – matching to family members.

<h3>
Bioinformatics</h3>
<div class="hatnote boilerplate further">
Further information: <a href="http://en.wikipedia.org/wiki/Bioinformatics" title="Bioinformatics">Bioinformatics</a></div>
<a href="http://en.wikipedia.org/wiki/Bioinformatics" title="Bioinformatics">Bioinformatics</a> involves the manipulation, searching, and <a href="http://en.wikipedia.org/wiki/Data_mining" title="Data mining">data mining</a>
 of biological data, and this includes DNA sequence data. The 
development of techniques to store and search DNA sequences have led to 
widely applied advances in <a href="http://en.wikipedia.org/wiki/Computer_science" title="Computer science">computer science</a>, especially <a href="http://en.wikipedia.org/wiki/String_searching_algorithm" title="String searching algorithm">string searching algorithms</a>, <a href="http://en.wikipedia.org/wiki/Machine_learning" title="Machine learning">machine learning</a> and <a href="http://en.wikipedia.org/wiki/Database_theory" title="Database theory">database theory</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-146">[146]</a>
 String searching or matching algorithms, which find an occurrence of a 
sequence of letters inside a larger sequence of letters, were developed 
to search for specific sequences of nucleotides.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-147">[147]</a> The DNA sequence may be <a href="http://en.wikipedia.org/wiki/Sequence_alignment" title="Sequence alignment">aligned</a> with other DNA sequences to identify <a href="http://en.wikipedia.org/wiki/Homology_%28biology%29" title="Homology (biology)">homologous</a> sequences and locate the specific <a href="http://en.wikipedia.org/wiki/Mutation" title="Mutation">mutations</a> that make them distinct. These techniques, especially <a href="http://en.wikipedia.org/wiki/Multiple_sequence_alignment" title="Multiple sequence alignment">multiple sequence alignment</a>, are used in studying <a href="http://en.wikipedia.org/wiki/Phylogenetics" title="Phylogenetics">phylogenetic</a> relationships and protein function.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-148">[148]</a> Data sets representing entire genomes' worth of DNA sequences, such as those produced by the <a href="http://en.wikipedia.org/wiki/Human_Genome_Project" title="Human Genome Project">Human Genome Project</a>,
 are difficult to use without the annotations that identify the 
locations of genes and regulatory elements on each chromosome. Regions 
of DNA sequence that have the characteristic patterns associated with 
protein- or RNA-coding genes can be identified by <a href="http://en.wikipedia.org/wiki/Gene_prediction" title="Gene prediction">gene finding</a> algorithms, which allow researchers to predict the presence of particular <a href="http://en.wikipedia.org/wiki/Gene_product" title="Gene product">gene products</a> and their possible functions in an organism even before they have been isolated experimentally.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-Mount-149">[149]</a>
 Entire genomes may also be compared, which can shed light on the 
evolutionary history of particular organism and permit the examination 
of complex evolutionary events.

<h3>
DNA nanotechnology</h3>
<div class="thumb tright">
<div class="thumbinner" style="width: 402px;">
<a class="image" href="http://en.wikipedia.org/wiki/File:DNA_nanostructures.png"><img alt="" class="thumbimage" data-file-height="693" data-file-width="1260" height="220" src="http://upload.wikimedia.org/wikipedia/commons/thumb/5/55/DNA_nanostructures.png/400px-DNA_nanostructures.png" width="400" /></a>
<div class="thumbcaption">

The DNA structure at left (schematic shown) will self-assemble into the structure visualized by <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Atomic_force_microscope" title="Atomic force microscope">atomic force microscopy</a> at right. <a href="http://en.wikipedia.org/wiki/DNA_nanotechnology" title="DNA nanotechnology">DNA nanotechnology</a> is the field that seeks to design nanoscale structures using the <a href="http://en.wikipedia.org/wiki/Molecular_recognition" title="Molecular recognition">molecular recognition</a> properties of DNA molecules. Image from <a class="external text" href="http://dx.doi.org/10.1371/journal.pbio.0020073" rel="nofollow">Strong, 2004</a>.</div>
</div>
</div>
<div class="hatnote boilerplate further">
Further information: <a href="http://en.wikipedia.org/wiki/DNA_nanotechnology" title="DNA nanotechnology">DNA nanotechnology</a></div>
DNA nanotechnology uses the unique <a href="http://en.wikipedia.org/wiki/Molecular_recognition" title="Molecular recognition">molecular recognition</a> properties of DNA and other nucleic acids to create self-assembling branched DNA complexes with useful properties.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-150">[150]</a>
 DNA is thus used as a structural material rather than as a carrier of 
biological information. This has led to the creation of two-dimensional 
periodic lattices (both tile-based and using the "<a href="http://en.wikipedia.org/wiki/DNA_origami" title="DNA origami">DNA origami</a>" method) as well as three-dimensional structures in the shapes of <a href="http://en.wikipedia.org/wiki/Polyhedron" title="Polyhedron">polyhedra</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-151">[151]</a> <a href="http://en.wikipedia.org/wiki/DNA_machine" title="DNA machine">Nanomechanical devices</a> and <a href="http://en.wikipedia.org/wiki/DNA_computing" title="DNA computing">algorithmic self-assembly</a> have also been demonstrated,<a href="http://en.wikipedia.org/wiki/DNA#cite_note-152">[152]</a> and these DNA structures have been used to template the arrangement of other molecules such as <a href="http://en.wikipedia.org/wiki/Colloidal_gold" title="Colloidal gold">gold nanoparticles</a> and <a href="http://en.wikipedia.org/wiki/Streptavidin" title="Streptavidin">streptavidin</a> proteins.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-153">[153]</a>

<h3>
History and anthropology</h3>
<div class="hatnote boilerplate further">
Further information: <a href="http://en.wikipedia.org/wiki/Phylogenetics" title="Phylogenetics">Phylogenetics</a> and <a href="http://en.wikipedia.org/wiki/Genetic_genealogy" title="Genetic genealogy">Genetic genealogy</a></div>
Because DNA collects mutations over time, which are then inherited, 
it contains historical information, and, by comparing DNA sequences, 
geneticists can infer the evolutionary history of organisms, their <a href="http://en.wikipedia.org/wiki/Phylogenetics" title="Phylogenetics">phylogeny</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-154">[154]</a> This field of phylogenetics is a powerful tool in <a href="http://en.wikipedia.org/wiki/Evolutionary_biology" title="Evolutionary biology">evolutionary biology</a>. If DNA sequences within a species are compared, <a href="http://en.wikipedia.org/wiki/Population_genetics" title="Population genetics">population geneticists</a> can learn the history of particular populations. This can be used in studies ranging from <a href="http://en.wikipedia.org/wiki/Ecological_genetics" title="Ecological genetics">ecological genetics</a> to <a href="http://en.wikipedia.org/wiki/Anthropology" title="Anthropology">anthropology</a>; For example, DNA evidence is being used to try to identify the <a href="http://en.wikipedia.org/wiki/Ten_Lost_Tribes" title="Ten Lost Tribes">Ten Lost Tribes of Israel</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-155">[155]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-156">[156]</a>

<h3>
Information storage</h3>
<div class="hatnote relarticle mainarticle">
Main article: <a href="http://en.wikipedia.org/wiki/DNA_digital_data_storage" title="DNA digital data storage">DNA digital data storage</a></div>
In a paper published in <a href="http://en.wikipedia.org/wiki/Nature_%28journal%29" title="Nature (journal)">Nature</a> in January 2013, scientists from the <a href="http://en.wikipedia.org/wiki/European_Bioinformatics_Institute" title="European Bioinformatics Institute">European Bioinformatics Institute</a> and <a href="http://en.wikipedia.org/wiki/Agilent_Technologies" title="Agilent Technologies">Agilent Technologies</a>
 proposed a mechanism to use DNA's ability to code information as a 
means of digital data storage. The group was able to encode 739 
kilobytes of data into DNA code, synthesize the actual DNA, then 
sequence the DNA and decode the information back to its original form, 
with a reported 100% accuracy. The encoded information consisted of text
 files and audio files. A prior experiment was published in August 2012.
 It was conducted by researchers at <a href="http://en.wikipedia.org/wiki/Harvard_University" title="Harvard University">Harvard University</a>, where the text of a 54,000-word book was encoded in DNA.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-157">[157]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-158">[158]</a>

<h2>
History of DNA research</h2>
<div class="hatnote boilerplate further">
Further information: <a href="http://en.wikipedia.org/wiki/History_of_molecular_biology" title="History of molecular biology">History of molecular biology</a></div>
<div class="thumb tright">
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<a class="image" href="http://en.wikipedia.org/wiki/File:Maclyn_McCarty_with_Francis_Crick_and_James_D_Watson_-_10.1371_journal.pbio.0030341.g001-O.jpg"><img alt="" class="thumbimage" data-file-height="2153" data-file-width="2740" height="173" src="http://upload.wikimedia.org/wikipedia/commons/thumb/e/ed/Maclyn_McCarty_with_Francis_Crick_and_James_D_Watson_-_10.1371_journal.pbio.0030341.g001-O.jpg/220px-Maclyn_McCarty_with_Francis_Crick_and_James_D_Watson_-_10.1371_journal.pbio.0030341.g001-O.jpg" width="220" /></a>
<div class="thumbcaption">

<a href="http://en.wikipedia.org/wiki/James_Watson" title="James Watson">James Watson</a> and <a href="http://en.wikipedia.org/wiki/Francis_Crick" title="Francis Crick">Francis Crick</a> (right), co-originators of the double-helix model, with Maclyn McCarty (left).</div>
</div>
</div>
DNA was first isolated by the Swiss physician <a href="http://en.wikipedia.org/wiki/Friedrich_Miescher" title="Friedrich Miescher">Friedrich Miescher</a>
 who, in 1869, discovered a microscopic substance in the pus of 
discarded surgical bandages. As it resided in the nuclei of cells, he 
called it "nuclein".<a href="http://en.wikipedia.org/wiki/DNA#cite_note-159">[159]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-160">[160]</a> In 1878, <a href="http://en.wikipedia.org/wiki/Albrecht_Kossel" title="Albrecht Kossel">Albrecht Kossel</a> isolated the non-protein component of "nuclein", nucleic acid, and later isolated its five primary <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Nucleobases" title="Nucleobases">nucleobases</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-161">[161]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-Yale_Jones_1953-162">[162]</a> In 1919, Phoebus Levene identified the base, sugar and phosphate nucleotide unit.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-163">[163]</a>
 Levene suggested that DNA consisted of a string of nucleotide units 
linked together through the phosphate groups. Levene thought the chain 
was short and the bases repeated in a fixed order. In 1937, <a href="http://en.wikipedia.org/wiki/William_Astbury" title="William Astbury">William Astbury</a> produced the first X-ray diffraction patterns that showed that DNA had a regular structure.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-164">[164]</a>

In 1927, <a href="http://en.wikipedia.org/wiki/Nikolai_Koltsov" title="Nikolai Koltsov">Nikolai Koltsov</a>
 proposed that inherited traits would be inherited via a "giant 
hereditary molecule" made up of "two mirror strands that would replicate
 in a semi-conservative fashion using each strand as a template".<a href="http://en.wikipedia.org/wiki/DNA#cite_note-165">[165]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-Soyfer-166">[166]</a> In 1928, <a href="http://en.wikipedia.org/wiki/Frederick_Griffith" title="Frederick Griffith">Frederick Griffith</a> in his <a href="http://en.wikipedia.org/wiki/Griffith%27s_experiment" title="Griffith's experiment">experiment</a> discovered that <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Trait_%28biology%29" title="Trait (biology)">traits</a> of the "smooth" form of Pneumococcus could be transferred to the "rough" form of the same bacteria by mixing killed "smooth" bacteria with the live "rough" form.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-167">[167]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-168">[168]</a> This system provided the first clear suggestion that DNA carries genetic information—the <a href="http://en.wikipedia.org/wiki/Avery%E2%80%93MacLeod%E2%80%93McCarty_experiment" title="Avery–MacLeod–McCarty experiment">Avery–MacLeod–McCarty experiment</a>—when <a href="http://en.wikipedia.org/wiki/Oswald_Avery" title="Oswald Avery">Oswald Avery</a>, along with coworkers <a href="http://en.wikipedia.org/wiki/Colin_Munro_MacLeod" title="Colin Munro MacLeod">Colin MacLeod</a> and <a href="http://en.wikipedia.org/wiki/Maclyn_McCarty" title="Maclyn McCarty">Maclyn McCarty</a>, identified DNA as the <a href="http://en.wikipedia.org/wiki/Griffith%27s_experiment" title="Griffith's experiment">transforming principle</a> in 1943.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-169">[169]</a> DNA's role in <a href="http://en.wikipedia.org/wiki/Heredity" title="Heredity">heredity</a> was confirmed in 1952, when <a href="http://en.wikipedia.org/wiki/Alfred_Hershey" title="Alfred Hershey">Alfred Hershey</a> and <a href="http://en.wikipedia.org/wiki/Martha_Chase" title="Martha Chase">Martha Chase</a> in the <a href="http://en.wikipedia.org/wiki/Hershey%E2%80%93Chase_experiment" title="Hershey–Chase experiment">Hershey–Chase experiment</a> showed that DNA is the <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Genetic_material" title="Genetic material">genetic material</a> of the <a href="http://en.wikipedia.org/wiki/Enterobacteria_phage_T2" title="Enterobacteria phage T2">T2 phage</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-170">[170]</a>

In 1953, <a href="http://en.wikipedia.org/wiki/James_Watson" title="James Watson">James Watson</a> and <a href="http://en.wikipedia.org/wiki/Francis_Crick" title="Francis Crick">Francis Crick</a> suggested what is now accepted as the first correct double-helix model of <a class="mw-redirect" href="http://en.wikipedia.org/wiki/Molecular_structure_of_Nucleic_Acids" title="Molecular structure of Nucleic Acids">DNA structure</a> in the journal Nature.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-FWPUB-6">[6]</a> Their double-helix, molecular model of DNA was then based on a single <a class="mw-redirect" href="http://en.wikipedia.org/wiki/X-ray_diffraction" title="X-ray diffraction">X-ray diffraction</a> image (labeled as "<a href="http://en.wikipedia.org/wiki/Photo_51" title="Photo 51">Photo 51</a>")<a href="http://en.wikipedia.org/wiki/DNA#cite_note-171">[171]</a> taken by <a href="http://en.wikipedia.org/wiki/Rosalind_Franklin" title="Rosalind Franklin">Rosalind Franklin</a> and <a href="http://en.wikipedia.org/wiki/Raymond_Gosling" title="Raymond Gosling">Raymond Gosling</a>
 in May 1952, as well as the information that the DNA bases are 
paired—also obtained through private communications from Erwin Chargaff 
in the previous years.

Experimental evidence supporting the Watson and Crick model was published in a series of five articles in the same issue of Nature.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-NatureDNA50-172">[172]</a>
 Of these, Franklin and Gosling's paper was the first publication of 
their own X-ray diffraction data and original analysis method that 
partially supported the Watson and Crick model;<a href="http://en.wikipedia.org/wiki/DNA#cite_note-NatFranGos-43">[43]</a><a href="http://en.wikipedia.org/wiki/DNA#cite_note-173">[173]</a> this issue also contained an article on DNA structure by <a href="http://en.wikipedia.org/wiki/Maurice_Wilkins" title="Maurice Wilkins">Maurice Wilkins</a> and two of his colleagues, whose analysis and in vivo B-DNA X-ray patterns also supported the presence in vivo
 of the double-helical DNA configurations as proposed by Crick and 
Watson for their double-helix molecular model of DNA in the previous two
 pages of Nature.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-NatWilk-44">[44]</a> In 1962, after Franklin's death, Watson, Crick, and Wilkins jointly received the <a href="http://en.wikipedia.org/wiki/Nobel_Prize_in_Physiology_or_Medicine" title="Nobel Prize in Physiology or Medicine">Nobel Prize in Physiology or Medicine</a>.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-174">[174]</a>
 Nobel Prizes were awarded only to living recipients at the time. A 
debate continues about who should receive credit for the discovery.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-175">[175]</a>

In an influential presentation in 1957, Crick laid out the <a href="http://en.wikipedia.org/wiki/Central_dogma_of_molecular_biology" title="Central dogma of molecular biology">central dogma of molecular biology</a>, which foretold the relationship between DNA, RNA, and proteins, and articulated the "adaptor hypothesis".<a href="http://en.wikipedia.org/wiki/DNA#cite_note-176">[176]</a>
 Final confirmation of the replication mechanism that was implied by the
 double-helical structure followed in 1958 through the Meselson–Stahl 
experiment.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-177">[177]</a>
 Further work by Crick and coworkers showed that the genetic code was 
based on non-overlapping triplets of bases, called codons, allowing <a href="http://en.wikipedia.org/wiki/Har_Gobind_Khorana" title="Har Gobind Khorana">Har Gobind Khorana</a>, <a href="http://en.wikipedia.org/wiki/Robert_W._Holley" title="Robert W. Holley">Robert W. Holley</a> and <a href="http://en.wikipedia.org/wiki/Marshall_Warren_Nirenberg" title="Marshall Warren Nirenberg">Marshall Warren Nirenberg</a> to decipher the genetic code.<a href="http://en.wikipedia.org/wiki/DNA#cite_note-178">[178]</a> These findings represent the birth of molecular biology.

<h2>
See also</h2>
<div class="noprint portal tright" style="border: solid #aaa 1px; margin: 0.5em 0 0.5em 1em;">
<table style="background: #f9f9f9; font-size: 85%; line-height: 110%; max-width: 175px;">
<tbody>
<tr style="vertical-align: middle;">
<td style="text-align: center;"><a class="image" href="http://en.wikipedia.org/wiki/File:TPI1_structure.png"><img alt="Portal icon" class="noviewer" data-file-height="914" data-file-width="1452" height="20" src="http://upload.wikimedia.org/wikipedia/commons/thumb/1/1c/TPI1_structure.png/32px-TPI1_structure.png" width="32" /></a></td>
<td style="font-style: italic; font-weight: bold; padding: 0 0.2em; vertical-align: middle;"><a class="mw-redirect" href="http://en.wikipedia.org/wiki/Portal:Molecular_and_Cellular_Biology" title="Portal:Molecular and Cellular Biology">Molecular and Cellular Biology portal</a></td>
</tr>
</tbody></table>
</div>
<div class="div-col columns column-width" style="-moz-column-width: 20em; -webkit-column-width: 20em; column-width: 20em;">
<ul>
<li><a href="http://en.wikipedia.org/wiki/Autosome" title="Autosome">Autosome</a></li>
<li><a href="http://en.wikipedia.org/wiki/Crystallography" title="Crystallography">Crystallography</a></li>
<li><a href="http://en.wikipedia.org/wiki/DNA-encoded_chemical_library" title="DNA-encoded chemical library">DNA-encoded chemical library</a></li>
<li><a href="http://en.wikipedia.org/wiki/DNA_microarray" title="DNA microarray">DNA microarray</a></li>
<li><a href="http://en.wikipedia.org/wiki/DNA_sequencing" title="DNA sequencing">DNA sequencing</a></li>
<li><a href="http://en.wikipedia.org/wiki/DNA,_RNA_and_proteins:_The_three_essential_macromolecules_of_life" title="DNA, RNA and proteins: The three essential macromolecules of life">DNA, RNA and proteins: The three essential macromolecules of life</a></li>
<li><a href="http://en.wikipedia.org/wiki/Genetic_disorder" title="Genetic disorder">Genetic disorder</a></li>
<li><a href="http://en.wikipedia.org/wiki/Haplotype" title="Haplotype">Haplotype</a></li>
<li><a href="http://en.wikipedia.org/wiki/List_of_nucleic_acid_simulation_software" title="List of nucleic acid simulation software">Nucleic acid modeling</a></li>
<li><a href="http://en.wikipedia.org/wiki/Meiosis" title="Meiosis">Meiosis</a></li>
<li><a href="http://en.wikipedia.org/wiki/Nucleic_acid_double_helix" title="Nucleic acid double helix">Nucleic acid double helix</a></li>
<li><a href="http://en.wikipedia.org/wiki/Nucleic_acid_notation" title="Nucleic acid notation">Nucleic acid notation</a></li>
<li><a href="http://en.wikipedia.org/wiki/Pangenesis" title="Pangenesis">Pangenesis</a></li>
<li><a href="http://en.wikipedia.org/wiki/Phosphoramidite" title="Phosphoramidite">Phosphoramidite</a></li>
<li><a href="http://en.wikipedia.org/wiki/Southern_blot" title="Southern blot">Southern blot</a></li>
<li><a href="http://en.wikipedia.org/wiki/X-ray_scattering_techniques" title="X-ray scattering techniques">X-ray scattering techniques</a></li>
<li><a href="http://en.wikipedia.org/wiki/Xeno_nucleic_acid" title="Xeno nucleic acid">Xeno nucleic acid</a></li>
<li><a href="http://en.wikipedia.org/wiki/Proteopedia" title="Proteopedia">Proteopedia</a> <a class="external text" href="http://www.proteopedia.org/wiki/index.php/DNA" rel="nofollow">DNA</a></li>
</ul>
</div>
<ul>
<li><a href="http://en.wikipedia.org/wiki/RNA" title="RNA">RNA</a></li>
</ul>

DNA

DNA From Wikipedia, the free encyclopedia For a non-technical introduction to the topic, see Introduction to genetics. For other uses, see DNA (disambiguation). The structure of the DNA double helix…