View source for Deoxyribonucleic acids

'''Editor-In-Chief:''' Henry A. Hoff

[[Image:DNA chemical structure.png|thumb|right|250px|This diagram shows the chemical structure of deoxyribonucleic acid, with colored labels identifying the four nucleobases, the phosphate, and deoxyribose components. Credit: Madeleine Price Ball, [[commons:User:Madprime|Madprime]].]]

'''Deoxyribonucleic acid''' ('''DNA''') is a polymer composed of nucleic acids linked together by a sugar-phosphate backbone.

The nucleic acids are inorganic acids with phosphoric acid as the only acid.

Attached to each sugar is a nucleobase, also called a nucleotide or "base".  Although the nucleotide is attached to the sugar via a covalent bond, the bases are connected by a hydrogen bond - making the molecule of DNA easy to separate with heat but also allows it to return to a double-stranded molecule spontaneously when cooled back down.  
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==Theoretical deoxyribonucleic acids==

'''Def.''' a "nucleic acid found in all living things (and some non-living, see virus);<ref name=DeoxyribonucleicAcidWikt1>{{ cite web
|author=[[wikt:User:Blurpeace|Blurpeace]]
|title=deoxyribonucleic acid
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=30 June 2009
|url=https://en.wiktionary.org/wiki/deoxyribonucleic_acid
|accessdate=24 May 2019 }}</ref> consists of a polymer formed from nucleotides which are shaped into a double helix; it is associated with the transmission of genetic information"<ref name=DeoxyribonucleicAcidWikt>{{ cite web
|author=[[wikt:User:SemperBlotto|SemperBlotto]]
|title=deoxyribonucleic acid
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=13 May 2005
|url=https://en.wiktionary.org/wiki/deoxyribonucleic_acid
|accessdate=24 May 2019 }}</ref> is called a '''deoxyribonucleic acid'''.

==Polyphosphoric acids==
{{main|Chemicals/Acids/Polyphosphorics|Polyphosphoric acids}}
'''Def.''' "a colourless liquid, H3PO4"<ref name=PhosphoricAcidWikt>{{ cite book
|author=[[wikt:User:SemperBlotto|SemperBlotto]]
|title=phosphoric acid
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=19 May 2005
|url=http://en.wiktionary.org/wiki/phosphoric_acid
|accessdate=2012-08-27 }}</ref> is called '''phosphoric acid''', '''orthophosphoric acid''', or '''monophosphoric acid'''.

An orthophosphoric acid molecule can dissociate up to three times, giving up an H<sup>+</sup> each time, which typically combines with a water molecule, H<sub>2</sub>O, as shown in these[chemical reactions:

:H<sub>3</sub>PO<sub>4</sub><sub>(s)</sub> &nbsp;&nbsp;+ H<sub>2</sub>O<sub>(l)</sub> ↔ H<sub>3</sub>O<sup>+</sup><sub>(aq)</sub> + H<sub>2</sub>PO<sub>4</sub><sup>−</sup><sub>(aq)</sub> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ''K''<sub>a1</sub>= 7.25×10<sup>−3</sup>

:H<sub>2</sub>PO<sub>4</sub><sup>−</sup><sub>(aq)</sub>+ H<sub>2</sub>O<sub>(l)</sub> ↔ H<sub>3</sub>O<sup>+</sup><sub>(aq)</sub> + HPO<sub>4</sub><sup>2−</sup><sub>(aq)</sub> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ''K''<sub>a2</sub>= 6.31×10<sup>−8</sup>

:HPO<sub>4</sub><sup>2−</sup><sub>(aq)</sub>+ H<sub>2</sub>O<sub>(l)</sub> ↔ H<sub>3</sub>O<sup>+</sup><sub>(aq)</sub> + &nbsp;PO<sub>4</sub><sup>3−</sup><sub>(aq)</sub> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ''K''<sub>a3</sub>= 4.80×10<sup>−13</sup>

The anion after the first dissociation, H<sub>2</sub>PO<sub>4</sub><sup>−</sup>, is the ''dihydrogen phosphate'' anion. The anion after the second dissociation, HPO<sub>4</sub><sup>2−</sup>, is the ''hydrogen phosphate'' anion. The anion after the third dissociation, PO<sub>4</sub><sup>3−</sup>, is the '''phosphate''' or '''orthophosphate''' anion. For each of the dissociation reactions shown above, there is a separate acid dissociation constant, called ''K''<sub>a1</sub>, ''K''<sub>a2</sub>, and ''K''<sub>a3</sub> given at 25 °C. Associated with these three dissociation constants are corresponding p''K''<sub>a1</sub>=2.12, p''K''<sub>a2</sub>=7.21, and p''K''<sub>a3</sub>=12.67 values at 25 °C. Even though all three [[hydrogen]] (H) atoms are equivalent on an orthophosphoric acid molecule, the successive ''K''<sub>a</sub> values differ since it is energetically less favorable to lose another H<sup>+</sup> if one (or more) has already been lost and the molecule/ion is more negatively charged.

For a given total acid concentration [A] = [H<sub>3</sub>PO<sub>4</sub>] + [H<sub>2</sub>PO<sub>4</sub><sup>−</sup>] + [HPO<sub>4</sub><sup>2−</sup>] + [PO<sub>4</sub><sup>3−</sup>] ([A] is the total number of moles of pure H<sub>3</sub>PO<sub>4</sub> which have been used to prepare 1 liter of solution), the composition of an aqueous solution of phosphoric acid can be calculated using the equilibrium equations associated with the three reactions described above together with the [H<sup>+</sup>][OH<sup>−</sup>] = 10<sup>−14</sup> relation and the electrical neutrality equation. Possible concentrations of polyphosphoric molecules and ions is neglected. The system may be reduced to a fifth degree equation for [H<sup>+</sup>] which can be solved numerically, yielding:

{| class="wikitable"
|-
!width="60" align="center" |'''[A] (mol/L)'''
!width="50" align="center" |'''pH'''
!width="110" align="center" |'''[H<sub>3</sub>PO<sub>4</sub>]/[A] (%)'''
!width="110" align="center" |'''[H<sub>2</sub>PO<sub>4</sub><sup>−</sup>]/[A] (%)'''
!width="110" align="center" |'''[HPO<sub>4</sub><sup>2−</sup>]/[A] (%)'''
!width="110" align="center" |'''[PO<sub>4</sub><sup>3−</sup>]/[A] (%)'''
|-
|1||1.08||'''91.7'''||8.29||6.20×10<sup>−6</sup>||1.60×10<sup>−17</sup>
|-
|10<sup>−1</sup>||1.62||'''76.1'''||23.9||6.20×10<sup>−5</sup>||5.55×10<sup>−16</sup>
|-
|10<sup>−2</sup>||2.25||'''43.1'''||'''56.9'''||6.20×10<sup>−4</sup>||2.33×10<sup>−14</sup>
|-
|10<sup>−3</sup>||3.05||10.6||'''89.3'''||6.20×10<sup>−3</sup>||1.48×10<sup>−12</sup>
|-
|10<sup>−4</sup>||4.01||1.30||'''98.6'''||6.19×10<sup>−2</sup>||1.34×10<sup>−10</sup>
|-
|10<sup>−5</sup>||5.00||0.133||'''99.3'''||0.612||1.30×10<sup>−8</sup>
|-
|10<sup>−6</sup>||5.97||1.34×10<sup>−2</sup>||'''94.5'''||5.50||1.11×10<sup>−6</sup>
|-
|10<sup>−7</sup>||6.74||1.80×10<sup>−3</sup>||'''74.5'''||25.5||3.02×10<sup>−5</sup>
|-
|10<sup>−10</sup>||7.00||8.24×10<sup>−4</sup>||'''61.7'''||'''38.3'''||8.18×10<sup>−5</sup>
|}

For large acid concentrations, the solution is mainly composed of H<sub>3</sub>PO<sub>4</sub>. For [A] = 10<sup>−2</sup>, the pH is close to p''K''<sub>a1</sub>, giving an equimolar mixture of H<sub>3</sub>PO<sub>4</sub> and H<sub>2</sub>PO<sub>4</sub><sup>−</sup>. For [A] below 10<sup>−3</sup>, the solution is mainly composed of H<sub>2</sub>PO<sub>4</sub><sup>−</sup> with [HPO<sub>4</sub><sup>2−</sup>] becoming non negligible for very dilute solutions. [PO<sub>4</sub><sup>3−</sup>] is always negligible. Note that the above analysis does not take into account ion activity coefficients; as such, the pH and molarity of a real phosphoric acid solution may deviate substantially from the above values.

'''Def.''' typically two to twenty, or three to seven, linked monophosphoric acids, or orthophosphoric acids, is called an '''oligophosphoric acid'''.

'''Def.''' "any of a class of inorganic polymers containing linked phosphate groups""any of a class of inorganic polymers containing linked phosphate groups"<ref name=PolyphosphateWikt>{{ cite book
|author=[[wikt:User:SemperBlotto|SemperBlotto]]
|title=polyphosphate
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=5 March 2006
|url=http://en.wiktionary.org/wiki/polyphosphate
|accessdate=2013-04-19 }}</ref>, or more than five linked phosphoric acids, is called a '''polyphosphate''', or '''polyphosphoric acid'''.

==Nitrogenous bases==
{{main|Chemicals/Bio-organics/Nitrogenous bases|Nitrogenous bases}}
Nitrogenous bases, found in cell nuclei, are nucleobases.

In normal spiral DNA the bases form pairs between the two strands: Adenine (A) with Thymine (T) and Cytosine (C) with Guanine (G). Purines pair with pyrimidines mainly for dimensional reasons - only this combination fits the constant width geometry of the DNA spiral.

Nitrogenous bases include purines: adenine, guanine, hypoxanthine, isoguanine, xanthine, and 7-methylguanine; and pyrimidines: cytosine, thymine, and isocytosine.

===Adenine===
[[Image:Adenine.svg|right|thumb|300px|Chemical Structure of Adenine is in vector format. Credit: [[c:user:Pepemonbu|Pepemonbu]].{{tlx|free media}}]]
[[Image:Adenine numbered.svg|thumb|left|250px|Adenine structure, with standard numbering of positions in red, is shown. Credit: [[c:user:Mikael Häggström|Mikael Häggström]].{{tlx|free media}}]]
Adenine has IUPAC Name of  9''H''-purin-6-amine. pKa=4.15 (secondary), 9.80 (primary)<ref>Dawson, R.M.C., et al., ''Data for Biochemical Research'', Oxford, Clarendon Press, 1959.</ref>

Adenine derivatives have a variety of roles in [[biochemistry]] including cellular respiration, in the form of both the energy-rich adenosine triphosphate (ATP) and the cofactors nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD), functions in protein synthesis and as a chemical component of DNA and RNA.<ref>[http://ghr.nlm.nih.gov/ghr/glossary/adenine Definition of Adenine from the Genetics Home Reference] - National Institutes of Health</ref>

The adjacent image on the right shows pure adenine, as an independent molecule.

Adenosine is adenine reacted with ribose.

Adenine attached to deoxyribose is deoxyadenosine.

Adenine forms several tautomers, compounds that can be rapidly interconverted and are often considered equivalent; however, in isolated conditions, i.e. in an inert gas matrix and in the gas phase, mainly the 9H-adenine tautomer is found.<ref>{{cite journal
|journal =Phys. Chem. Chem. Phys.
|year =2002
|volume =4
|issue =20
|pages =4877–4882
|title =Tautomers and electronic states of jet-cooled adenine investigated by double resonance spectroscopy
|author =Plützer, Chr. |author2=Kleinermanns, K.
|bibcode =2002PCCP....4.4877P
|doi =10.1039/b204595h
}}</ref><ref>{{cite journal  
|title = Experimental matrix isolation and theoretical ab initio HF/6-31G(d, p) studies of infrared spectra of purine, adenine and 2-chloroadenine,
|journal = Spectrochimica Acta Part A: Molecular Spectroscopy
|volume = 50
|pages = 1081–1094
|year = 1994
|note = 
|issn = 0584-8539
|doi = 10.1016/0584-8539(94)80030-8
|url = http://www.sciencedirect.com/science/article/B6THM-44JM8D7-9J/2/36ce6ab26fc6c5aae4a120f9e7bb7d88
|author1=M. J. Nowak |author2=H. Rostkowska |author3=L. Lapinski |author4=J. S. Kwiatkowski |author5=J. Leszczynski |issue = 6
|bibcode = 1994AcSpA..50.1081N }}</ref>
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===Adenine synthesis===

Both adenine and guanine are derived from the nucleotide inosine monophosphate (IMP), which in turn is synthesized from a pre-existing ribose phosphate through a pathway using atoms from the amino acids glycine, glutamine, and aspartic acid, and the coenzyme tetrahydrofolate.

==Sugars==
{{main|Chemicals/Bio-organics/Sugars|Sugars}}
[[Image:Ribose deoxyribose.png|thumb|right|250px|This diagram shows the pentoses ribose and deoxyribose in the α-D-ribofuranose form. Credit: [[commons:User:Boumphreyfr|Boumphreyfr]].]]
'''Def.''' "a derivative of the pentose sugar ribose<ref name=DeoxyriboseWikt>{{ cite book
|author=[[wikt:User:SemperBlotto|SemperBlotto]]
|title=deoxyribose
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=28 June 2005
|url=http://en.wiktionary.org/wiki/deoxyribose
|accessdate=2012-08-27 }}</ref> in which the 2' hydroxyl (-OH) is reduced to a hydrogen (H)"<ref name=DeoxyriboseWikt1>{{ cite book
|author=[[wikt:User:68.162.247.200|68.162.247.200]]
|title=deoxyribose
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=12 October 2005
|url=http://en.wiktionary.org/wiki/deoxyribose
|accessdate=2012-08-27 }}</ref> is called '''deoxyribose'''.

In the diagram of DNA at the page top, the pentose sugar deoxyribose is in a cyclic, furanose (5-membered ring) form. D-deoxyribose or L-deoxyribose is not demonstrated, but is determined by the hydroxyls being primarily below (D) or above (L) the plane of the ring. In [[Earth]]-based DNA, deoxyribose is in the dextro (D) configuration.

Deoxyribose may also occur in a pyranose (six-membered ring) form.

There are other pentose sugars including aldopentoses: apiose, arabinose, xylose, and lyxose, and ketopentoses: ribulose and xylulose. These may have a deoxy-form: deoxyapiose, deoxyarabinose, deoxyxylose, deoxylyxose, deoxyribulose, and deoxyxylulose. They may occur as a levo or dextro sugar and as a furanose or pyranose.

To occur in an Earth-like DNA, each of these six deoxypentoses and perhaps other sugars need to be dextro furanoses. Each is a DNA, for example, deoxyapionucleic acid.
{{clear}}

==Sugar phosphates==
{{main|Chemicals/Bio-organics/Sugars/Phosphates|Sugar phosphates}}
[[Image:Glucose-6-phosphate-skeletal.png|right|thumb|250px|This is a structural drawing of glucose-6-phosphate. Credit: [[commons:User:Yikrazuul|Yikrazuul]].]]
[[Image:Phytate.svg|right|thumb|250px|The diagram shows the chemical structure of phytate.]]
Any of the various sugars can have one or more phosphates attached as in glucose-6-phosphate diagrammed on the right.
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==Nucleosides==
{{main|Chemicals/Bio-organics/Nucleosides|Nucleosides}}
'''Def.''' "an organic molecule in which a nitrogenous heterocyclic base (or nucleobase), which can be either a double-ringed purine or a single-ringed pyrimidine, is covalently attached to a five-carbon pentose sugar (deoxyribose in DNA or ribose in RNA)"<ref name=NucleosideWikt>{{ cite book
|title=nucleoside
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=November 10, 2013
|url=https://en.wiktionary.org/wiki/nucleoside
|accessdate=2014-06-04 }}</ref> is called a '''nucleoside'''.

==Nucleotides==
{{main|Chemicals/Bio-organics/Nucleotides|Nucleotides}}
"All five nucleotides (including the RNA base "uracil") are synthesized through complex metabolic pathways involving several multi-subunit enzymes. The pathways differ for both purines and pyrimidines (uracil falling in the pyrimidine category since it is thymine's counterpart.)"<ref name=BiochemistryEnwikiversity>{{ cite book
|author=[[User:Biochemistry~enwikiversity|Biochemistry~enwikiversity]]
|title=Nucleotide Synthesis
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California USA
|date=2 March 2011
|url=https://en.wikiversity.org/wiki/Nucleotide_Synthesis
|accessdate=2017-01-21 }}</ref>

==Nucleic acids==
{{main|Nucleic acids}}
[[Image:Artificial nucleic acids.jpg|thumb|right|250px|These images show natural and artificial nucleic acid polymers. Credit: Irina Anosova, Ewa A. Kowal, Matthew R. Dunn, John C. Chaput, Wade D. Van Horn1, and Martin Egli.]]
[[Image:PNA en.svg|thumb|right|250px|Peptide nucleic acid is diagrammed. Credit: Mixtures.]]
"Synthetic genetics is a subdiscipline of synthetic biology that aims to develop artificial genetic polymers (also referred to as xeno-nucleic acids or XNAs) that can replicate ''in vitro'' and eventually in model cellular organisms."<ref name=Anosova>{{ cite journal
|author=Irina Anosova, Ewa A. Kowal, Matthew R. Dunn, John C. Chaput, Wade D. Van Horn1, and Martin Egli
|title=The structural diversity of artificial genetic polymers
|journal=Nucleic Acids Research
|date=15 December 2015
|volume=
|issue=
|pages=
|url=http://nar.oxfordjournals.org/content/early/2015/12/15/nar.gkv1472.full
|arxiv=
|bibcode=
|doi=10.1093/nar/gkv1472
|pmid=
|accessdate=2016-01-21 }}</ref>

'''Def.''' any "acidic, chainlike biological macromolecule consisting of multiply repeat units of phosphoric acid, sugar and purine and pyrimidine bases"<ref name=NucleicAcidWikt>{{ cite book
|title=nucleic acid
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=January 12, 2013
|url=http://en.wiktionary.org/wiki/nucleic_acid
|accessdate=2013-04-19 }}</ref> occurring in cell nuclei is called a '''nucleic acid'''.

'''Def.''' a nucleic acid "in which the sugar component is threose"<ref name=ThreoseNucleicAcidWikt>{{ cite book
|title=threose nucleic acid
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=November 14, 2012
|url=http://en.wiktionary.org/wiki/threose_nucleic_acid
|accessdate=2013-04-19 }}</ref> is called '''threose nucleic acid''', or '''threonucleic acid''' (TNA).

'''Def.''' a "synthetic organic polymer similar to DNA or RNA; the sugar backbone of nucleic acids is replaced by N-(2-aminoethyl)-glycine linked through peptide bonds - considered a possible precursor to RNA"<ref name=PeptideNucleicAcidWikt>{{ cite web
|author=[[wikt:User:70.24.247.54|70.24.247.54]]
|title=peptide nucleic acid
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=3 February 2012
|url=https://en.wiktionary.org/wiki/peptide_nucleic_acid
|accessdate=24 May 2019 }}</ref> is called '''peptide nucleic acid'''.

Additional DNAs may be
# deoxyapionucleic acid,
# deoxyarabinonucleic acid,
# deoxyxylonucleic acid (dXyNA),
# deoxylyxonucleic acid,
# deoxyribulonucleic acid, and
# deoxyxylulonucleic acid.

Synthesis of deoxyapionucleic acid has been accomplished.<ref name=Kataoka>{{ cite journal
|author=Mayumi Kataoka, Yasuo Kouda, Kousuke Sato, Noriaki Minakawaa and Akira Matsuda
|title=Highly efficient enzymatic synthesis of 3′-deoxyapionucleic acid (apioNA) having the four natural nucleobases
|journal=Chemical Communications
|date=14 August 2011
|volume=47
|issue=30
|pages=8700-2
|url=http://pubs.rsc.org/en/content/articlelanding/2011/cc/c1cc12980e#!divAbstract
|arxiv=
|bibcode=
|doi=10.1039/C1CC12980E
|pmid=
|accessdate=2016-01-19 }}</ref>

Deoxyxylonucleic acid and xylose nucleic acid have been produced.<ref name=Maiti/>

"[X]ylonucleic acid (XyloNA) [contains] a potentially prebiotic xylose sugar (a 3′-epimer of ribose) in its backbone."<ref name=Maiti>{{ cite journal
|author=Mohitosh Maiti, Munmun Maiti, Christine Knies, Shrinivas Dumbre, Eveline Lescrinier, Helmut Rosemeyer, Arnout Ceulemans and Piet Herdewijn
|title=Xylonucleic acid: synthesis, structure, and orthogonal pairing properties
|journal=Nucleic Acids Research
|date=13 July 2015
|volume=43
|issue=
|pages=7189-200
|url=https://nar.oxfordjournals.org/content/early/2015/07/13/nar.gkv719.full
|arxiv=
|bibcode=
|doi=10.1093/nar/gkv719
|pmid=
|accessdate=2016-01-21 }}</ref>

A "number of sugar-modified nucleic acid variants has been revealed as new genetic polymers, (2) some of them are endowed with catalytic activity (for e.g. FANA and HNA) (3). The structure of these artificial nucleic acids, however, mimics natural nucleic acid helicity (4)."<ref name=Maiti/>

"Although helices display a distinct pitch and curvature, they feature ca. 11–12 base pairs per turn, and χ/δ covariance plots indicate that the backbones of XNA:RNA or XNA:DNA heteroduplexes adopt an architecture that is either closely related to the A-form, as in the case of [1,5-anhydrohexitol nucleic acid (HNA)] HNA:RNA (96), [locked nucleic acid (LNA)] LNA:RNA (83), [cyclohexene nucleic acid (CeNA)] CeNA:RNA (85) and PNA:RNA (59), or between the A- and B-forms, as seen in the structures of DNA:RNA (97), [arabinonucleic acid (ANA)] ANA:RNA (79), [2′-deoxy-2′-fluoro-arabinonucleic acid (FANA)]FANA:RNA (79) and [peptide nucleic acid (PNA)] PNA:DNA (98)."<ref name=Anosova/>

Additional XNAs include bridged nucleic acid (BNA) glycol nucleic acid (GNA), FANA and peptide nucleic acid (PNA).

On the right is a diagram displaying various artificial and natural nucleic acid polymers.

"Representative structures illustrate the structural diversity and plasticity of natural and artificial nucleic acid (XNA) backbones. Structures are shown in alphabetic order. (A) Natural genetic polymers: B-form DNA (black), DNA:RNA hybrid and A-form RNA (gray). (B) Representative structures of XNA heteroduplexes with RNA or DNA. The RNA strand is shown in gray, the DNA strand in black and the orientation of the XNA strand is indicated. (C) XNA homoduplexes. Homo-XNA duplexes adopt a variety of structures. (D) Representative XNA-only heteroduplexes. FAF:FAF stands for FANA(F)-ANA(A)-FANA(F) XNA:XNA heteroduplex. Alt and chim indicate the alternated or chimeric order of FANA-segments in the duplex sequences respectively. The depicted duplexes have the following PDB ID codes in the Protein Data Bank (http://www.rcsb.org): B-DNA (3BSE); DNA:RNA (1EFS); A-RNA (3ND4); ANA(purple):RNA (2KP3); CeNA(blue):RNA (3KNC); FANA(violet):RNA (2KP4); HNA(yellow):RNA (2BJ6); LNA(cyan):RNA (1H0Q); PNA(orange):DNA (1PDT); PNA(orange):RNA (176D); CeNA:CeNA (blue, 2H0N); hDNA:hDNA (sky blue, 2H9S); FRNA:FRNA (magenta, 3P4A); GNA:GNA (red, 2XC6); HNA:HNA (yellow, 481D); LNA:LNA (cyan, 2×2Q); PNA:PNA (orange, 2K4G), TNA:TNA (green, coordinates not deposited in the PDB [...]); dXyNA:dXyNA (brown, coordinates not deposited in the PDB [...]); XyNA:XyNA (light green, 2N4J); FAF:FAF (FANA in violet, ANA in purple, 2LSC), FRNA:FANA (alt) (FRNA in magenta, FANA in violet, 2M8A); FRNA:FANA (chim) (FRNA in magenta, FANA in violet, 2M84)."<ref name=Anosova/>
{{clear}}

==Mitochondrial DNA==
[[Image:Mitochondrial DNA lg.jpg|thumb|250px|right|Mitochondrial DNAs are the small circular [[chromosomes|chromosome]] found inside mitochondria. Credit: National Human Genome Research Institute.]]
[[Image:Map of the human mitochondrial genome.svg|thumb|250px|right|Human mitochondrial DNA has 37 genes on their respective H- and L-strands. Credit: [[commons:User:Manudouz|Emmanuel Douzery]].]]
Mitochondrial DNA ('''mtDNA''' or '''mDNA''')<ref name=Sykes>{{ cite book
|author=Bryan Sykes
|title=Mitochondrial DNA and human history, In: ''The Human Genome''
|url=https://web.archive.org/web/20150907140051/http://genome.wellcome.ac.uk/doc_WTD020876.html
|publisher=Wellcome Trust
|accessdate=5 February 2012
|date=10 September 2003 }}</ref> consists of one or more strands making up the small circular [[chromosomes]] inside mitochondria. Mitochondria are passed only from mother to offspring.

The 16,569 base pairs of human mitochondrial DNA encode for 37 [[Draft:Genes|genes]].<ref name=Anderson>{{ cite journal |author=S. Anderson, A. T. Bankier, B. G. Barrell, M. H. L. de Bruijn, A. R. Coulson, J. Drouin, I. C. Eperon, D. P. Nierlich, B. A. Roe, F. Schreier, P. H. Sanger, A. J. H. Smith, R. Staden, I. G. Young
|title=Sequence and organization of the human mitochondrial genome
|journal=Nature
|volume=290
|issue=5806
|pages=457–65
|year=1981
|pmid=7219534
|bibcode=1981Natur.290..457A
|doi=10.1038/290457a0 }}</ref>

The guanine-rich strand is the heavy strand (or H-strand) which encodes 28 genes and the cytosine-rich strand is the light strand (or L-strand) which encodes the other 9 genes.<ref name=Anderson/>

The H (heavy, outer circle) and L (light, inner circle) strands are given with their corresponding genes.

There are 22 transfer RNA (TRN) genes for the following amino acids: F, V, L1 (codon UUA/G), I, Q, M, W, A, N, C, Y, S1 (UCN), D, K, G, R, H, S2 (AGC/U), L2 (CUN), E, T and P (white boxes).

There are 2 ribosomal RNA (RRN) genes: S (small subunit, or 12S) and L (large subunit, or 16S) (blue boxes).

There are 13 protein-coding genes: 7 for NADH dehydrogenase subunits (ND, yellow boxes), 3 for cytochrome c oxidase subunits (COX, orange boxes), 2 for ATPase subunits (ATP, red boxes), and one for cytochrome b (CYTB, coral box).

Two gene overlaps are indicated (ATP8-ATP6, and ND4L-ND4, black boxes).

The control region (CR) is the longest non-coding sequence (grey box). Its three hyper-variable regions are indicated (HV, green boxes).
{{clear}}

==Noncoding DNA==

"Non-coding DNA sequences do not code for amino acids. Most non-coding DNA lies between genes [intergenic] on the chromosome [...]. Other non-coding DNA, called introns, is found within genes. [...] Non-coding DNA [...] represents 98 percent of our genome sequence and it does all sorts of things, like regulate those genes to figure out where they should turn on, where they should turn off, how much we should turn on certain genes, how are we going to pack up the DNA into chromosomes, and so forth."<ref name=Margulies>{{ cite book
|author=Elliott Margulies
|title=Non-Coding DNA
|publisher=National Institutes of Health, National Human Genome Research Institute
|location=Bethesda, Maryland USA
|date=22 January 2017
|url=https://www.genome.gov/glossary/index.cfm?id=137&textonly=true
|accessdate=2017-01-22 }}</ref><ref name="Elgar & Vavouri">{{ cite journal |author=Elgar G, Vavouri T
|title=Tuning in to the signals: noncoding sequence conservation in vertebrate genomes
|journal=Trends Genet.
|volume=24
|issue=7
|pages=344–52
|year=2008
|month=July
|pmid=18514361
|doi=10.1016/j.tig.2008.04.005 }}</ref>

Over 80% of human DNA "serves some purpose, biochemically speaking".<ref name=Pennisi>{{ cite journal
| author = E. Pennisi
| title = Genomics. ENCODE project writes eulogy for junk DNA
| journal = Science
| volume = 337
| issue = 6099
| pages = 1159, 1161
| month = September
| year = 2012
| doi = 10.1126/science.337.6099.1159
| PMID = 22955811 }}</ref>

==Non-coding repetitive sequences==

Over 50% of human DNA consists of non-coding repetitive sequences.<ref name=Wolfsberg>{{ cite journal
|author=Wolfsberg T, McEntyre J, Schuler G
|title=Guide to the draft human genome
| journal=Nature
|volume=409
|issue=6822
| pages=824–6
|year=2001
|pmid=11236998
| doi = 10.1038/35057000 }}</ref>

==Non-coding RNA sequences==

Some DNA sequences that do not code protein may still encode functional non-coding RNA molecules, which are involved in the regulation of gene expression.<ref name=ENCODE>{{ cite journal
|author=The ENCODE Project Consortium
|title=Identification and analysis of functional elements in 1% of the human genome by the ENCODE pilot project
|journal=Nature
|volume=447
|issue=7146
| pages =799–816
|year=2007
|doi=10.1038/nature05874
|pmid=17571346
|pmc=2212820
|bibcode=2007Natur.447..799B }}</ref>

==Pseudogenes==
{{main|Genes/Pseudogenes|Pseudogenes}}
An abundant form of noncoding DNA in humans are pseudogenes, which are copies of genes that have been disabled by mutation.<ref name=Harrison>{{ cite journal
|author=Harrison P, Hegyi H, Balasubramanian S, Luscombe N, Bertone P, Echols N, Johnson T, Gerstein M
|title=Molecular Fossils in the Human Genome: Identification and Analysis of the Pseudogenes in Chromosomes 21 and 22
|  journal=Genome Res
|volume=12
|issue=2
| pages=272–80
|year=2002
|pmid=11827946
|doi=10.1101/gr.207102
|pmc=155275 }}</ref> These sequences are usually just molecular fossils, although they can occasionally serve as raw genetic material for the creation of new genes through the process of gene duplication and divergence.<ref name=Gerstein>{{ cite journal |author=Harrison P, Gerstein M |title=Studying genomes through the aeons: protein families, pseudogenes and proteome evolution
| journal=J Mol Biol
|volume=318
|issue=5
| pages=1155–74
|year=2002
|pmid=12083509
|doi=10.1016/S0022-2836(02)00109-2 }}</ref>

==Genes==

'''Def.''' "[a] unit of heredity; a segment of DNA or [[Draft:Human RNA|RNA]] that is transmitted from one generation to the next, and that carries [[Draft:genetics|genetic]] information such as the sequence of amino acids for a protein"<ref name=GeneWikt>{{ cite book
|title=gene
|publisher=Wikimedia Foundation, Inc
|location=San Francisco, California
|date=December 13, 2012
|url=http://en.wiktionary.org/wiki/gene
|accessdate=2012-12-13 }}</ref> is called a '''gene'''.

The genetic information in a genome is held within [[Draft:Genes|genes]], and the complete set of this information in an organism is called its genotype. A gene is a unit of heredity and is a region of DNA that influences a particular characteristic in an organism. Genes contain an open reading frame that can be transcribed, as well as regulatory sequences such as [[Gene promoter|promoter]]s and enhancers, which control the transcription of the open reading frame.

Only about 1.5% of the human genome consists of protein-coding exons.

==Telomeres==

Some non-coding DNA sequences such as telomeres and centromeres play structural roles in chromosomes.

Telomeres are usually lengths of single-stranded DNA containing several thousand repeats of a simple TTAGGG sequence.<ref name=Wright>{{ cite journal
|author=Wright W, Tesmer V, Huffman K, Levene S, Shay J
|title=Normal human chromosomes have long G-rich telomeric overhangs at one end
|journal=Genes Development
|volume=11
|issue=21
|pages=2801–9
|year=1997
|pmid=9353250
|doi=10.1101/gad.11.21.2801
|pmc=316649 }}</ref>

Telomeres and centromeres typically contain few genes, but are important for the function and stability of chromosomes.<ref name=Nugent>{{ cite journal
|author=Nugent C, Lundblad V
|title=The telomerase reverse transcriptase: components and regulation
|journal=Genes Dev
|volume=12
|issue=8
| pages=1073–85
|year=1998
|pmid=9553037
|doi=10.1101/gad.12.8.1073 }}</ref><ref name=Pidoux>{{ cite journal
|author=Pidoux A, Allshire R
|title=The role of heterochromatin in centromere function
| pmc=1569473
| journal=Philos Trans R Soc Lond B Biol Sci
|volume=360
|issue=1455
| pages=569–79
|year=2005
|pmid=15905142
| doi = 10.1098/rstb.2004.1611 }}</ref>

==Centromeres==

Centromeres are chromosomal loci that ensure delivery of a copy of a chromosome to each daughter upon cell division.  On the Spindle Apparatus, chromosome movement is run and maintained by the centromere during [[meiosis]] and [[mitosis]].<ref>Don W Cleveland, Yinghui Mao, Kevin F Sullivan, Centromeres and Kinetochores: From Epigenetics to Mitotic Checkpoint Signaling, Cell, Volume 112, Issue 4, 21 February 2003, Pages 407-421, ISSN 0092-8674, http://dx.doi.org/10.1016/S0092-8674(03)00115-6.
(http://www.sciencedirect.com/science/article/pii/S0092867403001156)</ref>

==Introns==

An '''intron''' is any nucleotide sequence within a gene that is removed by RNA splicing while the final mature RNA product of a gene is being generated.<ref name="Alberts">{{ cite book |author=Alberts, Bruce |title=Molecular biology of the cell |publisher=Garland Science |location=New York |date=2008 |pages= |isbn=0-8153-4105-9 |oclc= |doi= |accessdate=}}</ref><ref name="Stryer">{{ cite book |author=Stryer, Lubert; Berg, Jeremy Mark; Tymoczko, John L. |title=Biochemistry |publisher=W.H. Freeman |location=San Francisco |date=2007 |pages= |isbn=0-7167-6766-X |oclc= |doi= |accessdate=}}</ref> The term ''intron'' refers to both the DNA sequence within a gene and the corresponding sequence in RNA transcripts."<ref name=Kinniburgh>{{ cite journal|last=Kinniburgh|first=Alan|coauthors=mertz, j. and Ross, J.|title=The precursor of mouse β-globin messenger RNA contains two intervening RNA sequences|journal=Cell|year=1978|month=July|volume=14|issue=3|pages=681–693|url=http://www.cell.com/abstract/0092-8674(78)90251-9#|doi=10.1016/0092-8674(78)90251-9|pmid=688388}}</ref>

There are several families of internal nucleic acid sequences that are not present in the final gene product, including inteins, untranslated sequences ([Untranslated region] UTR), and nucleotides removed by RNA editing, in addition to introns.

Introns are extremely common within the nuclear genome of higher vertebrates (e.g. humans and mice), where protein-coding genes almost always contain multiple introns.

Some introns themselves encode specific proteins or can be further processed after splicing to generate noncoding RNA molecules.<ref name=Rearick>{{ cite journal |author=Rearick D, Prakash A, McSweeny A, Shepard SS, Fedorova L, Fedorov A |title=Critical association of ncRNA with introns |journal=Nucleic Acids Res. |volume=39 |issue=6 |pages=2357–66 |year=2011 |month=March |pmid=21071396 |pmc=3064772 |doi=10.1093/nar/gkq1080 |url=}}</ref> Alternative splicing is widely used to generate multiple proteins from a single gene. Furthermore, some introns represent mobile genetic elements and may be regarded as examples of selfish DNA.<ref name=Lambowitz>{{ cite journal |author=Lambowitz AM, Belfort M |title=Introns as mobile genetic elements |journal=Annu. Rev. Biochem. |volume=62 |issue= |pages=587–622 |year=1993 |pmid=8352597 |doi=10.1146/annurev.bi.62.070193.003103 |url=}}</ref>

The human genome contains an average of 8.4 introns/gene (139,418 in the genome).

Some introns are known to enhance the expression of the gene that they are contained in by a process known as intron-mediated enhancement (IME).

==Human DNA==
{{main|Keynote lectures/Human DNA}}
[[Image:DNA Structure+Key+Labelled.pn NoBB.png|thumb|right|250px|This diagram of the structure of DNA shows the four bases; adenine, cytosine, guanine and thymine, and the location of the major and minor groove. Credit: [[commons:User:Zephyris|Zephyris]].]]
"[H]uman DNA has millions of on-off switches and complex networks that control the genes' activities. ... [A]t least 80% of the human genome is active, which opposed the previously held idea that most of the DNA are useless."<ref name=McBournie>{{ cite book
|author=Bryan McBournie
|date=September 6, 2012
|title=Human genome study could unlock the biology of disease
|publisher=Sigma Xi
|url=http://alquemie.smartbrief.com/servlet/ArchiveServlet?issueid=176C7415-9260-447D-A04F-77F3B17D39AF&sid=5df40ccd%252dcb14%252d46a9%252d9d44%252d2dd7cc2984b5
|accessdate=2012-09-06 }}</ref>

"DNA contains genes, which hold the instructions for [life. But, these] take up only about 2 percent of the genome ... The human genome is made up of about 3 billion “letters” along strands that make up the familiar double helix structure of DNA. Particular sequences of these letters form genes, which tell cells how to make proteins. People have about 20,000 genes, but the vast majority of DNA lies outside of genes. ... [A]t least three-quarters of the genome is involved in making RNA [...] it appears to help regulate gene activity."<ref name=Ritter>{{ cite book
|author=Malcolm Ritter
|date=September 6, 2012
|title=Far from being mostly junk, human DNA is ‘a jungle’ of complex activity, huge project shows
|publisher=The Washington Post
|url=http://www.fftimes.com/node/254360
|accessdate=2012-09-06 }}</ref>
{{clear}}


==A DNAs==
[[Image:Adna3.ogv|right|250px|thumb|The A-DNA structure is shown. Credit: [[c:user:Mauroesguerroto|Mauroesguerroto]].]]
B-DNA is driven into the A form when under dehydrating conditions commonly used to form crystals, and many DNA crystal structures are in the A form.<ref name=Rosalind>{{Cite journal
|last=Rosalind|first=Franklin
|date=1953
|title=The Structure of Sodium Thymonucleate Fibres. I. The Influence of Water Content
|url=http://journals.iucr.org/q/issues/1953/08-09/00/a00979/a00979.pdf
|journal=Acta Crystallographica
|volume=6
|issue=8
|pages=673–677
|doi=10.1107/s0365110x53001939 }}</ref>

A-DNA is broader and apparently more compressed along its axis than B-DNA.<ref>{{Cite book|last=Dickerson|first=Richard E.|date=1992|title=DNA Structure From A to Z|url=https://ac.els-cdn.com/0076687992110076/1-s2.0-0076687992110076-main.pdf?_tid=53e46970-aa00-11e7-8a59-00000aacb361&acdnat=1507230584_5ff12415fca4b560400edc52c588d063|journal=Methods in Enzymology|volume=211|pages=67–111
|doi=10.1016/0076-6879(92)11007-6|pmid=1406328|isbn=9780121821128}}</ref>
{{clear}}

==B DNAs==
[[Image:Dnaconformations.png|thumb|right|250px|The structures of A-, B-, and Z-DNA are shown. Credit: [[c:user:Mauroesguerroto|Mauroesguerroto]].]]
[[Image:B&Z&A DNA formula.jpg|thumb|right|250px|The helix axis of A-, B-, and Z-DNA are compared. Credit: [[w:user:Lankenau|Lankenau]].]]
The ''B'' form described by James Watson and Francis Crick is believed to predominate in cells.<ref name=Richmond2003>{{cite journal 
|author=Richmond ''et al.''
|title=The structure of DNA in the nucleosome core 
|journal=Nature |volume=423 |pages=145–150 |year=2003 |pmid= 12736678 |doi=10.1038/nature01595 |last2=Davey |first2=CA |issue=6936
|bibcode = 2003Natur.423..145R }}</ref>

Other conformations are possible; A-DNA, B-DNA, C-DNA, E-DNA,<ref name=Vargason2000>{{cite journal 
|vauthors=Vargason JM, Eichman BF, Ho PS |title=The extended and eccentric E-DNA structure induced by cytosine methylation or bromination 
|journal=Nature Structural Biology |volume=7 |pages=758–761 |year=2000 |doi=10.1038/78985 |pmid=10966645 |issue=9}}</ref> <small>L</small>-DNA (the enantiomeric form of <small>D</small>-DNA),<ref name="Hayashi2005">{{cite journal 
|vauthors=Hayashi G, Hagihara M, Nakatani K | title=Application of L-DNA as a molecular tag 
| journal=Nucleic Acids Symp Ser (Oxf) |volume=49 |pages=261–262 |year=2005 |pmid=17150733 |doi=10.1093/nass/49.1.261 |issue=49}}</ref> P-DNA,<ref name="Allemand1998">{{cite journal 
|vauthors=Allemand JF, Bensimon D, Lavery R, Croquette V | title=Stretched and overwound DNA forms a Pauling-like structure with exposed bases 
|journal=PNAS |volume=95 |pages=14152–14157 |year=1998 |pmid=9826669 |doi=10.1073/pnas.95.24.14152 |issue=24 |pmc=24342
|bibcode = 1998PNAS...9514152A}}</ref> S-DNA, and Z-DNA have been described so far.<ref name=xiangjun>{{ cite web
|author=Xiang Jun
|title=List of 55 fiber structures |url=https://web.archive.org/web/20070526105110/http://rutchem.rutgers.edu/~xiangjun/3DNA/misc/fiber_model.txt |date=2007-05-26 }}</ref> In fact, only the letters F, Q, U, V, and Y are as of 17 February 2011 available to describe any new DNA structure that may appear in the future.<ref name="Bansal2003">{{cite journal 
|author=Bansal M 
|title=DNA structure: Revisiting the Watson-Crick double helix 
|journal=Current Science |volume=85 |issue=11 |pages=1556–1563 |year=2003}}
</ref><ref name="Ghosh2003">{{cite journal 
|vauthors=Ghosh A, Bansal M | title=A glossary of DNA structures from A to Z 
| journal=Acta Crystallogr D |volume=59 |issue=4 |pages=620–626 |year=2003 |doi=10.1107/S0907444903003251 |pmid=12657780}}</ref> There are also triple-stranded DNA forms and quadruplex forms such as the G-quadruplex and the i-motif.

{| class="wikitable"
|+ Structural features of the three major forms of DNA<ref name="Rich1984">{{cite journal 
|vauthors=Rich A, Norheim A, Wang AH |title=The chemistry and biology of left-handed Z-DNA 
|journal=Annual Review of Biochemistry 
|volume=53 |pages=791–846 |year=1984 
|pmid=6383204 |doi=10.1146/annurev.bi.53.070184.004043 
}}</ref><ref name="Sinden1994">{{cite book 
| title=DNA structure and function 
| last=Sinden 
| first=Richard R 
| date=1994-01-15 
| publisher=Academic Press 
| edition=1st 
| isbn=0-12-645750-6 
| pages=398
}}</ref><ref name="pmid7937803">{{cite journal 
| author=Ho PS 
| title=The non-B-DNA structure of d(CA/TG)n does not differ from that of Z-DNA 
| journal=Proc Natl Acad Sci USA |date=1994-09-27 |volume=91 |issue=20 |pages=9549–9553 
| pmid=7937803 |pmc=44850 |doi=10.1073/pnas.91.20.9549
|bibcode = 1994PNAS...91.9549H}}</ref>
!Geometry attribute
!A-DNA
!B-DNA
!Z-DNA
|-
|Helix sense ||align="center"| right-handed ||align="center"| right-handed ||align="center"| left-handed
|-
|Repeating unit ||align="right"| 1 bp ||align="right"| 1 bp ||align="right"| 2 bp
|-
|Rotation/bp ||align="right"| 32.7° ||align="right"| 34.3° ||align="right"| 60°/2
|-
|bp/turn ||align="right"| 11 ||align="right"| 10.5 ||align="right"| 12
|-
|Inclination of bp to axis ||align="right"| +19° ||align="right"| −1.2° ||align="right"| −9°
|-
|Rise/bp along axis ||align="right"| 2.3 Å (0.23&nbsp;nm)||align="right"| 3.32 Å (0.332&nbsp;nm)||align="right"| 3.8 Å (0.38&nbsp;nm)
|-
|Pitch/turn of helix ||align="right"| 28.2 Å (2.82&nbsp;nm)||align="right"| 33.2 Å (3.32&nbsp;nm)||align="right"| 45.6 Å (4.56&nbsp;nm)
|-
|Mean propeller twist ||align="right"| +18° ||align="right"| +16° ||align="right"| 0°
|-
|Glycosyl angle ||align="center"| anti ||align="center"| anti ||align="center"| C: anti,<br> G: syn 
|-
|Sugar pucker ||align="center"| C3'-endo ||align="center"| C2'-endo ||align="center"| C: C2'-endo,<br>G: C2'-exo
|-
|Diameter ||align="right"| 23 Å (2.3&nbsp;nm)||align="right"| 20 Å (2.0&nbsp;nm)||align="right"| 18 Å (1.8&nbsp;nm)
|}
{{clear}}

==Z DNAs==
[[Image:Z-DNA orbit animated.gif|right|thumb|300px| Credit: [[w:Zephyris|Richard Wheeler]].{{tlx|free media}}]]
The repeating polymer of inosine–cytosine produces a left-handed DNA.<ref name=Mitsui>{{cite journal |last1=Mitsui |first1=Y. |last2=Langridge |first2=R. |last3=Shortle |first3=B. E. |last4=Cantor |first4=C. R. |last5=Grant |first5=R. C. |last6=Kodama |first6=M. |last7=Wells |first7=R. D. |title=Physical and enzymatic studies on poly d(I–C)·poly d(I–C), an unusual double-helical DNA |journal=Nature |volume=228 |issue=5277 |pages=1166–1169 |year=1970 |pmid=4321098 |doi=10.1038/2281166a0}}</ref> The "reverse" circular dichroism spectrum for this DNA was interpreted to mean that the strands wrapped around one another in a left-handed fashion.<ref name=Mitsui/>

The ultraviolet circular dichroism of poly(dG-dC) was nearly inverted in 4&nbsp;M sodium chloride solution.<ref name=Pohl>{{cite journal |last1=Pohl |first1=F. M. |last2=Jovin |first2=T. M. |title=Salt-induced co-operative conformational change of a synthetic DNA: equilibrium and kinetic studies with poly(dG-dC) |journal=Journal of Molecular Biology |volume=67 |pages=375–396 |year=1972 |pmid=5045303 |doi=10.1016/0022-2836(72)90457-3 |issue=3}}</ref> This was the result of a conversion from B-DNA to Z-DNA, confirmed by examining the Raman spectra of these solutions and the Z-DNA crystals.<ref name=Thamann>{{cite journal |last1=Thamann |first1=T. J. |last2=Lord |first2=R. C. |last3=Wang |first3=A. H. |last4=Rich |first4=A. |title=High salt form of poly(dG–dC)·poly(dG–dC) is left handed Z-DNA: raman spectra of crystals and solutions |journal=Nucleic Acids Research |volume=9 |pages=5443–5457 |year=1981 |pmid=7301594 |doi=10.1093/nar/9.20.5443 |issue=20 |pmc=327531}}</ref> A crystal structure of "Z-DNA" was the first single-crystal X-ray structure of a DNA fragment (a self-complementary DNA hexamer d(CG)<sub>3</sub>), resolved as a left-handed double helix with two antiparallel chains held together by Watson–Crick base pairs.<ref name=Wang1979>{{cite journal 
|last1=Wang |first1=A. H. |last2=Quigley |first2=G. J. |last3=Kolpak |first3=F. J. |last4=Crawford |first4=J. L. |last5=van Boom |first5=J. H. |last6=van der Marel |first6=G. |last7=Rich |first7=A. |title=Molecular structure of a left-handed double helical DNA fragment at atomic resolution |journal=Nature |volume=282 |pages=680–686 |year=1979 |doi=10.1038/282680a0 |pmid=514347 |issue=5740 |bibcode=1979Natur.282..680W}}</ref>
{{clear}}

==Hypotheses==
{{main|Hypotheses}}
# As both ribose and deoxyribose nucleic acids exist, each pentose or hexose sugar should be usable to make a nucleic acid.

==Acknowledgements==

The content on this page was first contributed by: Henry A. Hoff.

Initial content for this page in some instances came from [http://www.wikiversity.org Wikiversity].

==See also==
{{div col|colwidth=12em}}
* [[Epigenomes]]
* [[Human DNA]]
{{Div col end}}

==References==
{{reflist|2}}

==External links==
* [http://www.bing.com/search?q=&go=&qs=n&sk=&sc=8-15&qb=1&FORM=AXRE Bing Advanced search]
* [http://www.genome.jp/ GenomeNet KEGG database]
* [http://books.google.com/ Google Books]
* [http://scholar.google.com/advanced_scholar_search?hl=en&lr= Google scholar Advanced Scholar Search]
* [http://www.ncbi.nlm.nih.gov/sites/entrez?db=gene Home - Gene - NCBI]
* [http://www.jstor.org/ JSTOR]
* [http://www.lycos.com/ Lycos search]
* [http://www.ncbi.nlm.nih.gov/sites/gquery NCBI All Databases Search]
* [http://www.ncbi.nlm.nih.gov/ncbisearch/ NCBI Site Search]
* [http://www.osti.gov/ Office of Scientific & Technical Information]
* [http://psycnet.apa.org/ PsycNET]
* [http://www.ncbi.nlm.nih.gov/pccompound PubChem Public Chemical Database]
* [http://www.questia.com/ Questia - The Online Library of Books and Journals]
* [http://online.sagepub.com/ SAGE journals online]
* [http://www.scirus.com/srsapp/advanced/index.jsp?q1= Scirus for scientific information only advanced search]
* [http://www.springerlink.com/ SpringerLink]
* [http://www.tandfonline.com/ Taylor & Francis Online]
* [https://www.genome.gov/glossary/ The Talking Glossary of Genetic Terms]
* [http://www.wikidoc.org/index.php/Main_Page WikiDoc The Living Textbook of Medicine]
* [http://onlinelibrary.wiley.com/advanced/search Wiley Online Library Advanced Search]
* [http://search.yahoo.com/web/advanced Yahoo Advanced Web Search]

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