Editing Nucleotide base (section)

{{Short description|Nitrogen-containing biological compounds that form nucleosides}}
{{Redirect|ACGT|the Japanese animation production company|A.C.G.T}}
{{Use dmy dates|date=June 2021}}
[[File:AGCT RNA mini.png|thumb|350px|Base pairing: two [[base pair]]s are produced by four nucleotide monomers, nucleobases are in blue. Guanine (G) is paired with cytosine (C) via three [[hydrogen bond]]s, in red. Adenine (A) is paired with uracil (U) via two hydrogen bonds, in red.]]
[[File:Blausen 0323 DNA Purines.png|thumb|230px|Purine nucleobases are fused-ring molecules.]]
[[File:Blausen 0324 DNA Pyrimidines.png|thumb|230px|Pyrimidine nucleobases are simple ring molecules.]]

'''Nucleotide bases'''<ref>{{Cite journal|url=https://doi.org/10.1351/goldbook.N04254|title=IUPAC - nucleotide bases (N04254)|last=The International Union of Pure and Applied Chemistry (IUPAC)|website=goldbook.iupac.org|doi=10.1351/goldbook.N04254 |doi-access=free}}</ref> (also '''nucleobases''', '''nitrogenous bases''') are [[nitrogen]]-containing biological compounds that form [[nucleosides]], which, in turn, are components of [[nucleotide]]s, with all of these [[monomer]]s constituting the basic building blocks of [[nucleic acids]]. The ability of nucleobases to form [[base pair]]s and to stack one upon another leads directly to long-chain helical structures such as [[ribonucleic acid]] (RNA) and [[deoxyribonucleic acid]] (DNA). Five nucleobases—[[adenine]] (A), [[cytosine]] (C), [[guanine]] (G), [[thymine]] (T), and [[uracil]] (U)—are called ''primary'' or ''canonical''. They function as the fundamental units of the [[genetic code]], with the bases A, G, C, and T being found in DNA while A, G, C, and U are found in RNA. Thymine and uracil are distinguished by merely the presence or absence of a methyl group on the fifth carbon (C5) of these heterocyclic six-membered rings.<ref>{{cite book|last=Soukup|first=Garrett A.|title=eLS|date=2003|chapter=Nucleic Acids: General Properties|publisher=American Cancer Society|language=en|doi=10.1038/npg.els.0001335|isbn=9780470015902}}</ref>{{page needed|date=January 2021}}
In addition, some viruses have [[2,6-diaminopurine|aminoadenine]] (Z) instead of adenine. It differs in having an extra [[amine]] group, creating a more stable bond to thymine.<ref>{{Cite web|url=https://www.sciencenews.org/article/virus-dna-z-bacteriophage-genetic-alphabet-bond-life|title=Some viruses thwart bacterial defenses with a unique genetic alphabet|date=5 May 2021}}</ref>

Adenine and guanine have a [[ring (chemistry)|fused-ring]] skeletal structure derived of [[purine]], hence they are called '''purine bases'''.<ref>{{Cite journal|url=https://goldbook.iupac.org/terms/view/P04953|title=IUPAC - purine bases (P04953)|last=The International Union of Pure and Applied Chemistry (IUPAC)|website=goldbook.iupac.org|doi=10.1351/goldbook.p04953|doi-access=free}}</ref> The purine nitrogenous bases are characterized by their single [[amino group]] ({{chem2|\sNH2}}), at the C6 carbon in adenine and C2 in guanine.<ref name="NIH.gov">{{cite journal | vauthors = Berg JM, Tymoczko JL, Stryer L | title = Section 25.2, Purine Bases Can Be Synthesized de Novo or Recycled by Salvage Pathways. | journal = Biochemistry. 5th Edition | url = https://www.ncbi.nlm.nih.gov/books/NBK22385/| access-date = 2019-12-11 }}</ref> Similarly, the simple-ring structure of cytosine, uracil, and thymine is derived of [[pyrimidine]], so those three bases are called the '''pyrimidine bases'''.<ref>{{Cite journal|url=https://goldbook.iupac.org/terms/view/P04958|title=IUPAC - pyrimidine bases (P04958)|last=The International Union of Pure and Applied Chemistry (IUPAC)|website=goldbook.iupac.org|doi=10.1351/goldbook.p04958|doi-access=free}}</ref>

Each of the base pairs in a typical double-[[helix]] DNA comprises a purine and a pyrimidine: either an A paired with a T or a C paired with a G. These purine-pyrimidine pairs, which are called [[complementarity (molecular biology)|''base complements'']], connect the two strands of the helix and are often compared to the rungs of a ladder. Only pairing purine with pyrimidine ensures a constant width for the DNA. The A–T pairing is based on two [[hydrogen bond]]s, while the C–G pairing is based on three. In both cases, the hydrogen bonds are between the [[amine]] and [[carbonyl]] groups on the complementary bases.

Nucleobases such as adenine, guanine, [[xanthine]], [[hypoxanthine]], purine, [[2,6-diaminopurine]], and 6,8-diaminopurine may have formed in outer space as well as on earth.<ref name="Callahan">{{cite journal | vauthors = Callahan MP, Smith KE, Cleaves HJ, Ruzicka J, Stern JC, Glavin DP, House CH, Dworkin JP | title = Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 108 | issue = 34 | pages = 13995–8 | date = August 2011 | pmid = 21836052 | pmc = 3161613 | doi = 10.1073/pnas.1106493108 | publisher = [[PNAS]] | bibcode = 2011PNAS..10813995C | doi-access = free }}</ref><ref name="Steigerwald">{{cite web |last=Steigerwald |first=John |title=NASA Researchers: DNA Building Blocks Can Be Made in Space |url=http://www.nasa.gov/topics/solarsystem/features/dna-meteorites.html |publisher=[[NASA]] |date=8 August 2011 |access-date=2011-08-10 }}</ref><ref name="DNA">{{cite web |author=ScienceDaily Staff |title=DNA Building Blocks Can Be Made in Space, NASA Evidence Suggests |url=https://www.sciencedaily.com/releases/2011/08/110808220659.htm |date=9 August 2011 |website=[[ScienceDaily]] |access-date=2011-08-09}}</ref>

The origin of the term ''[[Base (chemistry)|base]]'' reflects these compounds' chemical properties in [[acid–base reaction]]s, but those properties are not especially important for understanding most of the biological functions of nucleobases.