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'''Nucleobases''' (or Nucleotide bases) are the parts of [[RNA]] and [[DNA]] that may be involved in pairing (see also [[base pairs]]). These include [[cytosine]], [[guanine]], [[adenine]], [[thymine]] ([[DNA]]) and [[uracil]] ([[RNA]]). These are abbreviated as C, G, A, T, and U, respectively. They are usually simply called '''bases''' in [[genetics]]. Because A, G, C, and T appear in the DNA, these molecules are called '''DNA-bases'''; A, G, C, and U are called '''RNA-bases''', respectively.

Uracil replaces thymine in RNA. These two bases are identical except that uracil lacks the 5' methyl group. Adenine and guanine belong to the double-ringed class of molecules called [[purine]]s (abbreviated as R). Cytosine, thymine, and uracil are all [[pyrimidine]]s (abbreviated as Y).

A base [[covalent]]ly bound to the 1' [[carbon]] of a [[ribose]] or [[deoxyribose]] is called a ''[[nucleoside]]'', and a nucleoside with one or more [[phosphate]] groups attached at the 5' carbon is called a ''[[nucleotide]]''.

Apart from adenosine (A), cytidine (C), guanosine (G), thymidine (T) and uridine (U), DNA and RNA also contain bases that have been modified after the nucleic acid chain has been formed. In DNA, the only modified base is 5-methylcytidine (m5C). In RNA, there are many modified bases, including pseudouridine (Ψ), dihydrouridine (D), inosine (I), ribothymidine (rT) and 7-methylguanosine (m7G).<ref>http://www.mun.ca/biology/desmid/brian/BIOL2060/BIOL2060-22/CB22.html</ref><ref>http://www.biogeo.uw.edu.pl/research/grupaC_en.html</ref>

[[Hypoxanthine]] and [[xanthine]] are two of the many bases created through [[mutagen]] presence, both of them through deamination (replacement of the amine-group with a hydroxyl-group). Hypoxanthine is produced from [[adenine]], xanthine from [[guanine]].<ref>T Nguyen, D Brunson, C L Crespi, B W Penman, J S Wishnok, and S R Tannenbaum, [http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=48797 DNA damage and mutation in human cells exposed to nitric oxide in vitro], Proc Natl Acad Sci U S A. 1992 April 1; 89(7): 3030–3034</ref> Similarly, deamination of [[cytosine]] results in [[uracil]].

==Structure==
* The "skeleton" of adenine and guanin is [[purine]], hence the name '''purine-bases'''.
* The "skeleton" of cytosine, uracil and thymine is [[pyrimidine]], hence '''pyrimidine-bases'''.

===Main bases===
These are incorporated into the growing chain during RNA and/or DNA synthesis.
{| border="1" cellpadding="2" cellspacing="0" align="none" style="margin-left:1em"
|- align="center" valign="bottom"
| '''Nucleobase'''<br> || [[Image:Adenine chemical structure.png|58px|Chemical structure of adenine]]<br/>[[Adenine]]<br/> || [[Image:Guanine chemical structure.png|86px|Chemical structure of guanine]]<br/>[[Guanine]]<br/> || [[Image:Thymine chemical structure.png|63px|Chemical structure of thymine]]<br/>[[Thymine]]<br/> || [[Image:Cytosine chemical structure.png|51px|Chemical structure of cytosine]]<br/>[[Cytosine]]<br/> || [[Image:Uracil chemical structure.png|51px|Chemical structure of uracil]]<br/>[[Uracil]]<br/>
|- align="center" valign="bottom"
| '''Nucleoside'''<br><br> || [[Image:A chemical structure.png|95px|Chemical structure of adenosine]]<br/>[[Adenosine]]<br/>A || [[Image:G chemical structure.png|123px|Chemical structure of guanosine]]<br/>[[Guanosine]]<br/>G || [[Image:T chemical structure.png|87px|Chemical structure of thymidine]]<br/>[[Thymidine]]<br/>T || [[Image:C chemical structure.png|87px|Chemical structure of cytidine]]<br/>[[Cytidine]]<br/>C || [[Image:U chemical structure.png|87px|Chemical structure of uridine]]<br/>[[Uridine]]<br/>U
|-
|}

===Modified purine bases===
These are examples of modified adenosine or guanosine.
{| border="1" cellpadding="2" cellspacing="0" align="none" style="margin-left:1em"
|- align="center" valign="bottom"
| '''Nucleobase'''<br> || [[Image:Hypoxanthine chemical structure.svg|70px|Chemical structure of hypoxanthine]]<br/>[[Hypoxanthine]]<br/> || [[Image:Xanthine chemical structure.svg|80px|Chemical structure of xanthine]]<br/>[[Xanthine]]<br/> || [[Image:7methylguanine.png|86px|Chemical structure of 7-methylguanine]]<br/>[[7-Methylguanine]]<br/>
|- align="center" valign="bottom"
| '''Nucleoside'''<br><br> || [[Image:HX chemical structure.svg|95px|Chemical structure of inosine]]<br/>[[Inosine]]<br/>I || [[Image:X chemical structure.svg|105px|Chemical structure of xanthinosine]]<br/>[[Xanthinosine]]<br/>X || [[Image:7metylguanin.png|118px|Chemical structure of 7-methylguanosine]]<br/>[[7-Methylguanosine]]<br/>m7G
|-
|}

===Modified pyrimidine bases===
These are examples of modified cytidine, thymidine or uridine.
{| border="1" cellpadding="2" cellspacing="0" align="none" style="margin-left:1em"
|- align="center" valign="bottom"
| '''Nucleobase'''<br> || [[Image:Pseudouracil.PNG|51px|Chemical structure of uracil]]<br/>[[Uracil]]<br/> || [[Image:Dihydrouracil.PNG|51px|Chemical structure of dihydrouracil]]<br/>[[5,6-Dihydrouracil]]<br/> || [[Image:5-methylcytosine.png|65px|Chemical structure of 5-methylcytosine]]<br/>[[5-Methylcytosine]]<br/>
|- align="center" valign="bottom"
| '''Nucleoside'''<br><br> || [[Image:Pseudouridin.PNG|87px|Chemical structure of pseudouridine]]<br/>[[Pseudouridine]]<br/>Ψ || [[Image:Dihydrouridin.PNG|87px|Chemical structure of dihydrouridine]]<br/>[[Dihydrouridine]]<br/>D || [[Image:5methylcytidine.png|87px|Chemical structure of 5-methylcytidine]]<br/>[[5-Methylcytidine]]<br/>m5C
|-
|}

== Novel Bases ==
{{main|Nucleic acid analogues}}
A vast number of nucleobases analogues exist.
The most common application are used as fluorescent probes, either directly or indirectly, such as [[Aminoallyl nucleotide]] which are used to label cRNA or cDNA in [[microarrays]].
Several groups are working on alternative "extra" base pairs to extend the genetic code, such as [[isoguanine]] and [[isocytosine]] or the fluorescent 2-amino-6-(2-thienyl)purine and pyrrole-2-carbaldehyde.

In medicine, several [[nucleoside analogues]] are used as anticancer and antiviral agents. The viral polymerase incorporates these compounds with non-canon bases. These compounds are activated in the cells by being converted into nucleotides, they are adminisered as nuclosides as charged nucleotides cannot easily cross cell membranes.

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

== See also ==
* [[Nucleoside]]
* [[Nucleotide]]
* [[DNA]]
* [[RNA]]

==External links==
*[http://www.elmhurst.edu/~chm/vchembook/582dnadoublehelix.html Base pairing in DNA Double Helix (shows specific hydrogen bonds)]

{{Nucleic acids}}

[[Category:DNA]]
[[Category:RNA]]

[[cs:Nukleové báze]]
[[de:Nukleinbasen]]
[[es:base nitrogenada]]
[[eo:Nitrogena bazo]]
[[fr:Base azotée]]
[[it:base azotata]]
[[vi:nucleobase]]