Nucleobase

Nucleobases (or Nucleotide bases) are the parts of 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 pyrimidines (abbreviated as Y).

A base 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).^[1]^[2]

Hypoxanthine and 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.

Nucleobase	Uracil
Nucleoside	Thymidine T	Cytidine C	Uridine U

Modified purine bases

These are examples of modified adenosine or guanosine.

Nucleobase	Hypoxanthine	Xanthine	7-Methylguanine
Nucleoside	Inosine I	Xanthinosine X	7-Methylguanosine m7G

Modified pyrimidine bases

These are examples of modified cytidine, thymidine or uridine.

Nucleobase	5-Methylcytosine
Nucleoside	Pseudouridine Ψ	Dihydrouridine D	5-Methylcytidine m5C

Novel Bases

Main article: 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

^ http://www.mun.ca/biology/desmid/brian/BIOL2060/BIOL2060-22/CB22.html
^ http://www.biogeo.uw.edu.pl/research/grupaC_en.html
^ T Nguyen, D Brunson, C L Crespi, B W Penman, J S Wishnok, and S R Tannenbaum, 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