Nucleobases (or Nucleotide bases) are the parts of DNA and RNA that may be involved in pairing (see also base pairs). The main ones are cytosine, guanine, adenine (DNA and RNA), thymine (DNA) and uracil (RNA), 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.

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 purines (abbreviated as R). Cytosine, thymine, and uracil are all pyrimidines (abbreviated as Y).

The system of a base covalently 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 (?), dihydrouridine (D), inosine (I), ribothymidine (rT) and 7-methylguanosine (m7G).^[1][2]

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 carbonyl-group). Hypoxanthine is produced from adenine, xanthine from guanine.^[3] Similarly, deamination of cytosine results in uracil.

Structure

• The "skeleton" of adenine and guanine 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	Chemical structure of adenine Adenine	Chemical structure of guanine Guanine	Chemical structure of thymine Thymine	Chemical structure of cytosine Cytosine	Chemical structure of uracil Uracil
Nucleoside	Chemical structure of adenosine Adenosine A	Chemical structure of guanosine Guanosine G	Chemical structure of thymidine Thymidine T	Chemical structure of cytidine Cytidine C	Chemical structure of uridine Uridine U

Modified purine bases

These are examples of modified adenosine or guanosine.

Nucleobase	Chemical structure of hypoxanthine Hypoxanthine	Chemical structure of xanthine Xanthine	Chemical structure of 7-methylguanine 7-Methylguanine
Nucleoside	Chemical structure of inosine Inosine I	Chemical structure of xanthinosine Xanthinosine X	Chemical structure of 7-methylguanosine 7-Methylguanosine m7G

Modified pyrimidine bases

These are examples of modified cytidine, thymidine or uridine.

Nucleobase	Chemical structure of uracil Uracil	Chemical structure of dihydrouracil 5,6-Dihydrouracil	Chemical structure of 5-methylcytosine 5-Methylcytosine
Nucleoside	Chemical structure of pseudouridine Pseudouridine ?	Chemical structure of dihydrouridine Dihydrouridine D	Chemical structure of 5-methylcytidine 5-Methylcytidine m5C

Novel Bases

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 administered as nuclosides as charged nucleotides cannot easily cross cell membranes.

References

See also

• Nucleoside
• Nucleotide
• DNA
• RNA

External links

• Base pairing in DNA Double Helix (shows specific hydrogen bonds)

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