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AGCT RNA mini.png
Base pairing in RNA. Nucleobases in blue.

Nucleobases (or nucleotide bases/nitrogenous 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 most common 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 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.

Contents

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.
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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 xanthosine
Xanthosine
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 dihydrouracil
5,6-Dihydrouracil
Chemical structure of 5-methylcytosine
5-Methylcytosine
Nucleoside Chemical structure of dihydrouridine
Dihydrouridine
D
Chemical structure of 5-methylcytidine
5-Methylcytidine
m5C

Novel Bases

A vast number of nucleobase analogues exist. The most common applications 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

  1. ^ BIOL2060: Translation
  2. ^ Research
  3. ^ 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

See also

External links


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