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

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

Contents 1 Structure 1.1 Main bases 1.2 Modified purine bases 1.3 Modified pyrimidine bases 2 Novel Bases 3 References 4 See also 5 External links

• 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.

These are incorporated into the growing chain during RNA and/or DNA synthesis.

Nucleobase	Adenine	Guanine	Thymine	Cytosine	Uracil
Nucleoside	Adenosine A	Guanosine G	Thymidine T	Cytidine C	Uridine U

These are examples of modified adenosine or guanosine.

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

These are examples of modified cytidine, thymidine or uridine.

Nucleobase	Uracil	5,6-Dihydrouracil	5-Methylcytosine
Nucleoside	Pseudouridine Ψ	Dihydrouridine D	5-Methylcytidine m5C

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.

1. ^ http://www.mun.ca/biology/desmid/brian/BIOL2060/BIOL2060-22/CB22.html
2. ^ http://www.biogeo.uw.edu.pl/research/grupaC_en.html
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

• Nucleoside
• Nucleotide
• DNA
• RNA

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

v • d • e Major families of biochemicals
Peptides | Amino acids | Nucleic acids | Carbohydrates | Nucleotide sugars | Lipids | Terpenes | Carotenoids | Tetrapyrroles | Enzyme cofactors | Steroids | Flavonoids | Alkaloids | Polyketides | Glycosides
Analogues of nucleic acids:	Types of Nucleic Acids	Analogues of nucleic acids:
Nucleobases:	Purine (Adenine, Guanine) | Pyrimidine (Uracil, Thymine, Cytosine)
Nucleosides:	Adenosine/Deoxyadenosine | Guanosine/Deoxyguanosine | Uridine | Thymidine | Cytidine/Deoxycytidine
Nucleotides:	monophosphates (AMP, UMP, GMP, CMP) | diphosphates (ADP, UDP, GDP, CDP) | triphosphates (ATP, UTP, GTP, CTP) | cyclic (cAMP, cGMP, cADPR)
Deoxynucleotides:	monophosphates (dAMP, TMP, dGMP, dCMP) | diphosphates (dADP, TDP, dGDP, dCDP) | triphosphates (dATP, TTP, dGTP, dCTP)
Ribonucleic acids:	RNA | mRNA | piRNA | tRNA | rRNA | ncRNA | gRNA | shRNA | siRNA | snRNA | miRNA | snoRNA
Deoxyribonucleic acids:	DNA | mtDNA | cDNA | plasmid | Cosmid | BAC | YAC | HAC
Analogues of nucleic acids:	GNA | PNA | TNA | Morpholino | LNA