The Nitrogenous base reference article from the English Wikipedia on 24-Jul-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Nitrogenous base

Nitrogenous bases are the parts of RNA and DNA that are involved in pairing up (see also base pairs). These include cytosine, guanine, adenine, thymine (DNA) and uracil (RNA). These are abbreviated as C, G, A, T, and U, respectivly.

The four bases included in DNA are shown below. 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 nitrogenous 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

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