Nucleobases are the parts of RNA and DNA that may be 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, respectively. They are usually simply called bases in Genetics.



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.