Cytosine is one of the 5 main nitrogenous bases used in storing and transporting genetic information within a cell. It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attached (an amine group at position 4 and a keto group at position 2). The nucleoside of cytosine is cytidine.
Cytosine was first discovered in 1894 when it was isolated from calf thymus tissues. A structure was proposed in 1903, and was synthesized (and thus confirmed) in the laboratory in the same year.
Cytosine can be found as part of DNA, RNA or as a part of a nucleotide. As cytosine triphosphate (CTP), it can act as a co-factor to enzymes, and can transfer a phosphate to convert adenosine diphosphate (ADP) to adenosine triphosphate (ATP).