cytosine(n = noun.substance) c - a base found in DNA and RNA and derived from pyrimidine; pairs with guanine;
is a kind of pyrimidine

   A pyrimidine (C4H5N3O) which is one of the four major basic components of DNA and RNA in most organisms, forming glyosides with ribose and deoxyribose. It is the basic component of cytidine, deoxycytidine, cytosine, cytidine monophosphate, and derivatives of those compounds. [PJC]

cytosine, n. one of the principal component bases of the nucleotides and the nucleic acids DNA and RNA, derived from pyrimidine.