Cytosine is one of 5 nitrogenous bases that are absorbed to a 5 carbon sugar, pentose, and a phosphate accumulation to accomplish nucleotides. Nucleotides are the units that accompany calm to accomplish DNA and RNA molecules. The added bases, besides cytosine, that accomplish up a DNA atom are adenine, guanine and thymine. In an RNA molecule, uracil replaces thymine.

The bases are breach into two altered groups. Adenine and guanine are purine bases and cytosine, thymine and uracil are pyrimidine bases. The two groups alter in their basal structure. Purine bases are fabricated up of two rings of atoms, while pyrimidine bases are fabricated up of alone one ring. The bases are alleged nitrogenous bases as the rings accommodate nitrogen and carbon atoms.

The bases consistently alone brace with one added base. Purine bases band with alone pyrimidine bases. Purine bases never band with added purine bases and pyrimidine bases never band with added pyrimidine bases. Specifically, cytosine consistently pairs with guanine and adenine pairs with thymine or uracil, depending on whether in a DNA or RNA molecule. This bond is referred to as specific abject pairing.