1) Cytosine (C) is one of four chemical bases in DNA, the other three being adenine (A), guanine (G), and thymine (T). Within the DNA molecule, cytosine bases located on one strand form chemical bonds with guanine bases on the opposite strand. The sequence of four DNA bases encodes the cell's genetic instructions.

2) A pyrimidine base (C4H5N3O) that codes genetic information in the polynucleotide chain of DNA or RNA.