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.
Cytosine is one of the four building blocks of DNA and RNA. So it's one of the four nucleotides that's present both in DNA, RNA, and each cytosine makes up part of the code. Cytosine has the unique property in that it binds in the double helix opposite a guanine, one of the other nucleotides. Cytosine has one other interesting property that none of the other nucleotides have, is that very often in the cell, cytosine can have an extra chemical attached to them, a methyl group. And this DNA methylation at cytosines is thought helps regulate genes try to help turn them on and off.- Lawrence C. Brody, Ph.D.