Guanine (G) is one of four chemical bases in DNA, with the other three being adenine (A), cytosine (C), and thymine (T). Within the DNA molecule, guanine bases located on one strand form chemical bonds with cytosine bases on the opposite strand. The sequence of four DNA bases encodes the cell's genetic instructions.
Guanine is one of the building blocks of DNA. It's the G in the A, C, G, or T. Guanine in the double helix pairs with cytosine, so you will see CG pairs; one on one strand and one on the other. And the CG pairs happen to bind more tightly than the AT pairs, so long stretches of CG make stronger helixes than stretches of AT.- Lawrence C. Brody, Ph.D.