Guanine (G) is one of the four nucleotide bases in DNA, with the other three being adenine (A), cytosine (C) and thymine (T). Within a double-stranded DNA molecule, guanine bases on one strand pair with cytosine bases on the opposite strand. The sequence of the four nucleotide bases encodes DNA’s information.
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.
Division of Genomic Medicine