guanine

Guanine is the DNA letter, written G, that always pairs with cytosine. It is the partner half of the second of the two base pairs, just as adenine and thymine form the first.

Like adenine, guanine is a purine — a larger base with two fused rings. It pairs with cytosine through three hydrogen bonds, one more than the two bonds holding A to T. Those extra bonds make G-C pairs a little stickier, so DNA regions rich in G and C tend to hold together more tightly and need more heat to pull apart.

The reliable G-with-C pairing helps make each strand a faithful template for the other and keeps the helix a uniform width, since a large purine is always paired with a smaller pyrimidine. Counting bases in DNA reveals roughly equal amounts of guanine and cytosine, mirroring the equality of adenine and thymine.

Guanine also turns up beyond the genome — for example in GTP, an energy-carrying molecule, and in the modified base that caps messenger RNA. As a DNA letter, though, its defining job is to mark the spots that read C on the opposite strand.