cytosine

Cytosine is the DNA letter, written C, that always pairs with guanine. Whenever a strand reads G at some position, the partner strand reads C directly across from it, and vice versa.

Cytosine is a pyrimidine — a smaller, single-ringed base, like thymine. It bonds to guanine through three hydrogen bonds, which makes the C-G pair stronger than the two-bond A-T pair. This is why the proportion of C and G in a DNA molecule affects how much energy it takes to separate the two strands.

Cytosine is also the base most often chemically tagged to regulate genes. Cells can attach a small methyl group to certain cytosines, especially where a C sits next to a G in the sequence; this DNA methylation can switch nearby genes off without changing the underlying letters. Such marks are central to epigenetics.

A practical caveat follows from cytosine's chemistry: a methylated cytosine can spontaneously change into thymine over time, making certain sites unusually prone to mutation. So the same base that helps regulate genes is also a recurring source of genetic change.