Which base pairs with cytosine in a DNA double helix?

Guanine pairs with cytosine in a DNA double helix.

This pairing is held together by three hydrogen bonds, while adenine pairs with thymine through two hydrogen bonds. The specific matching rules allow one DNA strand to act as a template for copying its partner and for producing complementary RNA sequences.

Guanine and cytosine are both classified as nucleobases, but guanine is a purine with two fused rings and cytosine is a pyrimidine with one ring. Pairing one purine with one pyrimidine helps maintain a relatively uniform width along the double helix.

Uracil is commonly found in RNA in place of thymine, so it is not the normal partner of cytosine in DNA. In RNA, guanine still pairs with cytosine. The number of guanine–cytosine pairs also affects the melting temperature of a DNA molecule because three hydrogen bonds require more heat to separate than two.