Which base pairs with adenine in DNA?

In DNA, adenine pairs with thymine. This complementary pairing is part of the Watson-Crick base-pairing system that holds the two strands of the DNA double helix together.

Adenine and thymine connect through two hydrogen bonds, while guanine and cytosine connect through three. The pairing is chemically specific: adenine’s hydrogen-bonding sites match thymine’s in a way that produces the regular width and geometry of the double helix. Purines pair with pyrimidines, keeping the strands spaced correctly.

This complementarity is essential during replication. When the two DNA strands separate, the base sequence on each strand directs the construction of a matching partner. As a result, genetic information is preserved in two complementary copies rather than being stored on only one strand.

Uracil is the common mix-up. It pairs with adenine in RNA, where it takes the place of thymine. Cytosine pairs with guanine, so neither is adenine’s DNA partner. Although unusual nonstandard pairings can occur in special molecular contexts, thymine is the single canonical answer for ordinary double-stranded DNA.