Two hydrogen bonds form between adenine and thymine in DNA. This pairing is a standard Watson–Crick base pair, written A–T, in which adenine on one strand matches thymine on the opposite strand.
The two bases fit together because their hydrogen-bond donors and acceptors are positioned in complementary places. The bonds help hold the two DNA strands together, although the stacked arrangement of neighboring bases also contributes substantially to the double helix’s stability. Adenine and thymine are connected by two hydrogen bonds, while guanine and cytosine form three.
A common mistake is to assume that more hydrogen bonds automatically explain every difference in DNA stability. DNA regions rich in G–C pairs generally require more energy to separate because G–C pairs have three hydrogen bonds, but base stacking, sequence context, salt concentration, and other molecular effects also matter. Another mix-up comes from RNA: RNA usually pairs adenine with uracil rather than thymine. The pairing rules allow one DNA strand to serve as a template for copying the other, a foundation of replication and heredity.