Two hydrogen bonds connect an adenine–thymine base pair in DNA. These bonds form between specific nitrogen and oxygen atoms on the two bases and help hold the complementary strands together.
Adenine is a purine, meaning it has a two-ring structure, while thymine is a one-ring pyrimidine. Their shapes and hydrogen-bonding groups fit together in the DNA double helix. Guanine and cytosine form the other standard DNA pair, and they are connected by three hydrogen bonds.
The number of hydrogen bonds is often used as a quick way to remember the base-pairing rules: A–T has two, while G–C has three. However, hydrogen bonds are not the only source of DNA stability. Base-stacking interactions between neighboring aromatic bases contribute substantially to the helix’s overall stability.
This pairing is specific rather than interchangeable. Adenine does not normally pair with cytosine in standard DNA, and thymine does not normally pair with guanine. In RNA, uracil takes thymine’s place, so adenine pairs with uracil through two hydrogen bonds.