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Why does adenine pair with thymine in DNA?

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Diagram of a DNA polynucleotide chain showing linked deoxyribose sugars, phosphate groups, and nitrogenous bases (adenine, guanine, cytosine, thymine), with an inset comparing ribose and uracil in RNA. — Nucleic acid A portion of the polynucleotide chain of deoxyribonucleic acid (DNA), with an inset showing the corresponding pentose sugar and pyrimidine base in ribonucleic acid (RNA).

In DNA, adenine (A) pairs with thymine (T) because they are complementary nitrogenous bases that form a stable, specific "lock-and-key" fit within the double-helix structure.12

This pairing is facilitated by hydrogen bonds, which are weak electrostatic attractions between the bases. While individual hydrogen bonds are relatively weak, the cumulative effect of these bonds along the center of the DNA molecule provides the necessary stability to hold the two strands of the double helix together.32

The specific A-T pairing—along with the pairing of guanine (G) and cytosine (C)—was identified by James Watson and Francis Crick in 1953. Their model demonstrated that this arrangement is the only way to satisfy the molecular dimensions of the DNA structure. This complementary base-pairing is fundamental to the molecule's ability to serve as a template for replication, allowing genetic information to be accurately transmitted to the next generation.2