base pairing

Base pairing is the simple matching rule at the heart of how DNA works: A goes with T, and G goes with C. Like puzzle pieces that fit only one way, each base on one strand accepts only its correct partner across the helix. This single rule explains how genetic information is copied and read.

The pairing is held together by hydrogen bonds — weak attractions between specific atoms on the two bases. Adenine and thymine form two such bonds; guanine and cytosine form three. The geometry only works when a large purine pairs with a small pyrimidine, which is why A pairs with T and G pairs with C rather than in other combinations.

Because the rule is fixed, knowing the sequence of one strand tells you the sequence of the other: the two strands are complementary. This is the property that lets a cell copy DNA — separate the strands, and each one is a template for rebuilding its partner exactly.

Base pairing is not limited to copying DNA. It also lets RNA messages be read during protein synthesis and underlies many laboratory tools, from PCR primers finding their target to probes that detect a specific sequence. The same matching logic recurs throughout molecular biology.