complementary strand

Two strands of DNA fit together like a photograph and its negative: knowing one, you can work out the other exactly. The complementary strand is the partner strand whose bases are the matching opposites of yours — wherever you have an A, it has a T; wherever you have a G, it has a C.

This relationship comes straight from the base-pairing rules. Because A always pairs with T and G always pairs with C, the sequence of one strand completely determines the sequence of its complement. The two strands are not identical; they are mirror-matched, each carrying the same information in opposite form.

Complementarity is what makes faithful copying possible. When DNA is replicated, the two strands are pulled apart and each serves as a template; an enzyme reads it and lays down a new complementary strand, producing two identical double helices from one. The same idea lets a cell transcribe DNA into RNA.

One subtlety: because the two strands run in opposite directions, the complement is read in the reverse direction relative to its partner. When biologists write down the matching sequence, they usually give the reverse complement so the directions line up correctly.