Chargaff's rules

Chargaff's rules are a pair of patterns that show up whenever you carefully count the bases in DNA. The most famous one is simple: in any double-stranded DNA, the amount of adenine equals the amount of thymine, and the amount of guanine equals the amount of cytosine.

Erwin Chargaff and his colleagues measured base composition across many species in the late 1940s and found these equalities held even though the overall mix of bases varied from organism to organism. At the time, no one knew why A should match T and G should match C — the rules were an unexplained regularity.

The explanation arrived with the double helix. Because A always pairs with T and G always pairs with C across the two strands, every A on one strand is balanced by a T on the other, and every G by a C. Chargaff's first rule is exactly what you would expect from complementary base pairing, and it offered an important clue toward the helix's structure.

There is also a second, less famous Chargaff rule: roughly equal amounts of A and T, and of G and C, tend to appear even within a single strand of a large genome. This one is a looser statistical tendency rather than a strict consequence of pairing, and its causes are still discussed.