Chargaff's rules
/ SHAR-gaff /
Before anyone knew DNA was a double helix, a chemist noticed something oddly tidy when he measured the bases in DNA from many different organisms. No matter the species, the amount of A always matched the amount of T, and the amount of G always matched the amount of C. This curious bookkeeping is Chargaff's rules.
Erwin Chargaff's first rule states that in double-stranded DNA the amount of adenine equals the amount of thymine (A equals T) and the amount of guanine equals the amount of cytosine (G equals C); equivalently, total purines equal total pyrimidines. A second observation is that the overall base composition — how GC-rich the DNA is — varies a lot from one species to another, so the ratios are not the same fifty-fifty mix everywhere. Chargaff measured these patterns in the late 1940s without knowing why they held.
The rules mattered enormously because they were a clue, not yet an explanation. The fact that A equals T and G equals C makes perfect sense once you know that A pairs with T and G pairs with C: every A on one strand demands a T on the other. Watson and Crick used Chargaff's numbers as a vital constraint while building their model, and the base-pairing rule turned his mysterious equalities into an obvious consequence of structure.
If you measure a sample of double-stranded DNA and find it is 30 percent adenine, Chargaff's rule lets you predict the rest immediately: 30 percent thymine, and the remaining 40 percent split evenly into 20 percent guanine and 20 percent cytosine.
A = T and G = C — a clue that pointed straight at base pairing.
Chargaff's A=T, G=C rule holds for double-stranded DNA, where every base has a partner. It need not hold for single-stranded molecules like most RNA, which have no complementary partner strand forcing the counts to match.