Which scientist formulated the rules that A=T and G=C in DNA?

Erwin Chargaff formulated the rules that adenine equals thymine and guanine equals cytosine in DNA.

In the late 1940s and early 1950s, the Austrian-born American biochemist analyzed DNA from many organisms using techniques including paper chromatography and ultraviolet spectrophotometry. He showed that DNA base composition varies between species, but that double-stranded DNA consistently follows A=T and G=C. These findings became known as Chargaff's rules.

The pattern helped overturn the older idea that DNA was made from simple repeating sets of four bases. It also provided an important clue for the double-helix model proposed by James Watson and Francis Crick in 1953. Complementary base pairing explains the equality: adenine pairs with thymine, while guanine pairs with cytosine. Rosalind Franklin's X-ray diffraction work contributed crucial structural evidence, but she did not formulate Chargaff's rules.