A chemist counted DNA's letters and handed Watson and Crick the key

In the late 1940s, Erwin Chargaff measured the four bases in DNA from many species and found a strange constant: the amount of adenine always matched thymine, and guanine always matched cytosine. He didn't grasp why — but that pairing rule became the missing clue Watson and Crick used to work out DNA's double-helix structure in 1953.

— Erwin Chargaff, Chargaff's rules — Experimental Cell Research, 1950