Who realized that A is always equal to T and C is always equal to G?

The realization that adenine (A) is always equal to thymine (T) and cytosine (C) is always equal to guanine (G) is attributed to the work of Erwin Chargaff. In the 1950s, Chargaff conducted experiments that led to the formulation of Chargaff's rules, which highlighted the base pairing in DNA. This foundational discovery contributed to the understanding of the double helical structure of DNA by James Watson and Francis Crick.