Chargaff’s Rules and DNA Pairing

Chargaff’s rules and DNA base pairing explain how DNA strands match up. In double-stranded DNA, adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). Chargaff also found that A equals T, G equals C, and the total amount of purines (A and G) equals the total amount of pyrimidines (C and T).

DNA resembles a twisted ladder. And the rungs of the DNA ladder are complementary pairs of nitrogenous bases. According to base pairing rules, adenine, a purine, pairs with thymine, a pyrimidine, with two hydrogen bonds. And guanine, a purine, appears with cytosine, a pyrimidine, with three hydrogen bonds. But why do purines always pair with pyr...