Section of the Watson–Crick DNA structure showing hydrogen bonds connecting the pyrimidine bases thymine (T) and cytosine (C) with, respectively, the purine bases adenine (A) and guanine (G).
The precise hydrogen bond link between the bases made by Watson and Crick followed from the Chargraff base-pair rules and from J. O'Donohue's identification of the correct (keto) tautomeric form. Adapted with permission from Booth and Hey [6]. Copyright (1996) American Chemical Society.