On the hydrogen bonding scheme between purines and pyrimidines.
D000225:Adenine; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003596:Cytosine; D006147:Guanine; D013941:Thymine
D006147:Guanine; D003596:Cytosine; D000225:Adenine; D013941:Thymine