History of Phosphorus

by Eduard Farber

NH₂ /
C=O
\
NH₂

is involved, and they form pairs of oxidized and reduced states:

Purine	Pyrimidine
(reduced) Adenine	+	(oxidized) Thymine
(oxidized) Guanine	+	(reduced) Cytosine

Pyrimidine

Purine

Adenine