The pyrimidines have a basic six membered ring of benzene in which, pyrimidines-cytosine and thymine. Structurally cytosine has a single six membered ring with a hydroxyl group at the 2′ position and an amino group at the 6′ position (2 hydorxy, 6 amino pyrimidine).
Thymine also has a single six membered ring with hydroxyl groups at the 2′ and 6′ position and a methyl group attached to the 5′ position (2, 6 hydroxy, 5 methyl pyrimidine).
The purines have a double ringed structure. Attached to the six membered benzene right is a 5 membered imidazole ring. The imidazole ring is joined to the hexagonal ring at 4′ and 5′ positions.
The benzene ring has nitrogen in the 1′ and 3′ position (replacing Carbon). There are two types of purines viz Adenine and Guanine. Adenine has an amino group at the 6′ position (6 aminopurine), while guanine has an amino group at the 2′ position and a hydroxyl group at the 6′ position (2 amino, 6 hydroxy purine).