What are the positions of ' N ' atoms present in purine ring of nucleic acids?

The purine ring system consists of a pyrimidine ring fused to an imidazole ring with standardized nitrogen positions at 1, 3, 7, and 9.

The pyrimidine ring contributes nitrogen atoms at positions 1 and 3; the imidazole ring contributes nitrogen atoms at positions 7 and 9. Position 5 is occupied by a carbon atom.

Comparing with the options:

A) 1,3 and 5 – incorrect due to carbon at position 5

B) 1,3 and 9 – omits position 7

C) 1,5,7 and 9 – includes carbon position 5

D) 1,3,7 and 9 – matches all nitrogen positions correctly

The correct nitrogen positions correspond to option D.