These are adenine and guanine.
Purines adenine and guanine are double ringed structures that make up DNA. Purines have a characteristic double-ring structure composed of carbon, nitrogen and hydrogen atoms.
Each of purine's rings contain two nitrogen atoms making up a total of four within the double ringed structure. The remaining five positions within the rings are occupied by carbon atoms. The purine ring is encircled by hydrogen atoms which can be replaced by other atoms or groups of atoms to form different purines.
