Purines are characterized by containing two fused rings.

• A. They contain a single ring
• B. They contain two fused rings
• C. They contain three rings
• D. They are not nitrogenous bases

Answer: (B)

Purines have a bicyclic ring system: a six-membered ring fused to a five-membered ring. This two-ring, fused structure is the defining feature of purines (adenine and guanine) and explains why they are larger than the single-ring pyrimidines (cytosine, thymine, and uracil) with which they pair in nucleic acids. The fused rings provide the nitrogen-rich surface needed for hydrogen bonding patterns that help maintain the consistent width of the DNA double helix. So describing purines as containing two fused rings is accurate. A single ring would describe pyrimidines, three rings don’t occur for standard nucleobases, and purines are indeed nitrogenous bases.