Purines and pyrimidines differ in structure. Which statement is true?

• A. Purines are single-ring; pyrimidines are double-ring
• B. All bases pair in non-standard fashion
• C. Purines (A and G) have two rings; pyrimidines (C and T/U) have one ring; A pairs with T (or U in RNA) and G pairs with C
• D. RNA uses purine pairs with pyrimidines only

Answer: (C)

Purines and pyrimidines differ in how many rings their structures have, which helps keep the DNA/RNA backbone uniform in width. Purines (adenine and guanine) have two fused rings, while pyrimidines (cytosine and thymine in DNA, cytosine and uracil in RNA) have a single ring. This structural difference is matched by the base-pairing rules: adenine pairs with thymine in DNA (and with uracil in RNA), and guanine pairs with cytosine. This pairing pattern is what keeps the double helix stable and accurately encoded.

So the statement that correctly combines the ring structures with the standard pairing (A with T or U, and G with C) is true. The other options either reverse the ring counts, claim all bases pair in non-standard fashion, or overstate RNA pairing by implying only purine-pyrimidine pairing without acknowledging standard or occasional non-Watson-Crick interactions.