Purines can only H-bond to _____

• A. carbons
• B. amino groups
• C. pyrimidines
• D. sugars

Answer: (C)

Purines pair with pyrimidines in standard base pairing because the two-ring purine fits with the single-ring pyrimidine to keep the DNA/RNA helix width consistent. This pairing uses specific hydrogen-bonding patterns—adenine forms two hydrogen bonds with thymine (or uracil in RNA), and guanine forms three with cytosine. The partner must be a pyrimidine to provide the right arrangement of hydrogen-bond donors and acceptors and the correct geometry. Carbons or sugars don’t participate as hydrogen-bonding partners in base pairing, and while purines have an exocyclic amino group, that isn’t the complementary partner in canonical pairing.