Which statement about purines and pyrimidines is true?

• A. Purines bind to purines via hydrogen bonds.
• B. Purines pair with pyrimidines through hydrogen bonding.
• C. Pyrimidines are larger than purines.
• D. Both are identical in structure.

Answer: (B)

Purines pair with pyrimidines through hydrogen bonding, which keeps the DNA double helix width uniform and allows faithful replication. Purines (adenine and guanine) are two-ring structures, while pyrimidines (cytosine and thymine in DNA; uracil in RNA) are single-ring. In DNA, A pairs with T (or U in RNA) via two hydrogen bonds, and G pairs with C via three hydrogen bonds. This complementary pairing prevents purine–purine or pyrimidine–pyrimidine matches, which would distort the helix, and it explains why the statement about purines pairing with pyrimidines through hydrogen bonding is the true one.