Between purines and pyrimidines, which are larger and how do they pair in DNA?

• A. Purines are larger (adenine, guanine) and pair with pyrimidines (thymine, cytosine)—A with T, G with C.
• B. Purines are smaller and pair with purines.
• C. Pyrimidines are larger and pair with purines.
• D. Pyrimidines pair with purines and purines pair with pyrimidines.

Answer: (A)

Purines are larger because they have two-ring structures, while pyrimidines have a single ring. To keep the DNA double helix uniform in width, each purine pairs with a pyrimidine. In DNA, adenine pairs with thymine and guanine pairs with cytosine, with two hydrogen bonds in A–T and three in G–C, providing stability to the molecule.