Which statement correctly describes base pairing in DNA?

Which statement correctly describes base pairing in DNA?

• A. Purines pair with purines.
• B. Purines pair with pyrimidines.
• C. Pyrimidines pair with pyrimidines.
• D. There is no specificity in base pairing.

Answer: (B)

Base pairing in DNA follows a specific rule to keep the double helix uniform in width. Purines (A and G) pair with pyrimidines (T and C), so the pairing always bridges a two-ring structure with a single-ring structure. This purine–pyrimidine pairing creates the right distance between the two strands and is stabilized by hydrogen bonds (A with T via two bonds, G with C via three). That’s why the correct description is that purines pair with pyrimidines. The alternatives would disrupt the consistent width or lack any specific pairing, which isn’t how DNA is organized.

Base pairing in DNA follows a specific rule to keep the double helix uniform in width. Purines (A and G) pair with pyrimidines (T and C), so the pairing always bridges a two-ring structure with a single-ring structure. This purine–pyrimidine pairing creates the right distance between the two strands and is stabilized by hydrogen bonds (A with T via two bonds, G with C via three). That’s why the correct description is that purines pair with pyrimidines. The alternatives would disrupt the consistent width or lack any specific pairing, which isn’t how DNA is organized.