What base pairing pattern is observed in the DNA double helix?

• A. A pairs with T, and C pairs with G
• B. A pairs with G, and C pairs with T
• C. G pairs with A, and C pairs with T
• D. A pairs with U, and C pairs with G

Answer: (A)

Base pairing in the DNA double helix follows a precise complementary rule: adenine pairs with thymine and cytosine pairs with guanine. This keeps the two strands uniform in width because a purine is always paired with a pyrimidine. Adenine and thymine form two hydrogen bonds, while guanine and cytosine form three, which contributes to DNA’s stability. In RNA, thymine is replaced by uracil, so adenine pairs with uracil there, not in DNA. Pairings like A with G or C with T would disrupt the helix geometry and hydrogen-bonding pattern, so they’re not observed in DNA.