Which bases pair together in DNA, and why?

• A. Adenine pairs with Cytosine due to charged interactions
• B. A pairs with T and C pairs with G due to hydrogen bonding.
• C. G pairs with T due to complementary shape
• D. A pairs with G and C pairs with T due to hydrogen bonding

Answer: (B)

Base pairing in DNA is determined by hydrogen bonding between complementary bases, which keeps the two strands aligned with a uniform width. Adenine forms two hydrogen bonds with thymine, while cytosine forms three hydrogen bonds with guanine. This specific pairing explains why A pairs with T and C pairs with G. The mechanism relies on hydrogen bonds and the complementary shapes of the bases, not just charged interactions or simple shape alone; other pairings would mismatch the hydrogen-bonding patterns and distort the helix, making the structure unstable.