Why are the two strands of DNA antiparallel?

• A. Both strands run 5' to 3' in the same direction.
• B. The strands are parallel and identical in direction.
• C. One strand runs 5' to 3' in the opposite direction to the other strand, enabling complementary base pairing.
• D. The strands run 3' to 5' on both sides.

Answer: (C)

The arrangement of DNA strands is antiparallel, meaning one strand runs 5' to 3' while the other runs 3' to 5'. This orientation places complementary bases across from each other so A pairs with T and G pairs with C, held together by hydrogen bonds. It also fits with how DNA polymerase works: it can add nucleotides only to a free 3' end, so it reads the template strand in the 3' to 5' direction and builds the new strand in the 5' to 3' direction. The opposite directions of the two strands let replication proceed smoothly on both strands and maintain the stable double-helix geometry. If both strands ran in the same direction, proper base pairing and efficient synthesis would be disrupted.