Why does antiparallel orientation of the two DNA strands matter for replication?

• A. The antiparallel orientation allows both strands to be synthesized in the same direction.
• B. The antiparallel orientation prevents replication from occurring.
• C. One strand runs 5'→3' and the other 3'→5'; this polarity means DNA polymerases synthesize new strands 5'→3', producing continuous leading-strand synthesis and discontinuous lagging-strand synthesis.
• D. The antiparallel orientation has no impact on replication.

Answer: (C)

The key idea is the direction DNA is built and the orientation of the two strands. DNA polymerases can add nucleotides only to a free 3' end, so they synthesize new DNA in the 5' to 3' direction. The two strands in a DNA double helix run in opposite directions (antiparallel). As a result, one template is read in the 3' to 5' direction toward the replication fork, allowing the new strand to be made continuously in the same direction as the fork movement—that’s the leading strand. The other template runs 5' to 3' toward the fork, so it cannot be copied in one continuous piece in the same direction. Instead, synthesis occurs as short segments in the opposite direction (Okazaki fragments) on the lagging strand, which are later joined together. This setup, dictated by antiparallel orientation and the 5' to 3' polymerase activity, explains why replication involves both continuous leading-strand synthesis and discontinuous lagging-strand synthesis.