How does the antiparallel orientation of DNA strands affect replication?

• A. The two strands run 5' to 3' and 3' to 5', allowing polymerase to synthesize both directions.
• B. DNA polymerase can synthesize only 3' to 5' direction, requiring Okazaki fragments.
• C. The leading strand is synthesized continuously in the 5' to 3' direction, while the lagging strand is synthesized discontinuously as short fragments.
• D. Replication occurs in a single direction along the double helix.

Answer: (C)

DNA strands are antiparallel, meaning they run in opposite directions. DNA polymerase can only add nucleotides to a free 3' end, so new DNA grows in the 5' to 3' direction. At a replication fork, this creates two scenarios. The template that exposes a 3' end toward the fork allows continuous synthesis toward the fork, producing the leading strand in one smooth process. The opposite template runs 5' toward the fork, so it cannot be copied continuously in that direction; instead, it’s copied in short segments away from the fork. These short segments, called Okazaki fragments, are later joined to form the complete lagging strand. This combination—continuous synthesis on the leading strand and discontinuous synthesis on the lagging strand due to the 5' to 3' activity of polymerase on antiparallel templates—captures why replication is organized this way.