What is the leading strand?

• A. The leading strand is the new strand built toward the replication fork
• B. The strand built away from the replication fork in short fragments
• C. The parental strand used as a template for duplication
• D. The enzyme that seals nicks

Answer: (A)

The leading strand is the new DNA strand synthesized continuously in the 5' to 3' direction toward the replication fork. Because DNA polymerase can only add nucleotides to a 3' end, it can keep extending the strand as the fork progresses, producing a smooth, uninterrupted strand on the template that runs 3' to 5' toward the fork. The other template, facing the fork in the opposite orientation, is copied discontinuously in short segments away from the fork, which are later joined. This distinction—continuous synthesis toward the fork for the leading strand versus discontinuous synthesis away from the fork for the lagging strand—explains why one strand is built in one piece and the other in fragments.