Which statement describes replication fork and leading/lagging strand synthesis?

• A. The replication fork is where transcription occurs.
• B. The replication fork is where DNA is unwound; the leading strand is synthesized continuously toward the fork; the lagging strand is synthesized in short Okazaki fragments away from the fork.
• C. Both strands are synthesized continuously toward the fork.
• D. The leading strand is synthesized away from the fork.

Answer: (B)

This question tests how replication fork dynamics and strand synthesis work during DNA replication. The replication fork is the location where the double helix is unwound by helicase, exposing templates for the new strands. The leading strand is synthesized continuously toward the fork because DNA polymerase can add nucleotides only in the 5' to 3' direction, and the template runs 3' to 5' toward the fork, allowing uninterrupted synthesis. The lagging strand, on the other hand, has a template that runs 5' to 3' toward the fork, so synthesis must occur in short pieces away from the fork. These fragments, called Okazaki fragments, are initiated with RNA primers laid down by primase, extended by DNA polymerase, and later joined together by DNA ligase. This arrangement explains why one strand is made in a single, continuous fashion toward the fork while the other is synthesized discontinuously in fragments away from the fork.