Describe the difference between leading and lagging strand synthesis and the fragments formed.

• A. The leading strand is synthesized continuously toward the replication fork; the lagging strand is synthesized discontinuously as Okazaki fragments.
• B. The leading strand is synthesized in fragments; the lagging strand is continuous.
• C. Both strands synthesized continuously toward the fork.
• D. Only the lagging strand is synthesized; the leading strand is not.

Answer: (A)

The key idea is that DNA polymerase can only add nucleotides in the 5′ to 3′ direction, so the two template strands are copied in opposite ways as the fork opens. The leading strand is synthesized continuously toward the replication fork because its template runs 3′ to 5′ in that direction, allowing one polymerase to extend smoothly without interruption. The lagging strand, however, must be replicated away from the fork in short segments, because its template runs 5′ to 3′ toward the fork; these short pieces are called Okazaki fragments. Each fragment begins with an RNA primer laid down by primase; DNA polymerase then extends each fragment, primers are removed and replaced with DNA, and the fragments are linked together by DNA ligase to form a continuous strand. In short, the leading strand is a single uninterrupted synthesis toward the fork, while the lagging strand is discontinuous, made of Okazaki fragments that are later joined.