Distinguish leading and lagging strand synthesis.

Distinguish leading and lagging strand synthesis.

• A. Leading strand synthesized discontinuously away from replication fork; lagging synthesized continuously toward fork.
• B. Leading strand synthesized continuously toward replication fork; lagging synthesized discontinuously in Okazaki fragments away from fork.
• C. Both synthesized continuously toward fork.
• D. Both are synthesized discontinuously.

Answer: (B)

DNA polymerases can only add nucleotides in the 5' to 3' direction. As the replication fork opens, one template runs 3' to 5' toward the fork, so the new strand can be built continuously in the same direction as the fork movement—the leading strand. The opposite template runs 5' to 3' toward the fork, so synthesis must occur in short, separate segments away from the fork (Okazaki fragments). These fragments are later joined by ligase. This directional setup explains why the leading strand is synthesized continuously toward the fork while the lagging strand is synthesized discontinuously away from it.

DNA polymerases can only add nucleotides in the 5' to 3' direction. As the replication fork opens, one template runs 3' to 5' toward the fork, so the new strand can be built continuously in the same direction as the fork movement—the leading strand. The opposite template runs 5' to 3' toward the fork, so synthesis must occur in short, separate segments away from the fork (Okazaki fragments). These fragments are later joined by ligase. This directional setup explains why the leading strand is synthesized continuously toward the fork while the lagging strand is synthesized discontinuously away from it.