Which statement correctly describes the direction of synthesis for the leading and lagging strands during replication?

• A. Leading strand is synthesized discontinuously away from the fork; lagging strand is continuous toward the fork.
• B. Leading strand is synthesized continuously toward the fork; lagging strand synthesized discontinuously away from the fork.
• C. Both strands are synthesized in the same direction and continuously.
• D. Only lagging strand is synthesized; leading strand is not.

Answer: (B)

The main idea is that DNA polymerase adds nucleotides only in the 5' to 3' direction, and the two strands in the DNA duplex are antiparallel. As the replication fork opens, the template for the leading strand runs 3' to 5' toward the fork, so synthesis can proceed continuously in the same direction as the fork movement—toward the fork. The lagging strand template runs 5' to 3' toward the fork, which means synthesis must occur in short segments that are produced away from the fork, each starting with an RNA primer and extended 5' to 3' until the next primer is laid down. Those short fragments (Okazaki fragments) are later joined to form a continuous lagging strand.

So the correct description is that the leading strand is synthesized continuously toward the fork, while the lagging strand is synthesized discontinuously away from the fork. The other options contradict this arrangement by swapping directions, calling the lagging strand continuous, or claiming the leading strand is not synthesized at all.