Questions: The leading and the lagging strands differ in that the leading strand is synthesized at twice the rate of the lagging strand. the lagging strand is synthesized continuously, whereas the leading strand is synthesized in short fragments that are ultimately stitched together. the leading strand is synthesized by adding nucleotides to the 3' end of the growing strand, and the lagging strand is synthesized by adding nucleotides to the 5' end. the leading strand is synthesized in the same direction as the movement of the replication fork, and the lagging strand is synthesized in the opposite direction.

The answer is the last one: the leading strand is synthesized in the same direction as the movement of the replication fork, and the lagging strand is synthesized in the opposite direction.

Explanation for each option:

1. The leading strand is synthesized at twice the rate of the lagging strand.

   • This statement is incorrect. Both strands are synthesized at the same overall rate, but the mechanisms differ. The leading strand is synthesized continuously, while the lagging strand is synthesized in fragments.

2. The lagging strand is synthesized continuously, whereas the leading strand is synthesized in short fragments that are ultimately stitched together.

   • This statement is incorrect. It is the leading strand that is synthesized continuously, and the lagging strand is synthesized in short fragments called Okazaki fragments, which are later joined together.

3. The leading strand is synthesized by adding nucleotides to the $3^{\prime}$ end of the growing strand, and the lagging strand is synthesized by adding nucleotides to the $5^{\prime}$ end.

   • This statement is incorrect. Both the leading and lagging strands are synthesized by adding nucleotides to the $3^{\prime}$ end. DNA polymerase can only add nucleotides to the $3^{\prime}$ end of a growing DNA strand.

4. The leading strand is synthesized in the same direction as the movement of the replication fork, and the lagging strand is synthesized in the opposite direction.

   • This statement is correct. The leading strand is synthesized continuously in the direction of the replication fork movement, while the lagging strand is synthesized in the opposite direction in short segments.

In summary, the correct distinction between the leading and lagging strands is that the leading strand is synthesized continuously in the same direction as the replication fork, while the lagging strand is synthesized discontinuously in the opposite direction.