The leading and the lagging strands differ in that A. replication of the lagging strand begins after replication of the leading strand is completed. B. 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. C. the Okazaki fragments on the lagging strand are much shorter than the Okazaki fragments on the leading strand D. 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 correct answer is: B. 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 replication of DNA molecule or DNA synthesis is done by an enzyme - DNA polymerase that can work only in the 5' to 3' direction.

After helicase unzips the double-stranded DNA for replication, a replication fork is formed. The leading strand is synthesized in the same direction as the movement of the replication fork so it can occur continuously: the primase generates short strands of RNA-primers that bind to the single-stranded DNA to initiate DNA synthesis. On the other hand, the lagging strand is synthesized in the opposite direction comparing to the movement of replication fork and its replication is discontinuous, with short Okazaki fragments being formed and later linked together.