Why is the lagging strand synthesized discontinuously?

Why is the lagging strand synthesized discontinuously?

• A. Because polymerase can only synthesize in the 5' to 3' direction, requiring short fragments
• B. Because replication fork moves in two directions
• C. Because template is damaged
• D. Because primase attaches frequently

Answer: (B)

DNA polymerases can only synthesize DNA in the 5' to 3' direction, and the two strands are antiparallel. The leading strand template runs 3' to 5' toward the fork, so synthesis can proceed continuously toward the opening of the fork. The lagging strand template runs 5' to 3' toward the fork, meaning a single, continuous synthesis toward the fork would not be possible with 5' to 3' growth. To keep the correct direction of synthesis, the lagging strand is built in short chunks called Okazaki fragments, each initiated by an RNA primer laid down by primase. After the fragments are extended, the RNA primers are removed and filled in, and the fragments are joined by ligase. This fragmentary assembly on the lagging strand, not the fork’s bidirectional movement, explains why it’s synthesized discontinuously.

DNA polymerases can only synthesize DNA in the 5' to 3' direction, and the two strands are antiparallel. The leading strand template runs 3' to 5' toward the fork, so synthesis can proceed continuously toward the opening of the fork. The lagging strand template runs 5' to 3' toward the fork, meaning a single, continuous synthesis toward the fork would not be possible with 5' to 3' growth. To keep the correct direction of synthesis, the lagging strand is built in short chunks called Okazaki fragments, each initiated by an RNA primer laid down by primase. After the fragments are extended, the RNA primers are removed and filled in, and the fragments are joined by ligase. This fragmentary assembly on the lagging strand, not the fork’s bidirectional movement, explains why it’s synthesized discontinuously.