The leading strand is synthesized in which direction relative to the replication fork?

The leading strand is synthesized in which direction relative to the replication fork?

• A. 3' to 5' toward the fork
• B. 5' to 3' away from the fork
• C. 3' to 5' away from the fork
• D. 5' to 3' toward the fork

Answer: (D)

DNA polymerases can only add nucleotides to a growing strand at the 3' end, so synthesis proceeds in the 5' to 3' direction. At the replication fork, the two parental strands are antiparallel. The template for the leading strand runs 3' to 5' toward the fork, which allows continuous synthesis in the 5' to 3' direction toward the fork. That’s why the leading strand grows toward the fork. The lagging strand, in contrast, must be synthesized in short fragments away from the fork because its template runs 5' to 3' toward the fork, requiring discontinuous synthesis in the opposite direction.

DNA polymerases can only add nucleotides to a growing strand at the 3' end, so synthesis proceeds in the 5' to 3' direction. At the replication fork, the two parental strands are antiparallel. The template for the leading strand runs 3' to 5' toward the fork, which allows continuous synthesis in the 5' to 3' direction toward the fork. That’s why the leading strand grows toward the fork. The lagging strand, in contrast, must be synthesized in short fragments away from the fork because its template runs 5' to 3' toward the fork, requiring discontinuous synthesis in the opposite direction.