Which description correctly defines the leading strand during DNA replication?

• A. The strand synthesized discontinuously away from the replication fork
• B. The strand synthesized continuously toward the replication fork
• C. The strand left unpaired during replication
• D. The RNA primer strand

Answer: (B)

Synthesis of the leading strand occurs continuously toward the replication fork. DNA polymerase can only add nucleotides to a free 3' end, so the template for this strand must run 3' to 5' as the fork opens. As helicase unwinds the DNA, the strand with that orientation is copied in one uninterrupted run, keeping pace with the advancing fork. The other strand runs in the opposite orientation and is made discontinuously as short segments (Okazaki fragments) away from the fork, later joined together. RNA primers start the process, but the final leading-strand DNA is the continuous DNA, not the RNA primer itself.