Which DNA strand is synthesized discontinuously due to the reading direction limitation?

• A. Lagging strand
• B. Leading strand
• C. Template strand
• D. Coding strand

Answer: (A)

DNA polymerase can only synthesize DNA in the 5' to 3' direction, so the two antiparallel strands are copied differently as the replication fork opens. The strand whose template runs 3' to 5' toward the fork is copied continuously toward the fork—that’s the leading strand. The opposite template runs 5' to 3' toward the fork, so synthesis must proceed away from the fork in short segments (Okazaki fragments) each initiated by an RNA primer and then joined. This makes the lagging strand the one synthesized discontinuously due to the reading direction limitation. The other terms refer to different roles in transcription or to the continuously synthesized strand, not to the replication direction issue.