Which strand is synthesized as Okazaki fragments during lagging-strand synthesis?

• A. The leading strand contains Okazaki fragments.
• B. The lagging strand is synthesized continuously as a single strand.
• C. Okazaki fragments are never formed during replication.
• D. The lagging strand is synthesized discontinuously as Okazaki fragments.

Answer: (D)

During DNA replication, polymerase can only add nucleotides in the 5' to 3' direction. The two strands are antiparallel, so the fork lets one strand be copied continuously toward the fork (the leading strand). The other strand runs 5' to 3' away from the fork, so synthesis must occur in short, separate pieces—Okazaki fragments—that are later stitched together. RNA primers mark the start of each fragment, primers are removed and replaced with DNA, and ligase seals the gaps. This is why the lagging strand is synthesized discontinuously as Okazaki fragments, while the leading strand is synthesized continuously.