Which statement about priming on the lagging strand is correct?

Which statement about priming on the lagging strand is correct?

• A. The lagging strand uses continuous synthesis without primers
• B. RNA primers are never needed for replication
• C. An RNA primer is required only for the leading strand
• D. Each Okazaki fragment must be primed separately

Answer: (D)

Priming is needed to start DNA synthesis because DNA polymerase can only extend an existing 3' end; it cannot start from nothing. On the lagging strand, replication proceeds discontinuously in short chunks called Okazaki fragments, each oriented 5' to 3' away from the fork. For each fragment to begin, an RNA primer is laid down by primase, providing the necessary 3' end for DNA polymerase to extend. Therefore, every Okazaki fragment must be primed separately. After extension, the RNA primers are removed and replaced with DNA, and DNA ligase seals the gaps to make the strand continuous. This contrasts with the leading strand, which is synthesized continuously in the same direction as the fork from a single primer, but the key point for the lagging strand is that each fragment requires its own primer to initiate synthesis.

Priming is needed to start DNA synthesis because DNA polymerase can only extend an existing 3' end; it cannot start from nothing. On the lagging strand, replication proceeds discontinuously in short chunks called Okazaki fragments, each oriented 5' to 3' away from the fork. For each fragment to begin, an RNA primer is laid down by primase, providing the necessary 3' end for DNA polymerase to extend. Therefore, every Okazaki fragment must be primed separately. After extension, the RNA primers are removed and replaced with DNA, and DNA ligase seals the gaps to make the strand continuous. This contrasts with the leading strand, which is synthesized continuously in the same direction as the fork from a single primer, but the key point for the lagging strand is that each fragment requires its own primer to initiate synthesis.