Which of the following limitations of DNA polymerase is overcome in the process of lagging strand synthesis?

Question

BlackTom AI · Accepted Answer

DNA polymerase synthesises DNA only in the 5'→3' direction, so the lagging strand is replicated as discontinuous Okazaki fragments to accommodate that constraint.

1) Relevant knowledge points and concepts
- DNA polymerases add nucleotides to the 3' end of a growing strand, so polymerisation proceeds 5'→3'.
- The replication fork exposes two template orientations: the leading template is copied continuously in the fork direction; the lagging template has the opposite orientation and cannot be copied continuously in the fork direction by an enzyme that only works 5'→3'.
- Cells solve this by making short RNA primers (by primase), synthesising short DNA fragments (Okazaki fragments) in 5'→3' direction, removing primers, and joining fragments with DNA ligase.

2) Step-by-step reasoning
- At the replication fork one template runs 3'→5' in the fork direction (allowing continuous synthesis 5'→3' on the new strand = leading strand).
- The other template runs 5'→3' in the fork direction. Because DNA polymerase cannot synthesise 3'→5', it cannot produce a continuous complementary strand in the fork direction.
- The cell therefore synthesises multiple short segments: primase lays down RNA primers at successive points on the lagging template; DNA polymerase extends each primer 5'→3' to make Okazaki fragments moving away from the fork.
- Afterward polymerase(s) remove RNA primers, replace them with DNA, and ligase joins fragments to produce a continuous strand.

3) Intermediate steps shown
- Primase places RNA primer → DNA polymerase III (in bacteria) extends primer 5'→3' → forms Okazaki fragment → RNA primer removed and replaced by polymerase I → DNA ligase seals nick.

4) Why the correct answer is correct
- The discontinuous Okazaki fragment strategy directly addresses the directional limitation: although polymerase still works 5'→3', fragments let the whole lagging strand be replicated despite its opposite template orientation.

5) Why other options are incorrect
- a) Speed limited by nucleotide concentration: not specifically addressed by lagging-strand synthesis.
- b) Polymerase cannot initiate de novo, but that limitation is addressed by primase; the key challenge solved by the fragment strategy is directionality, not initiation (and primase-actuated initiation applies to both strands).
- d) Adding complementary bases to a template is the normal function of polymerase, not a limitation being overcome.

6) Conclusion
This explanation shows lagging strand synthesis overcomes the 5'→3' directionality constraint of DNA polymerase; therefore option c is correct, matching the provided answer.

Which of the following limitations of DNA polymerase is overcome in the process of lagging strand synthesis?单项选择题

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