52. In prokaryotes during replication, the lagging strand is synthesized in a series of short fragments known as Okazaki fragments, consequently requiring many primers. The RNA primers of Okazaki fragments are subsequently degraded by DNA polymerase I and the gap are filled. How DNA polymerase I fills the gap once the primer have been removed from lagging stand?

(1) DNA polymerase I has its own primer

(2) DNA polymerase I do not require primer

(3) DNA from leading stand serves as primer

(4) 3′-ends of existing Okazaki fragments on lagging stand serves as primer

 

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Introduction

During DNA replication in prokaryotes such as Escherichia coli, the lagging strand is synthesized discontinuously as short segments called Okazaki fragments. Each fragment begins with a short RNA primer synthesized by primase. After DNA synthesis, these RNA primers must be removed and replaced with DNA to create a continuous strand. This critical task is performed by DNA polymerase I, which has both exonuclease and polymerase activities. Understanding how DNA polymerase I fills the gap after RNA primer removal is essential to grasp the mechanism of lagging strand maturation.

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The Process of RNA Primer Removal and Gap Filling

1. RNA Primer Removal:

   • DNA polymerase I uses its 5′→3′ exonuclease activity to remove the RNA primers from Okazaki fragments.

   • This activity degrades the RNA primer nucleotides one by one from the 5′ end.

2. Gap Filling:

   • As the RNA primer is removed, DNA polymerase I simultaneously uses its 5′→3′ polymerase activity to synthesize DNA nucleotides, filling the gap left behind.

   • This synthesis requires a primer with a free 3′-OH group to initiate DNA polymerization.

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What Serves as the Primer for DNA Polymerase I?

• The 3′-ends of the existing Okazaki fragments on the lagging strand provide the necessary free 3′-OH group.

• DNA polymerase I extends from this 3′-OH end, replacing the RNA primer with DNA nucleotides.

• This coordinated action ensures that the lagging strand is synthesized continuously after primer removal.

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Why Other Options Are Incorrect

• (1) DNA polymerase I has its own primer:

  • Incorrect. DNA polymerase I requires a pre-existing primer with a free 3′-OH group; it cannot initiate DNA synthesis de novo.

• (2) DNA polymerase I does not require a primer:

  • Incorrect. Like all DNA polymerases, it requires a primer to extend from.

• (3) DNA from leading strand serves as primer:

  • Incorrect. The leading strand DNA does not serve as a primer for lagging strand synthesis.

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Summary Table

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Final Answer

(4) 3′-ends of existing Okazaki fragments on lagging strand serves as primer

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Keywords

DNA polymerase I, Okazaki fragments, RNA primer removal, lagging strand synthesis, 3′-OH primer, 5′→3′ exonuclease, gap filling, prokaryotic DNA replication, DNA polymerase primer requirement

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Conclusion

In prokaryotic DNA replication, after RNA primers of Okazaki fragments are removed by DNA polymerase I’s 5′→3′ exonuclease activity, the enzyme fills the resulting gap by extending from the 3′-ends of the adjacent Okazaki fragments. This ensures the lagging strand is synthesized into a continuous DNA strand, ready for final ligation by DNA ligase. This elegant mechanism highlights the coordination between primer removal and DNA synthesis during lagging strand replication.