19. In E. coli, though DNA polymerase I (POI l) plays an essential role in the replication process, it is not the major polymerase. Instead, the enzyme responsible for advancement of replication fork is POI Ill. From the four structures (A, B, C and D) given below, students made several interpretations about the shorter arm

beingextended by POI I and/or POI III.

Which one of the interpretation written below is correct?
(1) A will be extended by POI Ill but not by Pol I.

(2) Neither B nor C will be extended by either POl I or Pol III.

(3) C will be extended by both Pol I and POl III.

(4) D will be extended only by POl I, but not by POI III.

 

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Introduction

In Escherichia coli, DNA replication involves multiple DNA polymerases with distinct roles. DNA Polymerase III (Pol III) is the primary enzyme responsible for rapid and processive DNA synthesis at the replication fork, while DNA Polymerase I (Pol I) mainly functions in processing Okazaki fragments by removing RNA primers and filling gaps. Understanding which polymerase extends particular DNA structures is essential for grasping bacterial DNA replication mechanics.

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Roles of DNA Polymerase III and DNA Polymerase I

• DNA Polymerase III:

  • The main replicative polymerase.

  • Synthesizes both leading and lagging strands with high speed and processivity.

  • Extends DNA strands from primers or existing DNA ends efficiently.

• DNA Polymerase I:

  • Has 5′→3′ exonuclease activity to remove RNA primers.

  • Fills in the gaps left after primer removal.

  • Generally extends short DNA segments during Okazaki fragment maturation but is not the primary polymerase for elongation at the replication fork.

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Interpretation of DNA Structures and Polymerase Extension

Given four DNA structures (A, B, C, and D) with a shorter arm to be extended, interpretations about which polymerase can extend them are:

1. Structure A:

   • Extended by Pol III but not by Pol I.

   • Pol III can extend from a primer or DNA end efficiently. Pol I requires a nick or gap with a 5′ end suitable for its exonuclease activity.

2. Structures B and C:

   • Some DNA structures may lack suitable 3′ ends or primer-template junctions for extension by either polymerase.

   • However, structure C can be extended by both Pol I and Pol III if it presents a proper primer-template junction.

3. Structure D:

   • Extended only by Pol I, not by Pol III.

   • This typically applies to nicked DNA or gaps where Pol I’s exonuclease and polymerase activities act to remove primers and fill gaps.

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Correct Interpretation

• (1) A will be extended by Pol III but not by Pol I.
  This matches the known activities where Pol III is the main replicative polymerase extending primers, and Pol I is more specialized.

• (2) Neither B nor C will be extended by either Pol I or Pol III.
  This is incorrect since C can be extended by both.

• (3) C will be extended by both Pol I and Pol III.
  This is correct if structure C has a suitable primer-template junction.

• (4) D will be extended only by Pol I, but not by Pol III.
  This is correct for nicked or gapped DNA typical for Pol I action.

Among these, the best supported interpretation from molecular biology data is:

(1) A will be extended by Pol III but not by Pol I.

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Summary Table

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Conclusion

In E. coli DNA replication, DNA Polymerase III is the primary enzyme that extends DNA strands at the replication fork, including primer extension on the leading and lagging strands. DNA Polymerase I specializes in processing Okazaki fragments by removing RNA primers and filling the gaps. Therefore, structure A is extended by Pol III but not by Pol I, making option (1) the correct interpretation.