18. Which one of the tollowtng statements made about the is INCORRECT
    (1) The rate of forward movement otDnaB helicase along the template DNA increases 10-fold when DnaBand DNA POI III interact, thus ensuring that the helicase does not move ahead rapidly without the polymerase.
    (2) The transient interaction of the primase with the helicase allows activation of primase activity by 1000- fold, promoting RNA primer synthesis.
    (3) The length of the Okazaki fragments is typically restricted to 1000-2000 nucleotides.

(4) The E. coli ori C carries repeats of two sequence motifs: repeats of a 9-mer that collectively form the site at which the origin first becomes single-stranded, and repeats of a 13-mer to which the DnaA initiator protein

binds.

Introduction

Bacterial DNA replication is a highly coordinated process involving multiple proteins and DNA sequence elements. DNAB helicase unwinds the DNA, primase synthesizes RNA primers, Okazaki fragments form during lagging strand synthesis, and the origin of replication (oriC) contains specific sequence motifs critical for initiation. Understanding the interactions and functions of these components is essential for grasping bacterial replication mechanisms.

Statement Analysis

1. Statement (1): The rate of forward movement of DNAB helicase along the template DNA increases 10-fold when DNAB and DNA Pol III interact, thus ensuring that the helicase does not move ahead rapidly without the polymerase.

   • This is true. The interaction between DNAB helicase and DNA polymerase III holoenzyme coordinates helicase unwinding with DNA synthesis, preventing helicase from moving too far ahead of the polymerase.

2. Statement (2): The transient interaction of the primase with the helicase allows activation of primase activity by 1000-fold, promoting RNA primer synthesis.

   • This is true. Primase activity is greatly stimulated by its interaction with DNA helicase, enabling efficient primer synthesis at the replication fork.

3. Statement (3): The length of the Okazaki fragments is typically restricted to 1000-2000 nucleotides.

   • This is true for E. coli. Okazaki fragments in bacteria generally range from 1000 to 2000 nucleotides, reflecting the coordination between primase activity and lagging strand synthesis.

4. Statement (4): The E. coli oriC carries repeats of two sequence motifs: repeats of a 9-mer that collectively form the site at which the origin first becomes single-stranded, and repeats of a 13-mer to which the DnaA initiator protein binds.

   • This is incorrect.

   • In reality, DnaA binds to 9-mer repeats (DnaA boxes) within oriC, while the 13-mer repeats are AT-rich sequences that become unwound (duplex unwinding element, DUE) during initiation. The 13-mer repeats are not the binding sites for DnaA but are the region that melts to allow replication to begin.

Correct Identification of the Incorrect Statement

(4) is incorrect because it reverses the roles of the 9-mer and 13-mer repeats in oriC:

• 9-mer repeats (DnaA boxes): Binding sites for DnaA initiator protein.

• 13-mer repeats: AT-rich region that unwinds to form single-stranded DNA, not DnaA binding sites.

Summary Table

Conclusion

Among the statements about bacterial DNA replication, statement (4) is incorrect. The E. coli oriC contains 9-mer repeats (DnaA boxes) that bind the DnaA initiator protein and 13-mer AT-rich repeats that form the unwound single-stranded region, but the 13-mer repeats are not DnaA binding sites. The other statements about DNAB helicase activity, primase activation, and Okazaki fragment length are accurate and reflect current understanding of bacterial DNA replication.