3. In bacteria chromosomal DNA replication stops at
(1) one specific locus.
(2) several specific loci.
(3) a single locus, randomly.
(4) from several loci, randomly.

Introduction

In bacteria, DNA replication is a highly regulated process that ensures the entire genome is duplicated accurately before cell division. While the initiation of replication occurs at a single origin (oriC), the termination of replication is a more complex event. Rather than stopping at a single specific locus, bacterial chromosomal DNA replication typically terminates at several specific loci within a defined terminus region. This article explores the molecular basis of replication termination in bacteria, the role of ter sites, and the replication fork trap mechanism.

Bacterial DNA Replication Overview

• DNA replication initiates at the origin of replication (oriC).

• Two replication forks proceed bidirectionally around the circular chromosome.

• These forks eventually converge and terminate replication.

Termination at Several Specific Loci: The Terminus Region

• The terminus region of bacterial chromosomes contains multiple termination (ter) sites.

• These ter sites are specific DNA sequences that serve as binding sites for terminator proteins (e.g., Tus in E. coli).

• The ter sites are arranged to form a replication fork trap, which ensures replication forks stop within a defined region opposite oriC.

Mechanism of Replication Fork Trap

• Terminator proteins bind to ter sites and act as polar replication fork barriers.

• They allow replication forks to pass in one direction but block them in the opposite direction.

• This polarity ensures that replication forks are trapped and forced to meet within the terminus region.

• The fork trap prevents over-replication and ensures proper completion of chromosome duplication.

Evidence for Multiple Specific Termination Sites

• Studies in E. coli and B. subtilis show that replication forks pause and fuse within a region containing multiple ter sites rather than a single locus.

• The termination sites are conserved and function cooperatively to regulate fork progression.

• The dif site, involved in chromosome dimer resolution, is located within this terminus region, coordinating replication completion with chromosome segregation.

Why Not a Single Specific Locus?

• The presence of multiple ter sites provides a robust termination system.

• It accommodates variability in fork progression rates and prevents premature or incomplete termination.

• The fork trap system ensures that forks meet within a controlled region, maintaining genome stability.

Summary Table

Conclusion

Bacterial chromosomal DNA replication terminates at several specific loci within a defined terminus region, rather than at a single site. The coordinated action of multiple ter sites and terminator proteins forms a replication fork trap that ensures replication forks converge properly. This system is crucial for maintaining genome integrity and coordinating replication with cell division.

Answer:
The correct option is (2) several specific loci.