How does chromosomal replication differ from plasmid replication in bacteria?

• A. Chromosome replication uses a rolling-circle mechanism.
• B. Chromosome replication does not have a defined origin.
• C. Chromosome replication follows a defined origin and cell-cycle program; plasmids often replicate via rolling-circle or theta mechanisms and can be more variable.
• D. Plasmids always integrate into the chromosome.

Answer: (C)

Chromosomal replication in bacteria is tightly organized around a single, defined origin of replication and is coordinated with the cell cycle, so initiation happens once per generation and the replication forks proceed bidirectionally until termination. This ensures the chromosome is duplicated once and then properly segregated to daughter cells. Plasmids, on the other hand, are extrachromosomal elements with their own origins and often replicate autonomously, using modes such as rolling-circle or theta replication. This setup allows plasmids to control copy number independently of the chromosome, leading to variable copy numbers between cells and across plasmid types. Rolling-circle replication involves nicking one strand and producing a new copy as the circle is unrolled, while theta replication generates a two-armed bubble that expands and copies, similar in principle to chromosomal replication but not as tightly tied to the host cell cycle. So the key difference is that the chromosome follows a defined origin and cell-cycle program, whereas plasmids replicate with their own origins and more flexible, sometimes higher or variable, copy-number control.