What is the significance of an origin of replication on a plasmid?

• A. It signals RNA transcription termination.
• B. Origin of replication allows plasmid to replicate inside host cells and determines copy number.
• C. Origin of replication marks the start codon.
• D. Origin of replication is not necessary for plasmid maintenance.

Answer: (B)

The origin of replication is the site where copying of the plasmid DNA starts, so it lets the plasmid duplicate inside the host cell using the cell’s own replication machinery. Because the origin’s sequence and its control elements determine how often replication is initiated, it sets the plasmid’s copy number—some origins push for many copies per cell, others for only a few. This combination—the ability to replicate and the level of copy number—also underpins stable inheritance of the plasmid as cells divide, since more copies mean more chances the plasmid will be passed to daughter cells, while too many can burden the cell.

This isn’t about signaling transcription termination or marking a start codon, which are unrelated processes. And without a functional origin, the plasmid wouldn’t replicate at all and would eventually be lost from the population, so it is essential for maintenance.