How does plasmid copy number affect gene expression in a host?

• A. Higher copy number decreases expression.
• B. Higher copy number increases gene dosage and expression but can burden the host and destabilize plasmids.
• C. Copy number has no effect on expression.
• D. Copy number only affects replication rate but not expression.

Answer: (B)

The main idea is that plasmid copy number sets gene dosage in the cell. More plasmid templates mean more transcription opportunities, so you generally get more mRNA and, if the translation machinery isn’t limiting, more protein. In practice, increasing copy number often boosts expression, but it also taxes the cell’s resources. A heavy burden can slow growth, trigger stress responses, and reduce plasmid stability, making the plasmids harder to maintain and sometimes causing loss of plasmids from the population.

That balance is why the statement is best: higher copy number increases gene dosage and expression but can burden the host and destabilize plasmids. If the burden were too great, expression per cell can plateau or even drop as cells compensate or plasmids are lost.

Other ideas don’t fit as well because copy number does influence expression directly through the number of templates; it isn’t just about replication rate, and it typically doesn’t decrease expression on its own.