What does a selectable marker allow in plasmids?

• A. It allows identification of cells that carry the plasmid by enabling growth under selective conditions.
• B. It does not affect screening.
• C. It enhances transcription of the insert.
• D. It increases plasmid copy number.

Answer: (A)

Selectable markers let you identify cells that carry a plasmid by enabling them to grow under selective conditions. For example, an antibiotic resistance gene lets transformed cells survive on media containing that antibiotic, while cells lacking the plasmid die. This makes it easy to pick out the cells that actually carry the plasmid. The marker isn’t about boosting transcription of the inserted gene or increasing the plasmid copy number—that’s set by the promoter and the origin of replication, respectively. So the main purpose of a selectable marker is to distinguish transformed cells through selective growth.