Which statement best describes a selectable marker in plasmids?

Which statement best describes a selectable marker in plasmids?

• A. It increases the plasmid copy number
• B. It codes for a fluorescent protein used in screening
• C. It enables identification of cells that carry the plasmid by surviving under selective conditions (e.g., antibiotic)
• D. It anchors the plasmid to the host cell membrane

Answer: (C)

A selectable marker on a plasmid is a gene that lets only cells carrying the plasmid survive under certain conditions. In practice, this usually means an antibiotic resistance gene or another trait that provides a survival advantage when the culture is grown with a selective agent. When you plate cells on a medium that contains the selective agent, cells without the plasmid can’t grow, while those with the plasmid can, making it easy to identify the cells that took up the plasmid.

This differs from markers used for screening, like a fluorescent protein, which helps you visually identify clones but doesn’t inherently let the cells survive better under a specific condition. The idea of a selectable marker is tied to survival under selective pressure, not just a visual cue.

The other options don’t fit because increasing plasmid copy number is about how many copies of the plasmid exist in the cell, not about survival under selection; a fluorescent protein marker provides fluorescence for screening but doesn’t confer a survival advantage; and anchoring a plasmid to the host membrane isn’t how plasmids are maintained or selected for in cells.