What is a typical example of a selectable marker used in plasmids?

• A. Fluorescent reporter gene.
• B. A promoter.
• C. Antibiotic resistance gene.
• D. Origin of replication.

Answer: (C)

Selectable markers are genes that let cells carrying the plasmid be distinguished by growth under selective conditions. An antibiotic resistance gene is the classic example: bacteria with the plasmid can survive on media containing the antibiotic, while cells without the plasmid cannot grow. This makes it easy to isolate transformed cells. A fluorescent reporter gene is great for visualizing expression but doesn’t by itself enable survival-based selection. A promoter just controls when a gene is turned on, not survival, and the origin of replication is needed for plasmid copying but doesn’t provide a growth advantage under selection. So the antibiotic resistance gene is the typical selectable marker.