What is the purpose of the selection marker in a plasmid vector?

• A. It codes for the insulin protein.
• B. It ensures plasmid integrates into host genome.
• C. It inhibits bacterial growth.
• D. It allows identification of host cells that have taken up the plasmid.

Answer: (D)

The selection marker in a plasmid vector is used to tell you which cells have taken up the plasmid. It usually carries an antibiotic resistance gene, so when you grow the cells on a medium containing that antibiotic, only those that have the plasmid—and are expressing the resistance gene—survive. This makes it easy to identify and isolate the transformed cells from a mix of cells that didn’t acquire the plasmid.

The marker isn’t about producing a protein like insulin, and plasmids don’t have to integrate into the host genome to function as vectors. Also, the marker doesn’t inhibit growth; it enables growth under selective conditions by letting the resistant cells thrive.