Which component of an expression vector is used to select for cells that have taken up the plasmid?

• A. Selectable marker
• B. Strong promoter
• C. Tag sequence
• D. Origin of replication

Answer: (A)

The key idea is that a selectable marker is what lets you distinguish cells that carry the plasmid from those that don’t. A selectable marker is a gene that provides a trait the cells need to survive under selective conditions, typically an antibiotic resistance gene. When you grow transformed cells on a medium containing the antibiotic, only the cells that have taken up the plasmid—and thus express the resistance gene—will survive and form colonies. This makes it easy to identify and isolate cells with the plasmid.

The other components have different roles. A strong promoter drives high levels of transcription of the gene of interest, but it isn’t about enabling survival under selection. A tag sequence encodes a detectable or purifiable tag on the expressed protein, not a survival trait. The origin of replication allows the plasmid to replicate in the host, but it doesn’t provide a way to select for plasmid-containing cells.