What is a selectable marker and why is it used in cloning?

• A. A gene that encodes a fluorescent protein to mark cells.
• B. A gene that promotes plasmid replication.
• C. A gene (e.g., antibiotic resistance) that allows identification of cells that have taken up the recombinant plasmid.
• D. A region that disables gene expression.

Answer: (C)

A selectable marker is a gene that lets you identify and separate cells that have taken up the recombinant plasmid, usually by providing resistance to an antibiotic (or another selective condition). When you grow the cells on medium containing that antibiotic, only those that carry the plasmid survive, so you can easily isolate colonies that contain the recombinant DNA. This is crucial in cloning because it dramatically reduces the effort needed to find the cells that actually have the plasmid with your gene of interest. A fluorescent protein may let you see expression, but it isn’t a selection method—cells without the plasmid can still grow and show fluorescence if expressed, so it doesn’t enrich for transformed cells. A gene that helps plasmid replication or a region that shuts gene expression doesn’t provide the selective advantage needed to distinguish transformed cells, so they don’t fulfill the same purpose as a selectable marker.