What is the role of a selectable marker in cloning plasmids?

• A. Initiate replication of the plasmid
• B. Confer antibiotic resistance to identify transformed cells
• C. Provide a promoter for gene expression
• D. Encode fluorescent protein

Answer: (B)

Selectable markers in cloning plasmids provide a way to tell which cells actually received the plasmid. They usually encode antibiotic resistance, so when you grow transformed cells on medium containing the antibiotic, only those with the plasmid survive. This makes it practical to identify and isolate the transformed cells for further work.

The antibiotic resistance marker works because the plasmid carries a gene that allows the cell to inactivate the antibiotic or evade its effect. Without the marker, non-transformed cells die or fail to grow under selective conditions, making selection straightforward.

Other features of plasmids serve different roles. Initiating replication is controlled by the origin of replication, not the marker. A promoter drives expression of a gene, but it doesn’t distinguish transformed cells. A fluorescent protein can act as a reporter to show expression visually, but it isn’t as reliable for selecting transformed cells as antibiotic resistance.