What feature allows scientists to identify bacteria that have taken up the recombinant plasmid during transformation?

• A. Restriction enzymes used to cut DNA
• B. Promoter regions of genes
• C. Origin of replication in plasmid
• D. An antibiotic resistance gene that enables selection of transformants

Answer: (D)

The key idea is using a selectable marker carried on the plasmid to distinguish cells that have taken it up. An antibiotic resistance gene on the plasmid lets transformed bacteria survive when grown on a medium containing that antibiotic. Only those cells that received the plasmid—and thus express the resistance gene—live; non-transformed cells are killed or inhibited. This makes transformants easy to identify as colonies growing on the antibiotic-containing plate.

Other features on the plasmid aren’t about selecting transformants. The origin of replication is needed for the plasmid to replicate in the host, but it doesn’t tell you which cells actually took up the plasmid. Restriction enzymes are tools for cutting DNA during cloning, not for selecting transformants. Promoter regions control gene expression but don’t provide a survival advantage under selective conditions.