What is the purpose of including a marker gene such as antibiotic resistance in recombinant plasmids?

• A. To identify bacteria that contain the plasmid
• B. To induce gene expression
• C. To degrade antibiotics
• D. To increase growth rate

Answer: (A)

Selection of transformed cells using a marker gene is the main idea behind including an antibiotic resistance gene in recombinant plasmids. After introducing the plasmid into bacteria, you can grow the cells on a medium that contains the antibiotic. Only those cells that have taken up the plasmid—and thus carry the resistance gene—will survive, making it easy to identify and propagate the cells that actually contain the recombinant DNA. This selective step is what lets researchers efficiently isolate the plasmid-bearing bacteria for further experiments. It’s not about turning on gene expression, degrading the antibiotic, or increasing growth rate; it’s about providing a selectable trait to distinguish plasmid-containing cells from those that don’t.