Why are plasmids often constructed to contain antibiotic resistance genes?

• A. Act as marker gene
• B. It makes bacteria resistant to all antibiotics
• C. It increases plasmid size dramatically
• D. It causes rapid degradation of plasmid DNA

Answer: (A)

The main idea here is the use of a selectable marker in cloning. An antibiotic resistance gene on a plasmid lets you quickly identify cells that have taken up the plasmid: grow the cells on a medium containing the corresponding antibiotic, and only those with the plasmid survive. This makes it easy to isolate transformed cells and keep the plasmid in culture, which is essential for downstream experiments.

The other options don’t fit. A single resistance gene doesn’t make bacteria resistant to all antibiotics; resistance is usually specific to one antibiotic or a few. Adding the resistance gene adds only a small amount of DNA and doesn’t dramatically increase plasmid size. And antibiotic resistance genes don’t cause rapid degradation of plasmid DNA.