A gene is inserted into the middle of the ampicillin resistance gene of a plasmid that also has an intact tetracycline resistance gene. Which bacterial colonies contain the recombinant plasmid?
Answer and explanation
Correct answer: A. Those that grow on tetracycline but not on ampicillin
- Step 1: Bacteria with any plasmid are tetracycline resistant, so they grow on tetracycline.
- Step 2: If the insert interrupted the ampicillin resistance gene, bacteria with the recombinant plasmid lose resistance to ampicillin, while those with an unaltered plasmid keep it.
Answer
grow on tetracycline but not ampicillin.
Why not the others: Growth on both indicates an unmodified plasmid; growth on ampicillin only would need tetracycline to be disrupted; no growth means no plasmid.
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