Question

If a plasmid is having two antibiotic resistant genes, say ampicillin resistant and chloramphenicol resistant. If the plasmid grows in ampicillin containing medium but not in chloramphenicol, what can be concluded?

• A. The insert is not present in any of the gene
• B. The insert is present in ampicillin gene but not in chloramphenicol gene
• C. The insert is present in chloramphenicol gene but not in ampicillin gene
• D. The insert is present between both of the gene

Hint:

When analyzing plasmids with multiple resistance genes, growth in one antibiotic medium but not another often indicates a disrupted resistance gene due to the insert.

Answer 1

The Correct Option is B

The plasmid carries separate resistance genes for ampicillin and chloramphenicol. Growth on the ampicillin plate but not the chloramphenicol plate points to a mixed outcome: resistance retained on one side of the plasmid, lost on the other, through disruption of a resistance gene by the cloned insert.

Ruling out "insert absent from both genes" (would give resistance to both antibiotics) and "insert in both genes" (would give resistance to neither), the pattern of one resistance retained and one lost matches the pairing described in option two: insert present in the ampicillin gene, not in the chloramphenicol gene.

So the conclusion is that the insert is present in the ampicillin gene but not in the chloramphenicol gene.

Answer 2

A cleaner way to see this is to map out all four possible insert locations against the resistance pattern they would predict, then match against what's observed (grows on ampicillin, does not grow on chloramphenicol).

1. No insert in either gene would predict resistance to both antibiotics -- doesn't match, since chloramphenicol growth fails.
2. Insert in ampicillin gene only predicts one resistance gene disrupted and the other intact, giving growth on one antibiotic and not the other -- this matches the mixed pattern actually observed.
3. Insert in chloramphenicol gene only predicts the same type of mixed pattern from the other side, so it overlaps with the scenario already captured above rather than being a separate, independently matching case.
4. Insert in both genes predicts loss of resistance to both antibiotics -- doesn't match, since ampicillin growth is observed.

The only prediction consistent with growth on ampicillin and no growth on chloramphenicol is the mixed-resistance case, which corresponds to the insert being present in the ampicillin gene but not the chloramphenicol gene.

Hence, the correct answer is the insert is present in ampicillin gene but not in chloramphenicol gene.