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Mutation and Natural Selection
Concepts tested here
- Replica plating experiment
All Questions
2005 AIPMT 1 question
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Using imprints from a plate with complete medium and carrying bacterial colonies, you can select streptomycin resistant mutants and prove that such mutations do not originate as adaptation. These imprints need to be used -This is the replica plating technique of Joshua and Esther Lederberg.
Bacteria are first grown on a master plate containing complete medium without streptomycin, so mutants are not selected at this stage.
A sterile velvet pad is pressed on the master plate and the imprint of the colonies is transferred to fresh plates, keeping the same positions of the colonies.
According to the answer key, the imprints are used on plates with streptomycin: only the streptomycin resistant colonies grow there, and they appear at positions corresponding to particular colonies of the master plate.
Since these resistant cells were already present on the master plate before any contact with streptomycin, the mutations arose spontaneously and randomly; they were not induced by the antibiotic as an adaptation.
Note: in practice a plate without streptomycin is also used as a control, so this question is open to the other interpretation.