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 -
- On plates with minimal medium
- Only on plates with streptomycin
- Only on plates without streptomycin
- On plates with and without streptomycin
Answer
(2) Only on plates with streptomycin
Solution
This is the replica plating experiment of Lederberg and Lederberg.
Colonies on the master plate (complete medium, no streptomycin) are copied with sterile velvet onto plates containing streptomycin, where only the resistant colonies grow.
These occupy the same positions as particular colonies on the master plate, which were never exposed to the drug.
∴ Resistance arose by random mutation before contact with streptomycin, not as an adaptation to it. As per NCERT, variations (mutations) are random and pre-existing, and selection only picks them.
Answer: (2)