A researcher mixes a population of bacteria from separate colonies grown on master plates and stamps them onto plates containing different antibiotics using velvet. Colonies resistant to both drugs appear at predictable positions matching the master plate, even though the cells had never been exposed to antibiotics before. This classic experiment demonstrates that:

• A Resistance mutations arise spontaneously prior to drug exposure ✓
• B Antibiotics induce resistance mutations in sensitive cells
• C Resistance genes transfer between colonies during stamping
• D Velvet transfers antibiotic molecules from plate to plate

Explanation

The Lederberg and Lederberg replica plating experiment showed that pre-existing spontaneous mutants occupy fixed positions on the master plate and reproduce their pattern on selective plates they had never encountered. This refuted the adaptive induction hypothesis and supported random mutation followed by selection. No gene transfer occurs through sterile velvet.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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