7. You are doing a conjugation experiment with two strains of bacteria. The F+ strain
is bio+ leu+ pro+ lac+. The F- strain is bio- leu- pro- lac- Str+ (resistant to
Streptomycin). What kind of plates would you use to select recombinants?
a. Complete medium with streptomycin
b. Minimal medium
c. Minimal medium with 3 of the supplements
d. Minimal medium with 3 of the supplements and streptomycin
Bacterial Conjugation Experiment: Selecting Recombinants on Minimal Medium with Supplements and Streptomycin
Bacterial conjugation experiments use specific media to isolate recombinants from F+ (bio⁺ leu⁺ pro⁺ lac⁺) donor and F⁻ (bio⁻ leu⁻ pro⁻ lac⁻ Strʳ) recipient strains. The correct plates require counterselection against donors and positive selection for recipients that gained at least one donor gene. Option d ensures only true chromosomal recombinants grow.
Experiment Setup
F+ donors transfer chromosomal genes during conjugation, though less efficiently than Hfr strains. Recombinants form when recipient F⁻ cells integrate donor alleles (e.g., bio⁺), becoming prototrophic for that trait. Streptomycin (Str) kills Strˢ donors, as recipients carry Strʳ for counterselection. Minimal medium lacks bio, leu, pro, lac precursors, so auxotrophic parents cannot grow without supplements.
Option Analysis
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a. Complete medium with streptomycin: Permits all Strʳ cells, including unmated recipients; no selection for donor gene transfer.
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b. Minimal medium: Excludes all parents—donors lack Strʳ, recipients need all four supplements; no growth occurs.
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c. Minimal medium with 3 supplements: Allows growth if one donor gene transfers (e.g., bio⁺ with leu/pro/lac added), but lacks Str to kill donors.
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d. Minimal medium with 3 supplements and streptomycin (correct): Str eliminates donors; recipients need one donor gene (e.g., bio⁺) plus 3 supplements to grow, confirming recombination.
