182W10PS4Key

182W10PS4Key - 20182-2010-problem set 4: Bacterial Genetics...

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Unformatted text preview: 20182-2010-problem set 4: Bacterial Genetics Key1. Explain why different Hfr cells can have F plasmids integrated at different sites. Hfr cells are created when F plasmids integrate into the bacterial genome. F plasmids contain insertion sequences (IS sequences). The bacterial genome also contains IS sites, found at various positions along the bacterial chromosome. F plasmids can integrate at any of these bacterial IS sites through homologous recombination, resulting in Hfr strains with F plasmids integrated at different sites. 2. Since a mutation in an essential gene prevents a colony from growing, how would a bacteriologist study such a strain? Use a conditional lethal mutation such as a temperature-sensitive mutation.3. Describe how you would screen for Arg-cells arising from a wild-type culture of E. coli.To increase frequency of mutation, first expose wild-type culture to a mutagen. Then plate mutagenized cells onto minimal plates containing arginine. After colonies have grown up, replica plate onto minimal plates without arginine. Note position of cells that grow on first set of plates, but not the second set. Those are the Arg- mutants.4. Describe the difference between "selection" and "screen" with respect to mutant isolation methods....
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182W10PS4Key - 20182-2010-problem set 4: Bacterial Genetics...

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