5. Genetics of Bacteria and Viruses

In this chapter, we have focused on genetic systems present in bacteria and on the viruses that use bacteria as hosts (bacteriophages). In particular, we discussed mechanisms by which bacteria and their phages undergo genetic recombination, which allows geneticists to map bacterial and bacteriophage chromosomes. In the process, we found many opportunities to consider how this information was acquired. From the explanations given in the chapter, what answers would you propose to the following questions?How do we know that bacteriophages recombine genetic material through transduction and that cell-to-cell contact is not essential for transduction to occur?

How is the frequency of cotransduction related to the relative positions of genes on a bacterial chromosome? Draw a map of three genes and describe the expected relationship of cotransduction frequencies to the map.

A triple-auxotrophic strain of E. coli having the genotype phe⁻ met⁻ ara⁻ is used as a recipient strain in a transduction experiment. The strain is unable to synthesize its own phenylalanine or methionine, and it carries a mutation that leaves it unable to utilize the sugar arabinose for growth. The recipient is crossed to a prototrophic strain with the genotype phe⁺ met⁺ ara⁺. The table below shows the selected marker and gives cotransduction frequencies for the unselected markers.Selected Selected Colonies Containing the Marker Unselected Marker (%) _phe⁺ met⁺ ara⁺met⁺ 4 - 7phe⁺ - 2 51met⁺, phe⁺. - - 79ara⁺ 68 5 -Use the cotransduction data to determine the order of these genes.

A triple-auxotrophic strain of E. coli having the genotype phe⁻ met⁻ ara⁻ is used as a recipient strain in a transduction experiment. The strain is unable to synthesize its own phenylalanine or methionine, and it carries a mutation that leaves it unable to utilize the sugar arabinose for growth. The recipient is crossed to a prototrophic strain with the genotype phe⁺ met⁺ ara⁺. The table below shows the selected marker and gives cotransduction frequencies for the unselected markers.Selected Selected Colonies Containing the Marker Unselected Marker (%) _phe⁺ met⁺ ara⁺met⁺ 4 - 7phe⁺ - 2 51met⁺, phe⁺. - - 79ara⁺ 68 5 -Identify the compounds present in each of the selective media.

The phage P1 is used as a generalized transducing phage in an experiment combining a donor strain of E. coli of genotype leu⁺ phe⁺ ala⁺ and a recipient strain that is leu⁻ phe⁻ ala⁻. In separate experiments, transductants are selected for leu⁺ (Experiment A), for ala⁺ (Experiment B), and for phe⁺ (Experiment C). Following selection, transductant genotypes for the unselected markers are identified. The selection experiment results below show the frequency of each genotype.Experiment A Experiment B Experiment Cphe⁻ ala⁻ 26% leu⁻ ala⁻ 65% leu⁻ phe⁻ 71%phe⁺ ala⁻ 50% leu⁺ ala⁻. 48% leu⁺ phe⁻ 21%phe⁻ ala⁺ 19% leu⁻ ala⁺ 0% leu⁻ phe⁺ 0%phe⁺ ala⁺ 3% leu⁺ ala⁺ 4% leu⁺ phe⁺ 3%In Experiment B, why are there no transductants with the genotype leu⁻ ala⁺?

The phage P1 is used as a generalized transducing phage in an experiment combining a donor strain of E. coli of genotype leu⁺ phe⁺ ala⁺ and a recipient strain that is leu⁻ phe⁻ ala⁻. In separate experiments, transductants are selected for leu⁺ (Experiment A), for ala⁺ (Experiment B), and for phe⁺ (Experiment C). Following selection, transductant genotypes for the unselected markers are identified. The selection experiment results below show the frequency of each genotype.Experiment A Experiment B Experiment Cphe⁻ ala⁻ 26% leu⁻ ala⁻ 65% leu⁻ phe⁻ 71%phe⁺ ala⁻ 50% leu⁺ ala⁻. 48% leu⁺ phe⁻ 21%phe⁻ ala⁺ 19% leu⁻ ala⁺ 0% leu⁻ phe⁺ 0%phe⁺ ala⁺ 3% leu⁺ ala⁺ 4% leu⁺ phe⁺ 3%Diagram the crossover events that form each of the transductants in Experiment A.

The phage P1 is used as a generalized transducing phage in an experiment combining a donor strain of E. coli of genotype leu⁺ phe⁺ ala⁺ and a recipient strain that is leu⁻ phe⁻ ala⁻. In separate experiments, transductants are selected for leu⁺ (Experiment A), for ala⁺ (Experiment B), and for phe⁺ (Experiment C). Following selection, transductant genotypes for the unselected markers are identified. The selection experiment results below show the frequency of each genotype.Experiment A Experiment B Experiment Cphe⁻ ala⁻ 26% leu⁻ ala⁻ 65% leu⁻ phe⁻ 71%phe⁺ ala⁻ 50% leu⁺ ala⁻. 48% leu⁺ phe⁻ 21%phe⁻ ala⁺ 19% leu⁻ ala⁺ 0% leu⁻ phe⁺ 0%phe⁺ ala⁺ 3% leu⁺ ala⁺ 4% leu⁺ phe⁺ 3%Determine the order of genes on the donor chromosome.

The phage P1 is used as a generalized transducing phage in an experiment combining a donor strain of E. coli of genotype leu⁺ phe⁺ ala⁺ and a recipient strain that is leu⁻ phe⁻ ala⁻. In separate experiments, transductants are selected for leu⁺ (Experiment A), for ala⁺ (Experiment B), and for phe⁺ (Experiment C). Following selection, transductant genotypes for the unselected markers are identified. The selection experiment results below show the frequency of each genotype.Experiment A Experiment B Experiment Cphe⁻ ala⁻ 26% leu⁻ ala⁻ 65% leu⁻ phe⁻ 71%phe⁺ ala⁻ 50% leu⁺ ala⁻. 48% leu⁺ phe⁻ 21%phe⁻ ala⁺ 19% leu⁻ ala⁺ 0% leu⁻ phe⁺ 0%phe⁺ ala⁺ 3% leu⁺ ala⁺ 4% leu⁺ phe⁺ 3%What compound or compounds are added to the minimal medium to select for transductants in Experiments A, B, and C?