Question

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.
Table displaying transductant genotype frequencies for three experiments involving E. coli strains and phage P1.

Table displaying transductant genotype frequencies for three experiments involving E. coli strains and phage P1.

Diagram the crossover events that form each of the transductants in Experiment A.

Accepted Answer

Step 1: Understand the experimental setup. The phage P1 is used for generalized transduction, which involves transferring genetic material from a donor strain (leu⁺ phe⁺ ala⁺) to a recipient strain (leu⁻ phe⁻ ala⁻). The goal is to analyze the genotypes of transductants selected for leu⁺ in Experiment A and determine the crossover events responsible for their formation. Step 2: Analyze the genotype frequencies in Experiment A. The transductants selected for leu⁺ are categorized into four groups based on the unselected markers (phe and ala): phe⁻ ala⁻ (26%), phe⁺ ala⁻ (50%), phe⁻ ala⁺ (19%), and phe⁺ ala⁺ (3%). These frequencies provide clues about the proximity of the genes and the likelihood of crossover events. Step 3: Diagram the genetic arrangement of the donor strain. Assume the genes leu, phe, and ala are linked on the bacterial chromosome. Represent the donor strain as a linear sequence: leu⁺ - phe⁺ - ala⁺. The recipient strain has the corresponding alleles: leu⁻ - phe⁻ - ala⁻. The phage P1 transfers segments of the donor DNA to the recipient, and recombination occurs. Step 4: Identify crossover events for each transductant genotype. For example: (a) phe⁻ ala⁻ transductants (26%) likely result from no crossover, where only the leu⁺ gene is transferred. (b) phe⁺ ala⁻ transductants (50%) suggest a single crossover event between leu and phe, transferring both leu⁺ and phe⁺. (c) phe⁻ ala⁺ transductants (19%) indicate a single crossover event between phe and ala, transferring leu⁺ and ala⁺. (d) phe⁺ ala⁺ transductants (3%) result from a double crossover event, transferring all three genes (leu⁺, phe⁺, ala⁺). Step 5: Summarize the crossover events in a diagram. Create a visual representation showing the donor and recipient DNA, the location of the genes, and the crossover points for each transductant genotype. This will help illustrate how the different genotypes are formed during transduction.

비슷한 문제에 대한 검증된 영상 답변:

주요 개념

Generalized Transduction

Crossover Events

Selection of Transductants