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 with E. coli strains and phage P1.

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

In Experiment B, why are there no transductants with the genotype leu⁻ ala⁺?

Accepted Answer

Step 1: Understand the concept of generalized transduction. Generalized transduction occurs when a bacteriophage transfers random fragments of bacterial DNA from a donor strain to a recipient strain during infection. This process can result in the transfer of specific genes, such as leu⁺, phe⁺, or ala⁺, depending on the experimental selection criteria. Step 2: Analyze the data from Experiment B. In this experiment, transductants are selected for ala⁺, meaning only recipient cells that acquire the ala⁺ gene from the donor strain are able to grow. The table shows that there are no transductants with the genotype leu⁻ ala⁺. Step 3: Consider the genetic linkage between the leu and ala genes. Genetic linkage refers to the physical proximity of genes on the bacterial chromosome. If leu and ala are closely linked, it is likely that they are transferred together during transduction. This means that a recipient cell acquiring ala⁺ from the donor strain would also acquire leu⁺, resulting in the genotype leu⁺ ala⁺. Step 4: Evaluate the absence of leu⁻ ala⁺ transductants. The lack of leu⁻ ala⁺ transductants suggests that the ala gene is not transferred independently of the leu gene. Instead, the transfer of ala⁺ is always accompanied by the transfer of leu⁺ due to their close genetic linkage. Step 5: Conclude that the absence of leu⁻ ala⁺ transductants in Experiment B is due to the genetic linkage between leu and ala. This linkage ensures that any recipient cell acquiring ala⁺ from the donor strain also acquires leu⁺, preventing the formation of leu⁻ ala⁺ genotypes.

비슷한 문제에 대한 검증된 영상 답변:

주요 개념

Generalized Transduction

Selection of Transductants

Linkage and Genetic Mapping