The phage P1 is used as a generalized transducing phage in an exp...

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.

Experiment A Experiment B Experiment C
phe⁻ 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.

Accepted Answer

Hi, everybody. Here's our next problem. A group of researchers are studying the process of gene transfer through bacterial transduction. They have identified a population of trans tant which are bacteria that have acquired new genetic material through transduction. The researchers want to investigate how crossover events contribute to genetic diversity in these traduct. Which of the following statements is true about crossover events, inductive choice. A crossover events occur when traduct acquire genetic material from a different bacterial species. Choice B crossover events involve the exchange of genetic material between homologous regions of DNA. Choice C crossover events always result in the acquisition of new genes by traduct or choice D crossover events can only occur during bacterial conjugation, not transduction. Well, let's think through these answer choices. First, we want to make sure we've got our minds wrapped around transduction because there's several different ways that bacteria exchange genetic information. So we'll recall that transduction involves a viral infection of bacteria transferring the DNA. So you have P page infection of one strain of bacteria. And when the phage then moves on to infect a second strain, that age can bring portions of DNA from the first strain in effect. So from strain one two, strain two, since the age DNA incorporates into strain, the cells of strain one then gets repackaged into new P pages. Sometimes in that repackaging portions of the genome from strain one can come along for the ride into strain two. So that's transduction. And then we're talking about a crossover event and of course, crossovers, which can also happen during meiosis as we recall, involve swapping portions of DNA between chromosomes. And how does that occur that occurs when these regions are homologous. Since these homologous regions can pair up, that allows this crossing over which then allows swapping of genetic material. So with that in mind, let's evaluate our answer choices in choice. A it says that crossover events occur when traduct acquire genetic material from a different bacterial species. Well, this is not quite correct traduct can acquire new genetic material without crossing over. So if some of the genetic material from strain, one comes along for the ride into strain two, it's getting new genetic material. But if there's no homologous reasons in those two chromosomes or if they just don't happen to line up, you won't have a crossover event. So a crossover event doesn't necessarily occur. In most cases. Choice B says crossover events evolve the exchange of genetic material between homologous regions of DNA. And this is a correct description of crossover events. So we're going to go ahead and circle that. Our question just says which statement is true, it doesn't say what's the best answer or the best description. So if time is of the essence, I would go ahead and select choice B and keep going because that is a true statement. But in terms of our explanation, we're not concerned with time, we're concerned with being thorough. So let's look on at choice C crossover events always result in the acquisition of new genes. And this is not a true statement because they just sometimes result in the acquisition of new genes. If the genes brought along from the first strain are the same as the genes in the original strain. Even if there's a crossover event, you won't have the acquisition of a new gene, they'll just swap two of the same genes. So that's why 20 C isn't our true statement. And then finally, Joyce D crossover events can only occur during bacterial conjugation, not transduction. While this is not true, crossover events can occur in both of these situations. So again, our true statement about crossover events and traduct this choice B crossover events involve the exchange of genetic material between homologous regions of DNA. See you in the next video.

Key Concepts

Generalized Transduction

Crossover Events

Selection of Transductants

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