An interrupted mating study is carried out on Hfr strains 1, 2, a...

Question

An interrupted mating study is carried out on Hfr strains 1, 2, and 3 identified in Problem 17. After conjugation is established, a small sample of the mixture is collected every minute for 20 minutes to determine the distance between genes on the chromosome. Results for each of the three Hfr strains are shown below. The total duration of conjugation (in minutes) is given for each transferred gene.

Hfr strain 1 oriT met ala lac gal
Duration (min) 0 2 8 13 17
Hfr strain 2 oriT met leu thr azi
Duration (min) 0 2 7 10 17
Hfr strain 3 oriT gal pro trp azi
Duration (min) 0 3 8 14 19

For each Hfr strain, draw a time-of-entry profile like the one in Figure 6.11a.

Accepted Answer

Hi, everyone. Welcome back. Let's look at our next question. It says the conjugation process is utilized in interrupted mating experiments to map the bacterial chromosome's gene order. What is the advantage of using a selective medium in an interrupted mating experiment? So let's just recall kind of the basic outline of this process. And then we'll look at our answer choices and go through to see which one of them is correct. So as we know in an interrupted mating experiment, we have an H F R donor cell in which the F factor and its associated genes have been incorporated into the bacterial chromosome. And then we have an F minus recipient cell that doesn't have an F factor, the H F R cell because it has those genes incorporated into its main chromosome. That process of conjugation is relatively inefficient and slow. So we can interrupt that process along the way at different points. So at different points, a different number of those genes on the bacterial chromosome will have transferred over into the recipient cell by stopping at different points. We can then look at which genes transfer at earlier or later times, which shows us the order of those genes in the bacterial chromosome. And how do we know if a gene has transferred or not? Well, if you use genes in the H F R cell that give the cell the ability to bio synthesize different essential nutrients and you use a recipient cell but lacks those genes. You can then use as our question asks a selective medium to determine whether or not those genes have transferred by leaving out essential nutrients from the medium that can't be synthesized by the F minus cell, but can be synthesized. If it has received the necessary gene from the H F R cell, you can identify the recombinant cells because they will only grow if they have those cells. So with that kind of overview of the process in mind, let's take a look at our answer choices and see which one is the best description of the advantage of using the selective medium choice. A says to map the order of genes in the H F R strains. Well, this is not likely to be our answer. It is part of the process of mapping the order of genes, but we don't get that map and we don't get that order directly from the use of the selective medium. The selective medium is just a sort of yes or no of did the recipient cell get this particular gene or not? We then have to use further information to see the order based on whether the recipient cells received other genes. So choice A is not our best answer here. Then choice B says to make sure that only recombinant genotypes are found. And this is the purpose of the selective medium because only colonies will grow that have received a gene that allows them to synthesize the missing nutrient. So we'll go ahead and select that as our answer. But we'll just double check by looking at C N D to make sure nothing is a better description of this advantage. Choi says to make sure that conjugation has been completed. Well, you would only know that conjugation has been completed if you know whether the last gene or whether all the genes have transferred. And of course, our selective medium just kind of gives us a glimpse at one particular gene, whether it has transferred. So you'd have to kind of put together all the information to see, have all the genes transferred. So choice C would not be a specific advantage of the selective medium. And then finally, choice D is to eliminate all recipient cells. Well, this obviously is not correct. We don't want to eliminate all recipient cells. We just want to eliminate, eliminate cells that have not received the gene from the H F R cell. So we come back to our answer choice here and looking at what is the advantage of using a selective medium in an interrupted man experiment? Choice B to make sure that only recombinant genotypes are found. See you in the next video.

Key Concepts

Hfr Strains and Conjugation

Gene Mapping and Time-of-Entry

Time-of-Entry Profiles

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