A wild-type culture of haploid yeast is exposed to ethyl methanesulfonate (EMS). Yeast cells are plated on a complete medium, and 6 colonies (colonies numbered 1 to 6) are transferred to a new complete medium plate for further study. Four replica plates are made from the complete medium plate to plates containing minimal medium or minimal medium plus one amino acid (replica plates numbered 1 to 4) with the following results:
Identify the colonies that are auxotrophic (mutant). What growth information leads to your answer?

Question

A wild-type culture of haploid yeast is exposed to ethyl methanesulfonate (EMS). Yeast cells are plated on a complete medium, and 6 colonies (colonies numbered 1 to 6) are transferred to a new complete medium plate for further study. Four replica plates are made from the complete medium plate to plates containing minimal medium or minimal medium plus one amino acid (replica plates numbered 1 to 4) with the following results:

Identify the colonies that are auxotrophic (mutant). What growth information leads to your answer?

Diagram showing yeast colonies on a complete medium and their transfer to replica plates with varying minimal media conditions.

Identify the colonies that are auxotrophic (mutant). What growth information leads to your answer?

Accepted Answer

Welcome back. Let's look at our next problem. It says The researcher has a bacterial colony containing 500 cells that are a mixture of wild type and oxo trophic mutants to identify the oxo trophic mutants. The researcher performs replica plating. The colony is streaked onto a complete medium and allowed to grow afterward. A sterile velvet pad is placed onto the bacterial colony picking up some of the bacterial cells. The pat is then used to make a replica of a bacterial colony onto two different media, minimal media and minimal media supplemented with the nutrient that oxo traffic mutants require. What is the purpose of replica plating in identifying oxo trophic mutants in a bacterial colony? And our answer choices are a to maximize bacterial growth of all bacteria b to reduce the number of bacterial colonies c for better aeration and to promote mutation or d to allow comparison of the growth of bacterial colonies on different media. So let's just think through our scenario here, we have bacterial colonies that include wild type and mutant bacteria and they get streaked onto complete medium and that being complete medium, presumably all those different bacterial colonies wild type and mutant will grow. Then the velvet pad, velvet pad being pushed down on the bacteria is used to transfer bacteria in the same spatial arrangement. So wherever a bacterial colony is on the original plate, it will be in the same location on the new plate, which allows you to identify which colonies or which and those replica plates with the same spacial order. You have one with minimal media and one with minimal medium with specific nutrients. And your goal is to screen for an oxo trophic mutant that cannot synthesize one of those essential nutrients. So it will not grow on the minimum medium, but it will grow if the plate contains a specific nutrient it meets. So why do you do this replica plating? Well, let's look through our answer choices and think about this choice. A says to maximize bacterial growth of all bacteria. Well, this is not correct because when you put it out on minimal medium or minimal medium with just one nutrient, you are not going to maximize the growth of all bacteria only the bacteria that have the ability to synthesize those nutrients themselves. So choice A is not correct. Choice B says to reduce the number of bacterial colonies. Well, this will be an effect of doing this since some colonies are not a lot of colonies are not going to grow on the minimal medium, but that's not the purpose of what you're doing. So, choice B is not correct. And C was for better aeration and to promote mutation. Well, this is not the purpose. Um It doesn't make better aeration, doesn't make any difference to aeration. And mutation has already occurred. Your mixture already contained mutant copies mutant colonies. So choice C is not correct. So we're left with choice D to allow comparison of the growth of bacterial colonies on different media. And this is our correct answer because you'll be comparing the plates, the bacterial colony that grows on complete medium doesn't grow on minimal medium, but then does grow on the plate with a specific nutrient that allows you comparing those three plates, since the replica plate will have the colonies in the same geographical location. So you can match up the colonies over all three plates. Anything that grows allows you to determine which colonies have the oxo trophic mutation. And you need the minimal medium comparison since you want to make sure with the complete medium, since you want to make sure that the lack of growth in minimal medium is due to an inability to synthesize a nutrient, not just as some error in your growth process. So you'll make sure it grows on the complete medium so that you know nothing's wrong with your plate, nothing's wrong with your bacteria before you compare it to the minimal medium and the minimal medium with a specific nutrient. So again, what is the purpose of replica plating and identifying oxo trophic mutants in a bacterial colony. And that is choice D to allow comparison of the growth of bacterial colonies on different media. See you in the next video.

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Key Concepts

Auxotrophy

Ethyl Methanesulfonate (EMS)

Replica Plating