An Hfr strain with the genotype cys⁺ lue⁺ met⁺ strˢ is m...

Question

An Hfr strain with the genotype cys⁺ lue⁺ met⁺ strˢ is mated with an strain carrying the genotype cys⁻ lue⁻ met⁻ strᴿ. In an interrupted mating experiment, small samples of the conjugating bacteria are withdrawn every 3 minutes for 30 minutes. The withdrawn cells are shaken vigorously to stop conjugation and then placed on three different selection media, composed as follows:

Medium 1: Minimal medium plus leucine, methionine, and streptomycin
Medium 2: Minimal medium plus cysteine, methionine, and streptomycin
Medium 3: Minimal medium plus cysteine, leucine, and streptomycin

The following table shows the number of colonies growing on each selection medium. The sampling time indicates how many minutes have passed since conjugation began.

Sampling Time (minutes) Number of Colonies
Plate 1 Plate 2 Plate 3
3 0 0 0
6 0 0 0
9 0 62 0
12 0 87 0
15 51 124 0
18 79 210 62
21 109 250 85
24 144 250 111
27 152 250 122
30 152 250 122

Suppose a fourth selection medium containing leucine and streptomycin is prepared. At what sampling time do you expect the first-growing colonies to appear? Explain your reasoning.

Accepted Answer

Hi, everyone. This is our next problem. Suppose you want to study the transfer of a plasmid containing a gene for amici and resistance from a donor strain to a recipient strain. Unlike the donor strain, which is resistant to ampicillin, the resist recipient strain is sensitive to ampicillin and lack the plastic to test the transfer of the plasmid. The researcher plans to perform an interrupted mating experiment. The researcher wants to determine the minimum time interval required for the transfer of the plasma to occur. What will you expect from the number of trans conjugates over time? Provided that the transfer of the plasma is successful. Choice. A there's a gradual increase. B there's a gradual decrease. C the number of trans conjugates should be constant or D, none of the options is correct. So let's think about what's going on here. Recalling that we do have that choice. D where none of our options would be the correct answer. So we have our donor or F plus strain which is ampicillin resistant. And we're looking at the transfer over time to our F minus or recipient stream which is ampicillin sensitive. So we know that colonies will grow on selective medium with ampicillin if the plasmid has transferred. So the any colonies that grow are trans conjugates, colonies that grow are trans conjugates. So the interrupted mating experiment of course, stops the mating at different periods of time. So what you will see is as time goes on, there should be an increase. So as time increases, there should be an increase in the number of colonies that grow because as time goes on more and more of the recipient strain has received the plasmid. And the very earliest time at which a colony can grow would be that minimum time interval for the transfer of the plasmid. But our question is saying, what would we expect from the number of trans conjugates over time? And that would be choice A there is a gradual increase. So there'll be a minimum time which is the earliest time at which transfer can occur. But as the time gets longer and longer, there will be more and more of a chance that the plasmid will have transferred to the recipient strain and therefore a colony can grow. So over time, you'd see an increase in those number of colonies. So choice A is our answer here. So we can rule out choice D, none of the options is correct. Choice B gradual decrease. Well, since it's we're transferring the ability to grow on that selective medium, we would see an increase, not a decrease. So choice B is not correct. And finally, choice C, the number of trans con conjugates should be constant. This is not our answer because our recipient strain will be gradually acquiring that ampicillin resistance. So we'll go from no trans conjugates to at the very shortest time interval possible. We'll start to see some and as time intervals go up, we'll see more and more. So choice C is not correct either. So again, what will we expect from the number of trans conjugates over time? Provided that the transfer of the plasma is successful? Choice? A there is a gradual increase. See you in the next video.

Key Concepts

Hfr Strains and Conjugation

Interrupted Mating Experiment

Selective Media and Auxotrophy

Watch next