The locations of numerous lacI⁻ and lacIˢ mutations have been determined within the DNA sequence of the lacI gene. Among these, lacI⁻ mutations were found to occur in the 5′-upstream region of the gene, while lacIˢ mutations were found to occur farther downstream in the gene. Are the locations of the two types of mutations within the gene consistent with what is known about the function of the repressor that is the product of the lacI gene?

Video transcript

Hi everyone. Welcome back. Let's look at our next question. If a cell has an F. Factor copy of the laC opera on that has the eye superscript S mutation and a genomic copy with the O superscript C mutation, what would happen to this cell if lactose was present? Well this question kind of throws a lot of us. We have to think through this. Let's recall from our content video that the F factor is a plasma used to introduce D. N. A. Into bacteria. In this case scientists can study the interaction of these two different mutations by creating a partial deployed situation in the cell. You had your genomic DNA that was there in the bacteria. It has this particular mutation and then you introduced a second copy of that lack opera on with the F factor gets incorporated into the cell and that copy of the black opera has this I. S. Mutation. So we need to think about how these two mutations will affect combine and affect what goes on in the cell when lactose is present. So we'll start with just recalling what's the usual situation of lactose is present. Usually if lactose is present some of that lactose gets I summarized into a low lactose that binds to the repressor protein, the lack professor protein and the lac repressor is released. That allows transcription to take place. The lack Professor of course was bound to the operator region when it's released that unlocks the promoter when their oppressors browned. The operator of course it physically blocks the promoter. Once the lac repressor has moved away then the RNA polymerase can come in and transcribe the black jeans. So usually when lactose is present that releases the repressor from its binding site on the operator. So remembering that let's look at our situation with our mutations here. So first let's look at our I. S. Mutation on the F. Factor copy. We recall that the lack I. Gene is not one of the genes that produces the enzymes that digest lactose. The lack I. Gene is the gene for the lac repressor protein. So this I. S. Mutation causes a defect in the repressor protein that prevents a lactose from binding to it. So the result of that is if a lactose cannot bind the oppressor, the oppressor is going to stay bound to that operator that will block transcription of the genes so that the end result there is that the opera on will remain off even when lactose is present. Since the lactose, the aloe lactose cannot bind the oppressor. The oppressor can't be removed from its binding site. So that's the result of the I. S mutation. Let's look at R. O. C. Mutation on the genome A copy and see what its result is. Well the O. C mutation we have O. For operator is a mutation to the gene for the operator that impairs its finding to the laC repressor. So that means the laC repressor protein cannot block transcription. It doesn't bind the operator RNA polymerase has nothing getting in its way. The lack genes will just be continuously transcribed no matter what. So the result of the O. C. Mutation is that the opera on remains on. Even when there's no lack tous since that repressor protein cannot bind. So let's think of how these two mutations interact together, remembering that we're thinking about what will happen when lactose is present. Well, if the repressor cannot bind to the operator, it doesn't matter what's happening with the repressor protein, doesn't matter if it's normal, doesn't matter if it's mutated. The operator is not able to bind to it. So that means with both of these together, the opera on is continuously on. Since that repressor protein cannot bind at all. So in this case well lactose is present, it would be on anyway but regardless of the lactose level. So that's the sum total of both of these working together. Let's look through our answer choices and evaluate them, noting that we have choice D all of the above as an answer choice. So choice A. Says the lac Z. Y. A jeans in the genomic laptop Arran will be continuously expressed in this cell. Well those lack Y. Z. Y. A jeans are under the control of the operator and as we said with these two mutations together, the operation will continuously be on the oppressor will not be able to bind and therefore yes these genes will be continuously expressed no matter what. So this is correct but will recall that we have that all of the above choice. So we're not going to pick it as our answer just yet. Mark just mark that is correct. Choice B says the lack Y gene won't be translated and transcribed while black Y. Gene is one of those genes under this control. And as we saw the lack opera on is going to be continuously on. All those genes will be transcribed. So this is incorrect. We're gonna cross that out. That also allows us to eliminate choice B. Since not all the answers are correct and therefore we can go ahead and select A as our answer here. But just to be thorough we look down at choice C choi C says the lack I gene will not be transcribed. Well in this case we recall the lack I gene is not part of those genes for the lactose digesting enzymes. The lack I gene is for the lack of repressor protein. It's in its own separate location. It's not under the control of this opera on. It's not regulated and in fact it's continuously um transcribed at a low level. So that is not correct because it's going to keep on getting transcribed regardless of what's going on with those mutations. So finally we'll look back again if the cell has an F factor copy of the black operation with the I. S. Mutation and the genome a copy with the O. C mutation. What would happen to the self lactose was present? Choice A, the lac Z. Y. A. Jeans and the genomic lack opera on will be continuously expressed in this cell. See you in the next video.