For the genotypes and conditions (lactose present or absent) shown in the following table, predict whether functional enzymes, nonfunctional enzymes, or no enzymes are made.

Video transcript

Hi everyone. Here's our next question. It says what parameters will result in the optimum transcription of the LAC opera on And when we look over answer choices, we see there are different combinations of the presence and absence of glucose and lactose. Well, if we can't remember this off the top of our heads, um we're going to think about how these two factors affect the transcription of the black opteron. But a good way to start with that. If you're trying to recall this for a test is to think about what is the lack of opera and do and what would be its goal? So lack opera of course produces causes the transcription of the genes that help a bacterial cell to digest lactose and lactose is used as an alternate less preferred source of energy than glucose we recall. So obviously the conditions where you'd want to produce um the enzymes to digest lactose would be when you have lactose present. Because why would you waste energy producing those if there's no actress around and when glucose is absent because if glucose is present, you'd want to sell using that more efficient energy source rather than bothering to digest lactose. So if we keep those things in mind that can help us remember how they affect the transcription of the black Opera on. So we know that lactose and more specifically, it's a low lactose. Aloe here in parentheses, binds to the lac repressor protein and when it binds to the lac repressor protein, This protein is released from its finding to the operator. Now let's recall that when the black opera is an induce a ble opera on its default state is off. So when that lac repressor is in that bound state on the operator um it blocks transcription of those genes since the RNA polymerase cannot bind to the promoter when that repressor is present. So when lactose is present, some of its summarized into a low lactose binds to the lac repressor that releases it from its been into the operator, which then means that RNA polymerase combined to the promoter with the result that transcription can go forward. However, a transcription will go fairly slowly. I think the transcription goes under these circumstances that already preliminaries doesn't have a great binding to the promoter. So now we want to look at our other factor in transcription and that would be the absence of glucose and that's part of the positive regulation of that lack operation. So in glucose is absent, then C A. M. P levels are high. C A M P binds to the protein cap. And that then optimizes the binding of RNA polymerase. So we said that the army preliminaries cannot bind very well by itself to the promoter when cap is present and binds RNA polymerase binds more tightly and then we have high transcription levels. So when we're looking for what is the optimum transcription. Well it won't happen at all if lactose is absent. So we need lactose present. So let's look for those two choices which would be B or see here. But again, with lactose present, you will have transcription but just at a low level so that we need the absence of glucose which allows that optimal binding of RNA polymerase to have a high level of transcription. So you need glucose absent at the optimum transcription levels. So that means we can eliminate and we've already eliminated choice A. And choice D. Now we can eliminate choice B. Since that says glucose present and we would choose choice C glucose absent, lactose present. And again we double check. That makes sense with our understanding of what the lack operation is for. So that first kind of work through could give you kind of almost a shortcut to the answer if you were working quickly. But this is the way to think all the way through what's happening. That would mean these conditions would optimal eyes the transcription of it. So what parameters resulting in optimum transcription of the lac opera? Choice C, glucose absent lactose present in the next video