Contrast positive versus negative control of gene expression.

Video transcript

Hi everyone welcome back. Let's look at our next problem. It says the lack operandi exhibits and when we look at our answer choices um This actually shows us why it's important to kind of always look over the answer choices first and see what they are. Um Because we'll see that choice A. Is positive control of gene expression. Choice B. Is negative control of gene expression. But choice C. Is both positive and negative control of gene expression. So there exists the possibility that both A. And B. Are correct. So we want to notice that because that will prevent us from say seizing on if both are correct seizing on one of those picking that is our answer and moving on and then of choice. Choice D. Is neither positive nor negative control of gene expression. So let's think about our lack opteron um to evaluate which it is. And recall from our content video that the lack operation regulates the expression of the lack genes that allow bacteria to digest lactose. And for that regulation it wants to ensure that the lack genes are only expressed under two conditions. One is lactose present because obviously you don't want to expend the energy and resources to make uh genes make proteins to digest lactose. If there is no lactose and then finally you want glucose to be absent. Glucose is a better source of energy more efficient than lactose. So you don't want the bacteria wasting time on lactose if glucose is available. So to ensure that both of these conditions are met. Um The lack opera does exhibit two forms of control over gene expression. Um It does exhibit both positive and negative control. So I'm gonna go ahead and highlight that as our answer. But let's think about um what ways those both are exhibited when it comes to the lack opera on. So first we have positive control. Bring that over here and that comes about through the protein cap metabolite activator protein. And that's kind of a clue there that a and the activator. Because activators are the proteins through which you see positive control. So the cap protein helps regulate the genes only being expressed when glucose is absent when glucose is absent. We recall C A. M. P is released. This molecule binds to cap that allows it now to bind the cap binding site which is next to the promoter that enhances RNA polymerase binding which allows high gene expression. So without that there the gene is expressed only in low levels but with cap bound there are in a preliminary spines tightly. And you can see high levels of those genes being expressed. So that's the method of positive control. So that's a then think about how um negative control is seen in this this over here. So the form of negative control. You have the lac repressor. And again similar to how we had the clue of the activator protein telling us we had positive control repressor tells us that we have negative control. And the press the laC repressor binds to the operator when it is bound there physically blocks the promoter site. So that prevents RNA polymerase from binding there. Which means there's no gene expression of those black jeans. So it actually prevents the expression of the genes through the laC repressor protein. So again we see positive control through cap and negative control through the laC repressor protein. So our correct answer is C. Both positive and negative control of gene expression. So A and B are incorrect since they're not complete answers again. Um we want to be careful of that that we didn't select one of those and move on. And then of course he is incorrect since it says neither positive nor negative. So again, lack opera exhibits choice. See both positive and negative control of gene expression. See you in the next video.