Lac operon how does it work

If lactose is absent, then the repressor binds to the operator to prevent transcription. If either of these requirements is met, then transcription remains off. The cell can use lactose as an energy source by producing the enzyme b-galactosidase to digest that lactose into glucose and galactose. Only when both conditions are satisfied is the lac operon transcribed, such as when glucose is absent and lactose is present.

This process is beneficial and makes most sense for the cell as it would be energetically wasteful to create the proteins to process lactose if glucose were plentiful or if lactose were not available. Learning Objectives Describe the components of the lac operon and their role in its function.

Key Points The lac operon contains an operator, promoter, and structural genes that are transcribed together and are under the control of the catabolite activator protein CAP or repressor. The lac operon is not activated and transcription remains off when the level of glucose is low or non-existent, but lactose is absent.

The lac operon encodes for the genes needed to utilize lactose as an energy source. Key Terms operator : a segment of DNA to which a transcription factor protein binds repressor : any protein that binds to DNA and thus regulates the expression of genes by decreasing the rate of transcription. An operon is a functioning unit of genomic DNA that contains a group of genes controlled by a single promoter. The lac , or lactose, operon is found in E.

This operon contains genes coding for proteins in charge of transporting lactose into the cytosol and digesting it into glucose. This glucose is then used to make energy. The lac operon is exploited by molecular biologists, so we better understand how it works. Normally, the lac operon is turned off. A repressor protein binds the operator control region upstream of the operon preventing transcription. When lactose is present outside the cell, it crosses the cell membrane and acts as an inducer of the operon.

It does so once lactose is broken down to create allolactose. This leads to a positive feedback loop. This operon takes the stage when glucose levels are low. When the Allolactose is present, it will bind to the repressor, and then the repressor is going to leave the operator site. It's not going to be able to bind as well, and so let me draw that. So in this case, the operator, sorry, the repressor I should say.

The operator is where the repressor binds. So this is the repressor right over here. You have some Allolactose. We do that in white. You have some Allolactose that has bound to it, and because of that, it's not going to bind to the operator, and since it's not bound to the operator, well now, the RNA polymerase can actually transcribe these genes, and that's valuable because by transcribing these genes, we are going to be able to metabolize this lactose.

So lactose present, you have transcription. Transcription occurs. Now that's a very high level simple view of the lac operon, but there's more involved, because there's other sugars, in particular glucose, which is preferred by the cell. So, whoops, moving the wrong part. There you go. So let's think about what will happen in the presence of glucose and not in the presence of glucose. So let me write here. So glucose, and no glucose, actually let me do it, I'll do no glucose first.

So let's see, we have no glucose. And remember, glucose is preferred to lactose. Simpler sugar. If you have glucose around, why worry about the lactose?

And then here we have glucose. We have glucose around. And we could talk about both of these situations in the presence of lactose or not in the presence of lactose, but if we don't any lactose around, then were not gonna have the Allolactose around, and then you're just gonna have the repressor sit on the operator, and you're not going to have any transcription, and that's going to be whether or not we have glucose.

So I'm gonna think about no glucose, but we do have lactose, plus lactose, and in here, you have glucose plus lactose. Well, the lactose part, if we have lactose around then we're going to have the Allolactose around, and we just covered this scenario. The Allolactose binds to the lac repressor, keeps the lac repressor from binding to the operator, and so you have your RNA polymerase is able to actually perform the transcription.

But that's not it. In a situation with no glucose, you actually are going to also involve the CAP site. You're going to have an activator that's going to make this happen even more, because if you don't have glucose around, man, you really need that lactose.

And so, what you have is something called, so let me draw this, the Catabolite Activator Protein, right over here. You'll see that come up a lot in biology. In that presence, it is going to bind to this, the CAP site, and it is going to further activate the transcription. So in this situation, no glucose plus lactose, you're going to have even more transcription. So let me write this down. Lots of transcription.