In this video, we're going to continue to talk about the lac operon, specifically in the absence of lactose. Before we move on, recall from our previous lesson videos that the lac operon is only going to be on or expressed when those genes are needed. Recall that the genes in the lac operon, lacZ, lacY, and lacA, are needed to metabolize lactose and break it down in order to get energy from it. However, if lactose is absent, if there is no lactose in the environment, then those genes in the lac operon are not really needed because there's no lactose for those genes to break down. We can imagine that this means that the lac operon would be off or inactive in the absence of lactose, and that's exactly what we're going to see here.

When lactose is not available to metabolize because it's not available in the environment, then the repressor protein, lacI, is going to repress the expression of the genes in the lac operon. LacI, this repressor protein, is going to bind to the lac operator and block RNA polymerase, which, recall, is the enzyme responsible for transcription. It's going to block RNA polymerase and prevent RNA polymerase from initiating transcription. There will be no transcription because it is being blocked and inhibited.

If we take a look at our image down below, we can get a better understanding of the lac operon in the absence of lactose. In the absence of lactose, the repressor protein lacI is going to be active, and because it's active, it's going to bind to the operator and block transcription, preventing transcription and turning off the operon in the absence of lactose. Here, taking a look at this image, again, we're focusing on the lac operon in the absence of lactose. There's no lactose available in the environment.

Taking a look here at our image, notice we've got our lac operon over here, and we've got the lac regulatory gene over here, which has the lacI gene with its own promoter. And of course, the lacI gene with its own promoter is going to be transcribed and translated. In the absence of lactose, the lacI repressor protein is going to be active. Here we have an active lac repressor, and because it is a repressor and it is active, it is going to bind to the operator. You can see that the active lacI repressor is binding to the operator, and when it binds to the lac operator, as mentioned above in the text, it's going to block RNA polymerase and prevent RNA polymerase from binding and initiating transcription.

Transcription of these genes is not going to occur because the lacI repressor is bound and blocking the RNA polymerase in the absence of lactose. This is good for the cell because when there is no lactose available, when lactose is absent, then the cell has no need to express these genes because these genes are all involved with breaking down lactose. There is no lactose, so it's not going to waste energy in producing the products of these genes. This is why, in the absence of lactose, the lac operon is off or inactive and will not be expressed. This conclusion introduces what happens when the lac operon is in the absence of lactose. As we move forward, we'll be able to talk about the lac operon in the presence of lactose. So, I'll see you all in that video.