Review of the Lac Operon & Trp Operon - Online Tutor, Practice Problems & Exam Prep

Review of the Lac Operon & Trp Operon

In this video, we're going to do a review of the lac and trp operons as we complete the table that you see down below. And so notice in this table, this column right here is focusing on the lac operon and this column over here is focusing on the trp operon. And so in terms of the operon type, recall that the lac operon is an inducible operon, and that is because it is normally turned off, but it can be turned on. It can be induced. And so this would be the on state, and over here, this would be the off state. So it's normally or initially off, but it can be induced. It can be turned on. Now the trp operon over here, recall, is a repressible operon, which, recall, means that it is normally turned on, but it can be turned off. It can be repressed, and so, that is why the trp operon is a repressible operon. Now in terms of the number of genes, recall that the lac operon has 3 genes, which are lacZ, lacY, and lacA. And the trp operon, recall in terms of the number of genes, it has 5 genes, which are going to be trpA, trpB, trpC, trpD, and trpE. Now in terms of the functions of the genes within the operon, the functions of the 3 genes within the lac operon are to metabolize lactose, or in other words, to break down lactose in order to get energy out of it. And the functions of the 5 genes in the trp operon are to synthesize tryptophan, the amino acid tryptophan, synthesize or build tryptophan. Now recall that the repressor gene in the lac operon is lacI, and the repressor gene in the trp operon is trpR. Now the regulatory molecule in the lac operon is a derivative of lactose. You could say a lactose derivative. And, for those of you that may need to know this, it is called allolactose, and it acts as an inducer molecule to help induce transcription of the operon. Now the actually tryptophan itself. And so tryptophan is going to act as a corepressor to help activate the trpR repressor protein. And so that takes us to the effect of the regulatory Recall that in the lac operon, lactose is going to bind to the active lacI repressor and inactivate the lacI repressor. And so we have, the effect is inactivates the repressor protein. So it starts off with an active repressor that will repress transcription. And then upon the lactose derivative binding, it inactivates the repressor. So we have an inactive repressor over here. But when we take a look at the effect of the regulatory molecule in the trp operon, tryptophan is actually going to activate the repressor protein. And so notice that the repressor protein starts off in an inactive state, but upon tryptophan binding, the repressor ends up in an active state. And so you can see these, are basically opposite effects, and that's why one of them is inducible operon, the other one is a repressible operon. Now when the regulatory molecule is absent, when the lactose derivative or this inducer is absent, of course, that means that it is going the repressor, lacI, is going to be in its active state. That means it will repress transcription, and the operon will be turned off in the absence of lactose. But in the absence of the regulatory molecule in the trp operon, in the absence of tryptophan, what happens is the repressor protein, trpR, is going to remain inactive, and that means that it will not be able to repress transcription. And that means that the operon will be turned on without tryptophan. And then, of course, looking at the regulatory molecule when it is present, when lactose is present, that's going to help turn on the trp operon. And so you can see here that, in the presence of lactose, it is going to inactivate the repressor. It will no longer be able to repress transcription, and so transcription will be able to proceed and the operon will be turned on. Whereas in the case of the trp operon, when tryptophan is present, it is going to activate the trpR repressor, and that allows it to repress transcription, and so the operon will then be turned off. And so, basically, what you can see here is that the inducible, the lac operon, and the trp operon are practically opposites in many of these different categories. And this table here helps to be able to do a side by side comparison of these two operons. And so, now that we've completed this table, we've completed our review of the Lac and Trp operons, and we'll be able to get some practice applying these concepts as we move forward. So I'll see you all in our next video.

Which of the following statements is FALSE?

• A. The lac operon is an inducible operon that is normally turned off.
• B. The trp operon is a repressible operon that is normally turned on.
• C. Lactose is the inducer molecule for the lac operon.
• D. Tryptophan is the activator molecule for the trp operon.
• E. All of the above are true.

Which of the following statements is TRUE?

• A. Lac I is the inducer molecule for the lac operon.
• B. Tryptophan is an inducer molecule for the trp operon.
• C. In the presence of lactose, the lac operon is expressed.
• D. In the presence of tryptophan, the trp operon is expressed.