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How An Operon Controls Transcription in a Prokaryotic Cell

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  • 0:04 Role of an Operon
  • 1:59 Anatomy of an Operon
  • 4:21 Transcriptional Repression
  • 6:14 Transcription Induction
  • 8:03 Lesson Summary
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Lesson Transcript
Instructor: April Koch

April teaches high school science and holds a master's degree in education.

Is gene regulation really as simple as flipping a switch? What are the parts of an operon, and how do they function to control gene transcription? We'll study the lac operon to answer these questions.

The Role of an Operon

Gene expressiongenes

Scientists first understood gene regulation when they studied E. coli, a type of bacteria that lives in our large intestine. This bacterium makes enzymes that help us digest our food, like the lactose in milk. E. coli, remember, is a prokaryote - an organism whose cells lack a nucleus. Prokaryotic DNA is clustered into groups of genes called operons. When scientists first studied the lac operon found in E. coli, they observed gene regulation through transcriptional repression and induction.

We've talked about repression and induction before. Repression describes a decrease in gene transcription, whereas induction is an increase in gene transcription. The lac operon of E. coli is induced in the presence of lactose, and it's repressed when lactose is gone. Sometimes we describe induction and repression as 'turning on' and 'turning off' the operon. We can think of the operon as being controlled by a switch. Lactose turns it on, and the absence of lactose turns it off.

But, what is the true mechanism for transcriptional control? We know there's not really a switchboard down there, microscopically managing everything that happens in the bacterial DNA. So, what's the real story? How does repression and induction actually work? And what about lactose? It's only a sugar. Can it really have that much power over the expression of bacterial genes?

This lesson will walk you through the main parts of an operon, describing how each of the segments work. We'll take a closer look at the lac operon and how it responds to repressors and inducers. Then, we'll revisit repression and induction for a thorough understanding of how an operon controls transcription in the prokaryotic cell.

Anatomy of an Operon

Alright, so here's the lac operon from E. coli.

Image of lac operon
lac operon

Like all operons, it's a cluster of related genes that code for similar things. We call the individual genes structural genes. A structural gene is any gene that codes for a structural protein or an enzyme. Remember, enzymes are just one kind of protein. The structural genes in the lac operon code for three different enzymes. The first enzyme is called beta galactosidase. The second one is galactose permease, and the third is called thiogalactoside transacetylase. Whew! Let's just call them Betty, Gail, and Theo. All three of them are in charge of breaking down lactose in the intestine. Because Betty, Gail, and Theo are usually needed at the same time, then their genes are all transcribed at the same time. The Betty gene, the Gail gene, and the Theo gene are all side by side in the lac operon.

Structural genes
Image of lac operon genes

Now, let's look at what we have just in front of the structural genes. There's a short DNA segment here, and it's called the operator. This is the part of the operon that acts as a switch for transcription. It's just like an operator that works the switchboard for a telephone company. The operator controls whether or not transcription will occur. It does this by providing a binding site for the repressor, which blocks RNA polymerase from attaching to the promoter. Wait a second. RNA polymerase? Promoter? Haven't we seen those terms before?

The operator controls transcription by providing a binding site for the repressor.
operator provides binding site

Recall that genetic transcription begins when RNA polymerase locates the promoter on the sense strand of DNA. RNA polymerase starts building the mRNA strand right after the promoter. In a prokaryotic cell, transcription continues through the entire length of the operon. If we want all the structural genes to be transcribed and translated, then we need RNA polymerase to first attach to the promoter.

RNA polymerase attaches to the sense strand of DNA to begin transcription.
Promoter Sense Strand Diagram

The promoter lives just in front of the operator, and the operator sits just in front of the structural genes. All of these together make up the operon. Is there anything else? Oh, yes. Sitting just upstream from the operon is a regulatory gene that codes for the repressor. Remember, a repressor is a protein that regulates gene expression by blocking gene transcription. The repressor is a type of regulatory protein, so it makes sense that it's made by a regulatory gene.

Main parts of an operon
Image showing entire operon

Transcriptional Repression

I think that all these strange new terms will begin to make sense when we see some examples. Let's go back to repression and induction. Earlier, when we talked about the lac operon, we said that repression is the blocking of gene expression in response to a repressor. But, how does a repressor actually block transcription?

To understand repressors, you have to know what they look like. Here's a repressor for the lac operon. It's a funny-shaped protein with a hole in its side and a lumpy arm sticking out in front. It also has little feet designed for sticking to DNA.

Image of repressor
image of shape of repressor

When the lac repressor is active, it sticks its feet to the operator, and its lumpy arm ends up hanging over the promoter next door. Now, the promoter's blocked. So, what does that mean? It means that poor old RNA polymerase can't come in and attach to the promoter. If it can't attach to the promoter, then it can't begin building the mRNA. No transcription means no translation. So, Betty, Gail, and Theo will never be produced. We won't have any enzymes to help break down our lactose. The lac repressor has succeeded in blocking transcription of our lac operon.

The repressor binds to the operator, blocking the RNA polymerase from binding to the promoter.
image of repressor blocking RNA polymerase

Now that we know more about repressors, let's go ahead and expand our definition. We can say that a repressor is a protein that turns off transcription by binding to the operator and blocking the attachment of RNA polymerase to the promoter.

Does it really matter to us that we can't produce our enzymes now? I mean, how much do we care about Betty, Gail, and Theo? They're just enzymes that break down the sugar lactose. Do we really need them hanging around? Well, if we plan on having dairy anytime soon, then yes. Remember, we owe it to the lac operon of the E. coli in our intestine to help us digest the sugar lactose. So, if any lactose gets into our system, then we'd better have a way to get that repressor off the operon.

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