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Jacob Monod lac operon

Video transcript

- [Voiceover] So, hopefully by now you're familiar with the central dogma of molecular biology that tells us that DNA makes RNA in a process known as transcription and RNA makes protein in a process known as translation. Well let's take a look at two cells. Let's say that right over here we have an eye cell and let's say that we have a skin cell. And so, what makes the eye cell an eye cell and what makes the skin cell a skin cell? So, they both have the exact same DNA because all the cells in our body or in any organism have the same DNA. And so what makes an eye cell an eye cell and a skin cell a skin cell is which genes are expressed in that cell, so in the eye cell we have the expression of genes that make certain proteins that are unique to an eye cell and in a skin cell, we have genes that are expressed and they make proteins that are unique to a skin cell. And so the question I want to focus on is how does that happen? How do we regulate the expression of genes so that only those proteins that are necessary for the cell get expressed or are made. So let's just frame our question. How is gene expression regulated? So, let's look at the options that we have. Maybe gene expression is regulated at the protein level. What that means is that the DNA in each cell is all transcribed into RNA and then all of the RNA gets translated into proteins. So, according to this model, you'd have in each cell all of the proteins coded for by the entire human genome, if we're talking about humans, but then only those proteins that are necessary for the cell get activated. So, for example, in the eye cell, all the DNA gets transcribed into RNA and they all get transcribed into proteins, but only the specific eye proteins are activated and all the other proteins are inactivated. Maybe that's what happens. Maybe gene expression is regulated at the level of translation. That would mean that all the DNA in the cell is transcribed into RNA, but not all the RNA gets translated into protein. Just the RNA that make proteins of that particular cell would get translated. And the third option we have is maybe gene expression is regulated at the level of transcription. And that would mean that not all the DNA gets transcribed into RNA. Only the DNA that codes for proteins for that specific cell would get transcribed into RNA. So, for example in the eye cell, you'd only have DNA transcribed if that DNA is a gene that codes for a particular eye protein. And the answer to our question is that usually gene expression is regulated at the level of transcription. And if we think about it, this should make sense, because this is really the most effective way for a cell to make use of its resources. So, let's take a look again at our, you know, if gene expression was regulated at the protein level. Again, say we're talking about humans, ourselves. So, that would mean that in each of our cells, all the DNA gets transcribed into RNA and then all of that gets made into protein. So, we would have a tremendous amount of protein in our cells and actually thinking about it, I don't even think there's room in one cell for all of the proteins coded for in the genome. But even if there was, that would be a huge waste of energy. It takes a lot of ATP to put together proteins and so why would we want each cell to make a whole bunch of proteins that they'll never even use. So this is not very efficient. What about translation? If we regulated gene expression through translation? Well, it's more efficient than the protein level, but it's still also not that efficient because that means we have a bunch of RNA that would never even get translated, so making RNA doesn't take as much energy as making protein, but still, that would be a big waste of energy. And so it turns out that regulating gene expression at the transcription level is the most efficient because we're not making any RNA or any protein that we're not going to use. And we actually have a lot more to learn about how gene expression is regulated, but there's a particular model that we understand pretty well thanks to the work of two French scientists by the name of, one of them was Francois Jacob and the other one was Jacques Monod. And they discovered the mechanism of the lac operon. So, we call it the Jacob Monod lac operon. And the lac stands for the word lactose and the lac operon is found in the bacteria e. coli so it's a prokaryotic cell. And the picture that you're looking at is a sketch of the lac operon. It's a section of DNA in e. coli and let's just label the diagram so that we orient ourselves. So, let's say that this is the coding strand. Which means that this is the non-coding, or the template strand. And if you recall, it's actually the non-coding, or template strand, that gets transcribed, and that's the reason that I color coded the non-coding strand with various genes. Each of these colors represent a gene and we'll explain in a minute what they are. Now if I wanted to be more exact maybe I really should've also color coded the top because these two strands are complementary to each other. But I'm not gonna fill that out. Just use your imagination and remember these are complementary and I also just want to point out that I drew this transcription bubble because it's going to be easier for me to show you what's going on in that way, but the default is that these two strands are really stuck together and you usually do not have this bubble forming unless transcription is happening. So, just keep that in mind as we go along. Okay, so what is this lac operon? So, before we talk about the details, the lac operon has a couple of genes that will make enzymes that help e. coli break down lactose. So, let's take a look. So over here we have these three genes. They are called structural genes. It's not important for you to remember that. But, these three genes, this here is the lac z gene. This is the lac y gene, and this is the lac a gene. And so if you recall, the sugar lactose gets broken down into glucose, and galactose. So glucose and galactose are monosaccharides and lactose is a disaccharide. And the lac z, lac y, and lac a genes are all each going to code for an enzyme that helps in the breakdown of lactose, or in the metabolism of lactose. So, let's look at the lac z gene. The lac z gene codes for a protein, beta galactosidase. And beta galactosidase is the enzyme that actually breaks lactose down into glucose and galactose. The lac y gene codes for the enzyme lactose permease. And lactose permease helps the cell bring lactose into the cell. And the lac a gene also codes for an enzyme that helps in lactose metabolism. We just won't focus on it because it's not as important as the lac z and lac y gene. So, these genes are all needed for the metabolism of lactose and let's just label them, this part over here, right before these three genes, that's the start site. So, if RNA polymerase was transcribing, that start site tells it here's where you should begin to transcribe and then after the lac a gene we have a stop region so that tells RNA polymerase, stop transcribing. And normally, e. coli uses glucose as its energy source. That's the default. However, if glucose is not available or if it's suddenly inundated with lactose it will want to break down lactose. But, why should e. coli express lac z, lac y, and lac a in the absence of lactose, right? We just explained before that would be a huge waste of energy. So, the default situation is that these genes are not expressed. Let's see how that is. So over here we had a promoter site and the promoter site is the place that the RNA polymerase kind of just sits. So let's label our RNA polymerase. That's the enzyme that puts together RNA. So it just sits there. And after the promoter site, we have the operator site. And on the operator site, there is a protein that also just sits there, and this is called a repressor. And you can see the repressor is kind of blocking the RNA polymerase, so normally, the RNA polymerase would want to proceed in this direction and transcribe this entire area of the DNA, but the repressor doesn't allow it to do that. It just sits there and blocks transcription. And so, again, this is the default situation that's in the cell. It uses glucose as an energy source and transcription of these genes is blocked. Well, what happens when there's suddenly a lot of lactose in the cell? So let's just draw some lactose molecules. I'm gonna draw them as these little triangles although that does not adequately represent what lactose looks like, but for now it'll, we'll just draw it like that. So they have a bunch of lactose floating in the cell. So what's going to happen is that a lactose molecule will attach itself to the repressor protein. This changes the confirmation of the repressor protein somewhat. And that causes the repressor to come off the operator site. So let's just get rid of our repressor. And let's put it, well, over here, for now. Together with the lactose that was attached to it. Now, the path of RNA polymerase is open, so it moves in this direction and transcribes all these genes. We make beta galactosidase, we make lactose permease and we have all the enzymes that we need to metabolize lactose. Well, what happens when the level of lactose goes down and we broke down all of our lactose? So let's get rid of some of our lactoses. Well, now, well, they're all taken care of including this lactose that was attached to the repressor and when the lactose comes off of the repressor, it changes the confirmation of the repressor and causes it to go back onto the operator site. So let's put him back where he was. Now, you can see the RNA polymerase again is blocked and so there is no more transcription happening right over here. So let's just connect this to what we spoke about in the beginning. We said that the regulation of gene expression happens at the level of transcription. We only transcribe those genes that we need and this is exactly what's happening over here. In the absence of lactose, these genes are not expressed and receiving the energy. But when we have lactose around, these genes are expressed and we have the enzymes necessary for the metabolism of lactose.