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Current time:0:00Total duration:11:58

Jacob Monod lac operon

Video transcript

so hopefully by now you're familiar with the central dogma of molecular biology that tells us that DNA makes RNA and a process known as transcription and RNA makes protein in a process known as translation let's take a look at two cells let's see 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 in ISIL and what makes the skin Scout 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 and ISIL in a skin cell skin cell is which genes are expressed in that cell so in the eye so we have the expression of genes that make certain proteins that are unique to nice oh 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 each cell is all transcribed into RNA and then all 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 UN's 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 that transcribed into proteins but only the specific eye proteins are activated and all the 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 transcribe if that DNA is a gene that codes for your 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 gene expression was regulated at the protein level again see 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 ourselves and actually thinking about it I don't even think there's room in one cell for all the proteins coded for in the genome but even if there was that would be a huge waste of energy and 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 regulate a gene expression through translation well it's more efficient than the protein level but it's still so not that efficient because that means we have a bunch of RNA that would never even get translated so making RNA it 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 but the name of one of them was Francois Jacob and the other one was Jacques Manoj and they discovered the mechanism of the lac operon so we call it the jacoba nod lac operon and 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 the various genes each of these colors represent a gene and we'll explain in a minute what they are if I wanted to be more exact maybe I really should have also color code at the top because they these two strands are complementary to each other but I'm not going to fill 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 usually you 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 ecoli break down lactose so let's take a look so over here we have these three genes they're 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 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 Lakha y and Lac a genes are all each going to code for an enzyme that helps in the breakdown of lactose or any metabolism of lactose so let's look at the lac z gene black 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 help some bring lactose into the cell and the lac aging 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 Aleksey 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 aging we have a stop region so that's those RNA polymerase stopped transcribing and normally ecoli uses glucose as its energy source that's the default however if glucose is not available or if it's suddenly inundated with lactose that we'll want to break down lactose but why should equal i express lac z Lac why and lack a in the absence of lactose alright 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 have a promoter site and the promoter site is the place that the RNA polymerase kind of just sips 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 just this theorem blocks transcription and so again this is the default situation that's in the cell 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 going to draw them as these little triangles although that does not adequately adequately represent why lactose looks like but for now it'll we'll just draw like that so we have a bunch of lactose floating in a cell so it's going to happen is that a lactose molecule will attach itself to the repressor protein this changes the conformation 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 lactose is well now while 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 conformation of the repressor and causes it to go back onto the operator site so let's 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 right 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 we're saving the energy but when we have lactose around these genes are expressed and we have the enzymes the enzymes necessary for the metabolism of lactose