The Genetic Code

Hi in this video we're gonna be talking about the genetic code. So nucleotides and amino acids are not translated in a 1 to 1 method, which means that for every nucleotide there's not one amino acid, right? Because then there would only be four amino acids because there's four nucleotides. But we know that's not true. Four nucleotides and 20 amino acids. And so a combination of nucleotides equals one amino acid. And we call that combination of nucleotides a code on. And this code in represents a triplet code. So The there are three. I mean I like this three nucleotides that code for one amino acid. And those three nucleotides are called a code on then because there's three. Right? So you have 123 and then you repeat it right over and over and over again. And this represents a code on. And these are representing nucleotides because there are three. That means there are reading frames. So you can read 123. And that's one reading frame. You can read it 231. And that's a second reading frame or you can read it 312. And that's a third reading frame depending on which nucleotide you start at. And so the reading frame, there's usually one correct reading frame and that determines which nucleotide is number one, which which one starts the code on. Now the triplet code has a bunch of different characteristics about it that you need to know the first is that it's non overlapping. And so that means that the three nucleotides represent one single code on, not multiple code on. So if we go back to our example of of the 12 threes, right? This is a code on. This is a code on. This is a code on And this is a code on. What is not a code on. Is that if this if we say that 123 represents a code on, then 231 cannot represent a code on as well. Only one of them can because it's non overlapping. So if this 123 is already being used for one code on, it's not going to be used in any other code ins so there can't be a code on here. A code on here. And a code on here, it has to be one of them. One of them is chosen and that's called being non overlapping. The second characteristic is that the triplet code is degenerate. And that means that some amino acids are coded by more than one coat on. So because there are four nucleotides and there's three positions. We can say that there's 64 different code owns. But there's only 20 amino acids. And so that means that some of these co din's encode the same amino acid because there's many more cardin's than amino acids. So that's the second that's called degenerate. The third thing is that it's nearly universal. So the majority of the organisms on earth use this code and it's the exact same code. There are some organisms and some actual organelles that make slight changes in this code but it's only slight its usual one or two code ons. That may be code for a different amino acid than the rest of the organisms on earth. But for the majority of organisms on earth like nearly universal, everyone uses the same triplet code and the same nucleotides encode for the same amino acids. And then finally the triplet code has to be started and stopped. So there are Stark Oden's that is A U. G. And that starts the translation and then you have stop code ons. And those are three here and that will stop translation. So here we have an example of what the triplet code looks like. So here you have R. N. A. And these are the nucleotides that they are. You can see there's code on one and each three of these nucleotides represents a different code on. And each one of these nucleotides and codes for an amino acid. Um Here's a stop code on. So you A. G. So that means that it doesn't encode for amino acid but it tells it to stop. And then there's not an example of it here. But some of these nucleotides some of code ins can actually encode for more than one. You know acid. Now most of this is a review. You you probably know the majority of this from an intro bio class. But what I want to talk to you about is how it was discovered and there are a few different experiments That discovered it and I'm just going to mention the two most important ones and the first that you'll read about in your book is called the bacteria page are 11 Locusts and that was studied by printer and we've actually talked about this before and I believe the linkage are mapping um videos that we talked about. But This is a this is a different focus essentially. And so um what you know is that the R. 11 is a gene and the bacterial genome. Now these bacteriophages will cause license of the bacteria that they infect and those results in Plex. If you don't have any idea what I'm talking about. Go back and watch the some of the bacterial phase are working with viruses videos that will explain some of these terms. Now um bacteria pages there are many different types and so Brenner if you remember from the other experiments worked with two different pages each which infected and lies a different strain of E. Coli. So half of his one of these pages infected a coli type And it sliced it the other kind infected a coli type K. 12 and it would infect it and cause license. And you can measure that you can look at it and if these are wild type viruses they infect their gonna burst cause license and create those plaques. Now at the time that Britain was studying this there had been mutations identified that prevented this slicing. So these were already mutant viruses they were already in laboratory settings and they prevented licensing. So what Brenner did to this is he actually added a chemical called pro flavin. So he got those mutant viruses and he added pro flab into them. And pro flavin is a chemical that causes a single nucleotide mutation. So he took the mutant viruses and he added a single mutant by adding this chemical onto it. So he created one more mutation and he found that some of the bacteria for ages that had been treated with pro flavin would actually revert is the term that he used and resemble wild type. It wouldn't be perfect it wouldn't be exact but it would be really really close to wild type. And so he called these reversion mutants or they had reverted from their mutant and they couldn't lies bacteria. Then after the single mutation they can live bacteria. And so he studied this a lot. He thought about it a lot. And what actually happened was the original mutation um disrupted the reading frame. And then pro flavin came in and corrected it. So what does that look like? So here we have our wild type. So here's the D. N. A. Sequence. Here's the M. R. N. A. After it's been transcribed and then here's the translated polyp peptide. And when this poly peptide has this and this is just an example this isn't actually the sequence in the bacteria of age. But I'm just giving you an example of it. So what happens is if the polyp peptide looks like this you get a bacterial page that will lies the bacteria. Now if you get the mutant form, what happens because there's actually some type of mutation that goes on. So here we have T. G. C. T. G. C. Well what we have here is T. G C. G. C. So we've missed there's a deletion here and T. Is missing. So when this gets transcribed it's also missing in the M. RNA. And that means when it's translated you're gonna have a different pollen peptide sequence. So you start out correct but then you have all of these different amino acids that aren't supposed to be there. And in this case this doesn't life bacteria because now you've messed up the reading frame right? So before you have T G C T G C T G C. And that's your reading frame but now you have T G C G C T G C T G C T. You're reading frame has been messed up. So when you add pro flavin which its whole purpose is to add another nuclear tied you get the same phenotype at the beginning, it doesn't necessarily fix it right away. But what happens is that it can add a nucleotide. And when it adds this nucleotide which in this case is a c It corrects the reading frame. So here's your M. RNA. But what you get for your after translation is you get the correct nucleotide, you get a couple of raw ones and then it replaces the reading frame and replaces the correct or the wild type polyp peptide sequence. And so this is the reverted form because it resembles wild type but it's not perfect right? It has a couple of different mutations here but it resembles wild type and therefore it can license bacteria again. And so this was one of the major studies that looked at, you know, just a single nucleotide can actually revert to the wild type version. And the only way it would do that is through this reading frame method. Now there was another big huge set of experiments that really decoded, allowed for decoding the triplet code. And so this was done by these two scientists. And what they did is they created these RNA and they synthesized them in the laboratory. And it was a new technique and they created these things called RNA polymers and it just repeated. Well only one nucleotide. So if we're just to skip to the example here here's RNA home a polymer. It has all your cells, right? Just repeated over and over and over again and they synthesized this in the laboratory setting. So then they actually put this into a coli and allowed a coli to create the protein made from this just repeating your sl. And what they found is that if you just have a repeating your sl molecule then it's just going to create a polyp peptide containing all phenomenon colony. So now they knew that this coded for this and so they did this with everyone. So they had an RNA polymer with a with G. With T. And they got all four of these codes. So what was a a what was G. G. G. And what was T. T. T. Then after they had that they needed to start doing combinations of nucleotides. And so that was called RNA hetero polymers. And they did they repeated two or more nucleotides over and over. And they did some more complicated things that are not going into that had to do with calculating the ratio of A S. T. G. S or T. S. Or how many ever nucleotides they added. But it's essentially using these methods they were able to actually decipher the entire triplet code what every single code on encoded for which amino acid. And that was huge because with that triplet code we can now make a lot of predictions about um which sequences would produce what type of proteins or poly peptide. So, super, super important. So with that let's now move on