Hi, in this video we're going to be talking about the genetic code. So, nucleotides and amino acids are not translated in a one-to-one method, which means that for every nucleotide there's not one amino acid. Right? Because then there would only be 4 amino acids since there are 4 nucleotides. But we know that's not true. There are 4 nucleotides and 20 amino acids, so a combination of nucleotides equals 1 amino acid. And we call that combination of nucleotides a codon, and this codon represents a triplet code. So there are 3 nucleotides that code for 1 amino acid, and those 3 nucleotides are called a codon. Then because there are 3, you have 1, 2, 3, and then you repeat it over and over again, and this represents a codon, and these are representing nucleotides. Because there are 3, 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 1, which starts the codon.
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, meaning that the 3 nucleotides represent 1 single codon, not multiple codons. So if we go back to our example of the 1, 2, 3's, right, this is a codon, this is a codon, this is a codon, and this is a codon. What is not a codon is if we say that 123 represents a codon, then 231 cannot represent a codon as well. Only one of them can because it's non-overlapping. If 123 is already being used for 1 codon, it is not going to be used in any other codons. So there can't be a codon here, a codon here, and a codon here, it has to be one of them. The second characteristic is that the triplet code is degenerate, which means that some amino acids are coded by more than 1 codon. So because there are 4 nucleotides and there are 3 positions, we can say that there are 43=64 different codons, but there are only 20 amino acids. And so that means that some of these codons encode the same amino acid because there are many more codons than amino acids.
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 usually 1 or 2 codons that might code for a different amino acid than the rest of the organisms on Earth. But for the majority of organisms on Earth, nearly universally, everyone uses the same triplet code, and the same nucleotides encode the same amino acids. Finally, the triplet code has to be started and stopped. So there are start codons, AUG, that starts the translation. And then you have stop codons, and those are 3 here, which will stop translation. So here we have an example of what the triplet code looks like. You have RNA, and these are the nucleotides that they are. You can see there's codon 1, and each of these three nucleotides represents a different codon, and each one of these nucleotides encodes for an amino acid. Here's a stop codon, UAG, so that means that it doesn't encode an amino acid, but it tells it to stop. And then there's not an example of it here, but some of these nucleotides, some codons can actually encode for more than one amino acid.
Most of this is a review. 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. The first that you'll read about in your book is called the bacteriophage r11 locus, and that was studied by Brenner. We've actually talked about this before, in the linkage or mapping videos that we talked about, but this is a different focus essentially. And so, what you know is that r11 is a gene in the bacteriophage genome. Now, these bacteriophages will cause lysis of the bacteria that they infect, and those result in plaques. If you don't have any idea what I'm talking about, go back and watch some of the bacteriophage or working with viruses videos which will explain some of these terms. Bacteriophages, there are many different types, and so Brenner, if you remember from the other experiments, worked with two different phages, each of which infected and lysed a different strain of E. Coli. So half of his, one of these phages, infected E. Coli type B, and it lysed it. The other kind infected E. Coli type K12, and it would infect it and cause lysis, and you can measure that. And you can look at it, and if these are wild type viruses as they infect, they're going to burst, cause lysis, and create those plaques.
