hello and welcome to learn a level biology for free with miss estrich in this video i'm going to be going through the features of the genetic code if you are new here then make sure you click subscribe to keep up to date with all the latest videos so first of all there's three key features of the genetic code that you need to learn about for aqa biology that is it's degenerate universal and non-overlapping and we're going to go through each of those three terms individually a couple of other things i'm going to point out as well straight away the start and stop codons and a start codon is the three bases at the start of a gene on dna and therefore it's copied onto mrna as well you don't actually need to know what those three bases are they are tac on dna and therefore they get copied into aug on mrna and this codes for the amino acid methionine but that later gets removed now you don't actually need to know as i said the three bases or the amino acids that it codes for the key thing that you need to know is the start codon is the first codon so triplet of bases on the dna and mrna and it initiates translation the stop codon in contrast is the final three bases on the end of dna and therefore it's also copied onto mrna again you do not need to know the different stop codons and there are actually three it's these three at the bottom on dna and it doesn't actually code for an amino acid this time but what you do have to know is what a stop codon does and what it is able to do is because it does not code for an amino acid there is no complementary anticodon with a particular amino acid so as a result in translation which is the second stage of protein synthesis when the ribosome is moving along the mrna when it reaches the stop codon it causes the ribosome to detach and therefore translation stops so that's why it's called a stop codon because in that way it means translation can no longer continue and therefore the synthesis of that one polypeptide chain coded for by that one gene is complete so if we go through those three features then we're going to look at degenerate first and the first thing you need to be aware of is the fact there are 20 different amino acids and the genetic code is able to code for all of those 20 amino acids so we need to look at how that is possible and there are four dna bases guanine cytosine thymine and adenine and therefore it has to be that the genetic code is three bases out of those four coding for the different combinations to create at least 20 codes and you can prove this mathematically using the formula four to the power of n four because we have four options of possible bases n is referring to the number of bases that make up your code so i'm going to show you how this was discovered mathematically and that will then finally lead us to what we mean by degenerates so if it was that the genetic code was just one base coded for one of the amino acids that'd be four to the power of one and therefore there's only four possible combinations there's only four codes that means you could only code with four amino acids and that is insufficient to code for 20. if it was two bases in the code that would be four to the power of two which would give us 16 possible combinations and again that is insufficient to provide 20 codes for amino acids so then finally if we get to three bases that would be four to the power of three and that is 64. so that means the genetic code with four possible bases involved and a combination of three of those bases each time you are able to come up with 64 different combinations of codes and that would mean it could code for 64 amino acids and that is more than you need that is enough to code for the 20 amino acids that are present so that's one of the ways that they realize that the genetic code is three bases codes for one amino acid however as i said 64 combinations is more than the 20 that are needed to code for amino acids and this then results in the first feature of the genetic code the fact that it is degenerate and what that means is each of those 20 amino acids are coded for or most of them are coded for by more than one triplet of bases so there's more than one code to code for each amino acid so i've given one example here so tyrosine is coded for both of these two triplets ata and atg and i've put this genetic wheel over on the side just so you can have a look at some of the other options and it's worth noting at this point you do not have to know the names of any of the amino acids or any of the codes which code for them for aqa what you do need to know is how you would use a genetic code wheel or table so if you were given the wheel the way it works is you start in the middle and you read outwards so for example g c c that triplet code codes for alanine or ala for short so you can use the wheel to work out a triple of bases which amino acid it codes for or you can do the opposite you might be looking at serine scr and wanting to know which triplets code for searing so a g c and a g u and with the u straight away hopefully you've realized that these genetic code wheels are the mrna codes so the codon is on the mrna it's not for the dna so you would need to just work out if you were asked to give the dna code work out what the original dna was so that is what degenerate means there's more than one triple of bases which codes for the same amino acid and we've looked at the mathematical reason behind that now this is an advantage because what that means is if there was a point mutation meaning just one base was changed then it would still potentially code for the same amino acid so for example in this tyrosine example if there was a mutation a substitution mutation where this adenine was substituted for guanine because the genetic code is degenerate it would still code for tyrosine and therefore even though mutation had occurred it will have no effect on the final protein so we call it a silent mutation so that's why this is such an important feature the next feature universal what this means is the same triple of bases so on this genetic code wheel will code for the same amino acid in almost every living thing so that's what we mean by universal this genetic code is exactly the same whether you are an animal plant bacterium and this advantage has been incredibly helpful in gene technologies because this is why genetic engineering has been possible between different species so for example in the creation of the mass production of insulin using bacteria that is possible because when you remove the human gene for insulin and insert into bacterial dna to make the insulin because the genetic code is universal the bacteria is able to make exactly the same protein insulin so the last feature is non-overlapping and what this means is each base in a gene is only part of one triple of bases that codes for an amino acid and i'm just going to put boxes around to indicate what we mean by that so this adenine is only part of this triplet here this cytosine is only part of this triplet and this guanine is only part of this triplet so each triplet is read as a discrete unit there's no overlap and by that what we mean is acg is one triplet then we don't overlap any of those bases we have to go on to the next set of three gct next set of three tca in comparison to if we went acg then overlapped and said c g g was the next triplet overlapped again and said g g c it was the next triplet so the exact language which uses that each base is only part of one triplet and they're read as discrete units and the advantage of this is if there is a point mutation and this time it does result in a different amino acid being coded for at least it's only affecting one triple of bases and therefore one amino acid so it should hopefully minimize the impact on that polypeptide chain whereas if it was an overlapping code it would affect multiple triplets of bases and therefore multiple amino acids could potentially be coded for incorrectly and that would be a much more drastic mutation having a bigger impact on the final protein structure so those are the three features last couple of things linked to the genetic code and we're going to go through and one of them is introns versus exons this is just quick definitions that you need to know so introns are sections of dna that actually don't code for amino acids and over 90 percent quite a lot over 90 percent of your dna are introns and sometimes you might see introns being described as junk dna and the reason about 98 of your dna is junk is because it does actually end up getting cut out and there are important features of this intro of these introns but you don't actually learn about them at a level if you are interested to research further then research alternative splicing and that's the reason that's the importance of them so introns are sections of dna that don't code for amino acids you only find them in eukaryotic dna you do not find them in prokaryotic dna and they aren't in mrna because they get spliced out before you get your final mrna and then you are just left with exons and exons is the term given for the sequences of bases which do code for amino acids final idea that links to this topic is the genome compared to the proteome and you could be asked for the one mark definition of either of these or you could be asked to compare these two terms so the genome is an organism's complete set of dna within one cell so it's all of the genes that that organism has in contrast the proteome is the full range of proteins in one cell so the gene names the full range of genes proteome is the full range of proteins in a cell so some of the differences your genome should never change the only reason it would is if a mutation occurs whereas the proteome of a cell does change quite often and it will change between cells because specialized cells will be creating different proteins and also your cells will respond by producing which proteins they currently need and that means that although you have the genes for every protein in every cell you do not use all of those genes and that comes up in the gene regulation or protein synthesis regulation looking at turning on and off or switching on and off genes the other thing to point out is the genome of an all of different organisms will vary very very widely so for example humans we have about three billion dna base pairs in our genome compared to bacteria which on average only have six hundred thousand so different species have very very different genomes so that is it for the genetic code i hope you found it helpful if you have please give it a thumbs up [Music] you