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Main content
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Video transcript

so we already know that chromosomes are made up of really long strands of DNA all wound up into our into themselves something like I'm just kind of drawing it as a random long strand of DNA all wound up in itself and on that strand you have sequences which we call genes so that might be one gene right over there this might be a nother gene that might be a gene right over there and each of those genes can code for specific polypeptides or specific proteins and the key question is is how do you go from the information encoded in these genes encoded as sequences of DNA how do you go from that how do you go from the gene which is encoded in DNA how do you go from that to protein which is made up of polypeptides which are made up of amino acids and this is often called the central dogma of biology but we already saw in the video of transcription that the first step is to go from the gene to messenger RNA that the RNA the messenger RNA you can view it as a transcript we have rewritten the information now as RNA and then the next step which we were going to dive into in this video is going from that messenger RNA to protein and this process is called translation because we're literally translating that information into a polypeptide sequence and you can see a little bit visually here and this is all review we cover a lot of this in the video on transcription and the overview of video on transcription and translation is if you look at a eukaryotic cell and the bacteria in a prokaryotic cell it's analogous you just don't have the nuclear membrane and you're not going to do the processing step that I'm going to talk about a little bit and we went in detail on the video and transcription but you start with the DNA you have your RNA polymerase is the main actor that's able to transcribe the RNA from that if we're talking about a eukaryotic cell what you end up with we wouldn't call mRNA we would call that it is pre mRNA REM are n/a which then it needs to be processed the introns need to be taken out we add a cap and a tail here and if we're talking with a UK XL will then formally call that mRNA and then it can travel and this is where we get into the translation step it can travel to a ribosome which is where it will be translated into a polypeptide sequence and you see the analogous thing happening here in this in this in this bacterial or this a prokaryotic cell right over here except you don't see the nuclear membrane because this is prokaryotic and you don't see that processing step so you could just consider this straight this is M RNA right over there so the questions are well how does this thing happen and what what even is a ribosome so let's zoom in a little bit on a ribosome right over here and there's a couple of interesting actors one as you can imagine is the ribosome itself and it is made up of it is made up of proteins proteins plus ribosomal RNA so in the video on transcription we're already familiar with messenger RNA and we often view RNA like DNA as primarily encoding information it's acting as a transcript for a gene but it doesn't have to only encode information it can also so it's proteins plus that's not a T there this is a plus it can also provide a functional structural role which it does in ribosomal RNA and this big you know this looks like a an oversized hamburger bun or something right over here this is a super oversimplification of what a ribosome looks like and encourage you to do a web search for image searches for ribosomes and then you can you can get a more appreciation of how how beautiful these structures are and how intricate they actually are so this is the site and you can broadly think of the ribosome as having this you know this is the top bun and the bottom bun and the and it's going to travel along the mRNA from the five prime end to the three prime end reading it and taking that information and turning it into a sequence of amino acids so how does that truly happen well each each of these three every three nucleotides every three nucleotides there you recall that a codon so that's a codon this is let me do this in a color that is visible on both white and black so this next three nucleotides is a codon this is a codon this is a codon and what's actually the information is actually encoded in the nitrogenous bases so this first codon right over here we see it's Aug so the nitrogenous bases are adenine uracil and guanine and this has this this codon it codes for the amino acid and the amino acid methionine but this is also this is a good one to know Aug let me write it over here Aug Aug is known as the start codon start codon this is where the ribosome will initially attach to start translating that messenger RNA and so we we've the way that this this drawing is that we are just starting to translate this messenger RNA so how does that actually happen how do we get from these three letter sequences to specific amino acids well let's think about it how many how many possible three-letter sequences are there well there are there are four possible nitrogenous bases there so there's four possible so if you if you have a codon and it has three three places there's four possible things that could be in the first place there's four possible things that could be in the second place and there's four possible things that could be in the third place so there are 64 64 possible permutations four times four times four permutations so you could think of it there's 64 different codons different ways of arranging the a the u and the G and that's good because there are many amino acids and this is actually overkill because there's actually 22 standard amino acids 22 standard standard amino amino acids and 21 that are we found in eukaryotic cells so we have more than enough more than enough permutations to cover the different amino acids and you it's not hard to find tables that will actually show us what the different sequences what they actually what they actually code for so you can see here you can get take you can take the first letter the second letter and then the third letter figure look at the different sequences and you can say okay look at that Aug atony uracil guanine that codes for methionine all right over here you could and you could do that with any of you could say cytosine uracil uracil that codes for leucine and you can see that it's it's not just one amino acid per codon that here you have four codons all code for all code for leucine and so it turns out that sixty-one of the codons let me write this down so sixty-one of the codons of the possible 64 code for amino acids amino amino acids and three play a role that essentially tells the the the ribosome to stop three codons three codons are stop codons and you can see them right over here UAA UAG UGA that's how the ribosome knows to stop translating so Aug that's a start codon and it codes for methionine so that lets you know that well these polypeptide chains are going to start with methionine and then these characters tell it where to stop but how do how does the amino acid actually get how do they all get tied up together to form this polypeptide and how do they get matched up how do they actually get matched up with the appropriate codon and that's where we have another RNA based actor and this is tRNA so T RNA the T stands for transfer turns for RNA that there's a bunch of different T RNAs that each can bind to specific amino acids and on parts of those tRNA they have what are called anti codons that pair with the appropriate code so this tRNA and that's not what it looks like I'll show you in a second what it looks like that's a tRNA molecule T RNA at one end of the molecule it's binding to the appropriate amino acid methionine right over here and it and then at the other end of the molecule although that's it's in the middle of the tRNA actual chain you have your anticodon and your anticodon matches up to the appropriate codon and so this is how if they bump into each other the right way and the ribosome is going to facilitate it that the Aug is going to be associated with the methionine and if we look at what tRNA actually looks like and this is still just a visualization so this is a this is a strand of tRNA you get a sense of okay it's a sequence of RNA right over here this is it I guess you say think of its it's 2 dimensional structure but then it wraps around itself to form this fairly complex molecule and the anticodon which is right here it's kind of in the middle of the sequence it forms the basis for this end of the molecule that's the part that's going to pair with the codon on the MRNA and then at the at the other end of the molecule at the other end of the molecule is where you actually bind to the appropriate amino acid so I know what you're thinking alright I see that the ribosome it knows where to start it starts at the start codon I see how the appropriate tRNA can bring the appropriate amino acid but how does the chain actually form and you can view this in three steps and associate with those three steps or three sites on the ribosome and the three sites we call this the a site a then your can be able to see it if I write it in black a or yellow alright let me write it in blue so that is the a site this is the P site and this is the e e site and I'll talk in a second why we call them a P and E so the a site is where the the appropriate tRNA initially bounds the tRNA that's bound to an amino acid and so you can see we're starting the translation process the next thing that's going to happen is another tRNA the one that is that matches that has an anticodon that matches the uau that's going to bond over here on the a site and it's bringing the appropriate amino acid with it it's bringing the tyrosine with it so why is that called the a site well a stands for amino acyl you can do or an easy way to remember it it's the it's the tRNA it's a place where the tRNA that's bound to an amino acid just what amino acid is going to bind on the ribosome and so once that happens once this character comes here let me draw that once this character comes right over here there's going to be a you a and it's bound to the tyrosine well then you can have a peptide bond form between the two amino acids and the ribosome and the ribosome itself can move to the right so this this this this [Music] tRNA will then be in the e site this tRNA will then be in the P site and then the a site will be open for another amino acid carrying tRNA so what does P and E set is what are the p and e sites stand for well you can see a little bit more clearly right over here so the P site is where you have the polypeptide chain actually forming and so the P site is often or well one way to remember it is is that's where you have the polypeptide chain and now you have a new you have a new a site where you can bring in a new amino acid and then the ribosome is going to shift is is once this is bound the ribosome the the peptide bond forms and then the ribosome can shift to the right when the ribosome shifts to the right we're going to be in this position where the thing that was here that was in the a site now the polypeptide is attached to it it is going to now be in the P site and the thing that was in the P site is now going to be in the e site it is now ready to exit and that's why it's called the e site because that's the site from which you exit and so this is going to keep happening until we get to one of the stop codons and when you get to one of the stop codons then the appropriate polypeptide is going to be released and we will have created this thing that could either be a protein or part of a protein so this is very exciting because this is happening in your cells as we speak this is and in fact if you think about things like antibiotics the way that they work are is that the way that antibiotics work is that ribosomes in prokaryotes are different enough than ribosomes in plants and animals or in eukaryotes that we can find molecules that that hurt the function of ribosomes in prokaryotes but don't do it to eukaryotes and so if you have bacteria in your bloodstream and if you take the appropriate antibiotic it could disrupt this translation process in the bacteria but not in your cells that you want to keep
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