in this lecture i'm going to talk about the processes of transcription and translation how your dna becomes proteins remember that's the whole purpose of your dna your dna is the recipe for how to make a protein how to assemble that protein put this amino acid first this one second and so on and remember that this all starts in the nucleus that dna never leaves the nucleus so in the nucleus of a eukaryotic cell that dna goes through transcription to become messenger rna and it's that messenger rna that leaves the nucleus and goes through translation on the ribosomes to become a protein and we're going to talk in a little more detail about those two processes so this first one transcription takes place in the nucleus and then the translation takes place of course on the ribosomes so really we have an intermediate between dna and protein we don't just go directly from our dna to making a protein this is a good plan why number one that dna is protected in the nucleus our one master recipe for how to assemble that protein is safe in the nucleus protected from the cytoplasm all the chemistry going on up here the second thing is if every single time we needed to make a protein we only had one coffee of the recipe and we have all these chefs out here all these ribosomes and they're only getting one recipe at a time that's very much a waste of time it would take forever to make enough enzyme let's use enzymes as our example would take forever to make enough enzymes to carry out a chemical reaction in the cell we need to make a lot of protein sometimes very quickly and this allows us to do that because you can make thousands of these messenger rna from this one dna recipe and send those out to thousands of ribosomes and make a lot of protein very quickly let's talk real quickly about the major differences between copying the dna to make a piece of messenger rna versus dna replication first of all remember in dna replication we copied that dna in its entirety all 23 pairs of chromosomes were copied so they could be divided into two separate cells now what we're doing instead is we're only copying one region of the dna so if this was one of our chromosomes all stretched out we're only going to copy a region of dna called a gene and that gene is a region of dna that codes for one protein think of it this way if you have a recipe book with thousands of recipes and you only want to make a quiche you are not going to start at the very beginning and make every single recipe in that entire recipe book one of which would be quiche you only want to cook the one recipe the same thing here we only want to make a piece of messenger rna that represents this one protein because we're only wanting to make one protein so that is one major difference is rather than copying all the dna we're only going to make a piece of messenger rna that represents one gene the second way it's different is that this is happening continuously in the cell how many times did we replicate the dna in its entirety only once during the s phase of the cell cycle this on the other hand is happening all the time when a cell is doing its cell job it needs to make a lot of protein and that's what is happening continuously in the cell so that's a big difference too so let's look a little bit more at the details first of all of transcription a couple of things you need to know one is a reminder about rna structure so rna structure is similar to dna structure in that it's a string of nucleotides we have ribose we have a base and then we have a phosphate group this base can be one of four bases it can be adenine guanine cytosine and uracil we represent that by just the first letter of the base a g c and u this list should look familiar with the exception of one base three of these are the same in dna this last one uracil is the only one that's different so in rna these are our four bases in dna remember we have a g c and t there is also a complementary base pairing that occurs here because what's going to happen is we're going to make a piece of messenger rna that is complementary to the dna strand in other words if we're reading this dna strand this would be the complementary messenger rna strand what base was complementary to a when we were copying the dna remember it was t thymine but in in rna we don't have thymine instead we have uracil so complementary to adenine is uracil so a will get copied as u on the messenger rna g will be c that's the same as what happens with dna c is g that's the same also and t becomes a so that piece of messenger rna isn't an exact copy it is complementary so during this transcription this messenger rna that gets produced is complementary to the dna strand here's what else is significant when we copied the dna in its entirety remember that we read both sides and we copied both sides we made a complimentary strand for each side that's not what happens in transcription in transcription we only read one side of the dna template so we're only making a piece of messenger rna that is complementary to one side of that dna strand not both sides and that piece of messenger rna is not going to attach like this complementary strand of dna does it's going to feed off separately and leave the nucleus to go out to a ribosome to assemble that protein let's look at an example of this happening this is a very generalized example but what's going to happen is we're going to have a region of dna that a certain enzyme recognizes as the start of a gene the start of that gene is going to be indicated by a region of dna called the promoter sequence that promoter sequence indicates that that's the start of a gene and then a very important enzyme comes in and starts reading one side of that dna strand to make a complementary strand of messenger rna it's going to assemble a strand of nucleotides that are complementary to the dna strand that enzyme is called rna polymerase remember that dna polymerase is what assembled the dna polymer now rna polymerase is going to assemble a rna polymer it's going to make that piece of messenger rna that is complementary to one side of the dna so let's just look real quickly at this example there's going to be a promoter sequence the rna polymerase comes in and it's going to start reading one side it's going to read the three prime to five prime size and it's going to assemble a piece of messenger rna that's complementary so looking again at our complementary bases if there's an a on the dna what's the complementary base on the messenger rna it would be you that c becomes a g the other z g a becomes u u u c becomes g g g becomes c skipping all the way down here t becomes a so we assemble a piece of messenger rna that is complementary to the piece of dna that represents one gene so the key player here is rna polymerase there is going to be a start signal and a stop signal and assembling this piece of rna it's also important to realize that messenger rna is really only one type of rna there are several more two of which you need to know about in this course so when we're looking at rna you now know a little bit about messenger rna mrna m stands for messenger you also need to know something about trna and rna r stands for ribosomal and this is a major component of the ribosome that we're going to talk about when we discuss protein translation in a minute and then trna this means transfer rna transfer rna is going to play a very important role as well so looking at this dna sequence again we're going to make a complementary strand of messenger rna and really this piece of messenger rna should look like the other side of the dna the only difference is anywhere there's a t it's a u noun so this looks exactly like the complementary strain of dna except binding is replaced with uracil once this messenger rna is assembled it goes to the cytoplasm and that's when we start the process of translation protein translation how do we translate that messenger rna into an amino acid sequence a couple of important terms to know three bases in a row on the messenger rna are called a codon it's a triplet of bases so looking at this example ugg that's our first codon uu that's our next codon when i say three bases it needs to be in a row it wouldn't be you you you here each of these is called a codon and each codon represents one amino acid one codon equals one amino acid on the resulting protein and guess what that codon codes for the same amino acid every single time for every living organism on earth whether it's a mushroom or a sunflower or a fish or a human we all use the same code if you look right here ggc this codon becomes glycine it becomes glycine every single time no matter what organism we're talking about and that process is called translation you can look at something called a genetic code to see what those codons become for every organism on earth and here's that genetic code you can see that aaa on the messenger rna always becomes lysine you can also see that there's some redundancy there are four different codons that give you glycine every time that's why you can have a variety of different base sequences on the dna that give you the same amino acid sequence for a protein and why we're all slightly different in our dna sequence in fact no two people have the exact same dna sequence unless you're identical twins so this is the genetic code it applies to every organism on earth it's also important to note this is the base sequence on the messenger rna which is complementary to the original dna so aaa on the messenger rna what was that on the dna it was ttt remember ttt on the dna made a complementary strand of messenger rna and now when that gets read by the ribosome it says put a lysine here how does that all happen how do we go from a codon on the messenger rna to having this chain of amino acids the polypeptide gene well for that we need to look a little more closely at a ribosome this is transcription you don't need to know this level in detail so i'm trying to skip forward there we go that's what i want to show you okay so this is a piece of messenger rna and this is inside a ribosome this triplet of bases on the messenger rna is called a codon this is going to dock in the ribosome and these molecules are going to bring in the correct amino acid and then it's going to move forward and we're going to bring in another one and it'll move forward and we'll bring in another one and this chain of amino acids starts growing so we're talking about the process of protein translation so one codon on the messenger rna equals one amino acid on the polypeptide chain remember polypeptide chain is the term we use for the polymer of amino acids it then gets folded into its tertiary structure and that's when it's a functioning protein so looking at this ribosome there are some important molecules that are bringing in those amino acids those amino acids are carried by something called transfer rna so in this cartoon form this transfer rna trna is carrying an amino acid how does it know which amino acid to bring well if it's carrying an amino acid it has a sequence of nucleotides at the bottom here that are complementary to the codon that determines which amino acid goes there and it's called the anticodon it's opposite of the codon what do i mean by that let's quickly go back to our codon table let's go back to our example of lysine aaa is the codon on the messenger rna that codes for lysine so we have a ribosome and we have this piece of messenger rna that's feeding through and a a a is the tripleta basis that comes in this is our messenger rna we know that that transfer rna that comes in needs to be carrying lysine how does this one carrying lysine know that this is where it docks to bring in that amino acid to the growing polypeptide chain it knows that because this transfer rna has an anti codon that's complementary to this codon what's complementary to aaa what's the complementary base to a on messenger rna or on any rna what is the complementary base to a it's you so that means this anticodon will have the sequence u u if it has the sequence u uu uu is complementary and it knows to bring that lysine into this spot and link it to that growing polypeptide chain so transfer rna has an anti codon that is complementary to the codon so going back to this picture here we see the anticodon this would be you you and the anticodons aaa this guy who's about to come in a dock with the glycine ggc so the anticodon would be ccg and that transfer rna is carrying the amino acid that is coded for by ggc which is glycine and that's how translation works in the simplest form there is a start sequence and there is a stop sequence on the messenger rna let's go back again to our codon table so we can see that aug is always the start sometimes that gets edited out but it's always the start you can see there are three possible stop symbols not symbols but stop codons okay let's do an example let's put that codon table to use i'm going to give an imaginary dna sequence we're going to transcribe a piece of messenger rna and then we're going to translate that into a protein obviously in reality this would be thousands of bases long i'm just going to make a short one on the board obviously okay so let's pretend this is our dna sequence and again there's dna before this there's dna after this we're just looking at one gene on the dna one little piece of that whole strand of dna that codes for one protein okay then the complementary stranded dna on this side would be this okay remember that we have a region up here called the promoter sequence that indicates it's the start of a gene and rna polymerase is going to come in and it's only going to read one side and it's going to start making a piece of messenger rna that is complementary to the one side of the dna the only difference is we don't have thymine anymore we have uracil so it's going to look very much like this side of the dna except that it's not going to have t so anywhere there's a t there's going to be a u g g c c u a u this is our strand of messenger rna that now snakes away and it goes out to the cytoplasm and a ribosome is going to start reading that so a u g is our first codon g c c is our next codon and then u a u is our next codon and it would just keep going so this is our messenger rna now we need to determine what the resulting amino acid sequence would be remember coded for by the dna a complementary strain of messenger rna three bases at a time each three bases code for one amino acid same amino acid for every organism on earth let's start with aug start it's also methionine we're going to just use the three-letter abbreviation so this is our polypeptide chain this is our amino acid sequence amino acid sequence of our polypeptide chain first amino acid in our chain mint next one gcc so we go gcc alanine i'm just going to use the three letter abbreviation aloe ua uau tyrosine tyr and so on we would eventually get to one of those three stop signals so eventually we would need uaa or something similar to say stop stop adding amino acids to this growing polypeptide chain that is the end so that is how a dna sequence becomes messenger rna and then becomes the amino acid sequence of our polypeptide gene i want to show you a quick slide of what happens when something goes wrong let's say that we have just one base on the dna that's different so here we have a t instead of a c what's going to happen is now on the messenger rna we have an a instead of a g and that results in a different amino acid instead of glycine here we have serine here this has the wrong amino acid sequence that protein is not going to fold correctly it is not going to have the same structure and therefore it will not have the same function just one base being wrong on the dna can be devastating it can cause a condition in which a very critical protein doesn't function correctly and that individual can die or can be very very sick as a consequence i want to show you a quick video clip of transcription and translation similar to the dna replication video you watched it's from the same pbs video it happens very quick but i just i just want to show you this whole thing in action because again on the board it's just a static flat process but it's actually happening continuously all day long in your cell so it's a very important process to understand