hello bisque 130 this is the beginning of recorded lecture 35 so in the previous recorded lecture we briefly got started on this chapter just to introduce or reintroduce some important terms and Concepts regarding just DNA and nucleotide structure uh now with that background with that terminology we're going to talk about DNA replication so not for the not for the last time not for the first time uh once again we are going to have to compare procaryotes and ukar um lucky for us the way they replicate their DNA uh is is pretty similar so technically this section is called DNA replication in procaryotes but everything that I'm about to say I'm heading just underneath you know calling it just DNA replication it's true for all of these cells when I finish describing the basic process of DNA replication then we will talk about how it is different from procaryotes versus ukar but on the whole it is the same so DNA replication making more DNA this is going to be part of binary fision this is going to be part of the eukaryotic cell cycle cells need to be able to to double up their DNA to copy over their DNA to replicate their DNA all of this all of this is going to take place at a location in the cell called the replication fork and oh boy it looks like there's a lot going on here um uh by the end of this lecture um we will hopefully understand everything that's going on here I'll I'll go through this entire thing one little piece at a time uh it's not as intimidating as it maybe seems uh what I'm introducing now is just the idea of replication fork being the phrase to describe this location and yeah it kind of looks like a fork in the road here as you go from two strands down to one strand yeah replication fork is just where this is happening so DNA replication takes place at the replication fork now like I said there's a lot going on here we could break this down one step at a time so I'm going to number these just so we could count through this talking about the first enzyme involved in this process an enzyme called DNA here hel case so we could see DNA helicase sometimes just called helicase we could see it labeled here uh but again this figure is very busy looking so here's another one that's much more simplified showing just helicase now all all these figures are going to depict the many enzymes involved in DNA replication as you know triangles or circles or rectangles or little blobs I I really just want you to to have in your head that any enzyme we talk about is going to have some big complicated three-dimensional structure these are these are proteins with tertiary structure these things uh these these things are more complicated than just a triangle or a blob or whatever they will be drawn as a triangle or blob or whatever but yeah the complicated proteins anyway so what is the job of helicase well if we are going to replicate DNA what we're going to do is we're going to take a single strand of d DNA and knowing the rules of Base complimentarity we're going to build another strand complimentary to this one if there's a if there's an a here we're going to build a t if there's a g here we're going to put in a c and so on so if we're going to build a fresh strand of DNA complementary to another strand we have to make it single stranded you can't go through that exercise with double stranded DNA so it makes sense that the first step here is to make single stranded DNA so helicase unwinds the DNA Helix again going to be a lot of enzymes here most of their names make a lot of sense so DNA helicase unwinds the DNA double helix and of course it breaks hydrogen bonds between the bases that's what's holding together this double helix so it has to break apart those Bonds in order to make them single stranded and yes in doing so it creates two importantly two singl stranded templates of DNA we are actually going to replicate both of these so that's what we've got so far DNA helic case unwinding the DNA double helix there's another enzyme that is happening simultaneously it's it's doing its job simultaneously this is an enzyme called topoisomerase that's a long one it's the the green little circle here and it's really hard to tell what's Happening Here what Tombo isomerase is doing is it's it's helping to relieve tension in this Helix as it unwinds so this is this can to be very difficult to to visualize but here's here's my shot at it so you could think of the DNA double helix as you know two long strings or rubber bands that have been wrapped around one another forming this double helix now as DNA helicase unwinds this double helix or of rips these two parts up uh these two strand ends apart from one another if the other end is held fast and it is held fast that's going to cause the overall uh structure to just s sort of coil up like this there's going to be tension in this structure if you start to unzip it like this uh and this is this is not good having this this amount of tension it's going to break apart it's going to lose its structural Integrity so as DNA helicase is sort of unzipping these two rubber bands what the enzyme Topo isomerase is going to do again big complicated protein I've drawn as a green circle um is going to cut the DNA and then let it sort of uncoil unwind a little bit relieve that tension and then seal it back together again so that we have you know nice straight DNA double helix after this unzipping has happened to prevent this from getting out of control this sort of coiling up if this is difficult to wrap your bra around that's fine again this is very difficult to visualize the only thing you need to know about Topo isas is that it relieves tension in the DNA double helix I'm not g you know even I'm not even calling it number two in this process I'm sort of lumping it with DNA helicase because it's doing it's doing this as helicase is doing its thing Topo isomerase relieves tension in the DNA Helix that's all you really need to know about Topo isomerase okay now we have singl stranded DNA that is you know not tensioned up uh after the side of unwinding ready to start building DNA complimentary to this right well not quite single stranded DNA as it turns out is not very stable DNA doesn't like to be single stranded and it wants to find Partners it wants to zip right back up again so as helicase unzips the DNA we have our next protein coming in here proteins called single strand binding proteins um get little purple things right here again it's kind of hard to see what they're doing here is a better figure showing these things yep we got the DNA dou Helix there's helicase here and yes these are the single strand or single stranded binding proteins uh they they do exactly what their name implies these are proteins that bind to single stranded DNA exactly like you see here um and their their purpose is to stabilize the single strand DNA and to prevent it from Just zipping right back up again so in summary protein number two or you know step number two again you know I'm never going to ask a test question what's step four the numbers are just so we can walk through this uh one piece at a time single strand binding proteins bind single stranded DNA and they prevent rewinding of this DNA double helix okay now we're ready to start building DNA right well the the enzyme that's going to build DNA is called DNA polymerase um there are going to be a couple of these so there Roman numerals Associated here but the the enzyme that actually constructs DNA is called DNA polymerase but we're not ready for it yet there's a problem with DNA polymerase if we have you know a fresh you know single stranded DNA right here ready to be replicated ready to you know put in a nucleotide complimentary to this nucle nucleotide and then a nucleotide complimentary to this one build a strand of DNA up here DNA polymerase cannot get started DNA polymerase and you know Paul is a very common abbreviation for polymerase in figures and notes and stuff like that DNA polymerase cannot start a new strand of DNA it cannot plop down that first nuclea tide if this is a if this is an a you know if you're going to start building there needs to be a t right here compliment to it DNA PA just it can't it can't start so before we get to the action of DNA polymerase we need something that can start DNA polymerase it it can build off of something it just can't put down the first one so before we get to DNA Paul our next step is actually an enzyme called primase primase is an enzyme that builds a primer uh we could see this here yep another another figure you know there's the DNA double helix and you know helicase is not pictured topoisomerase is not pictured single strand binding proteins are not pictured again try to keep this simple uh We've unzipped it and yes here is the enzyme primase and here's the enzyme primase what primas does is it builds a short primer as the name implies that is made of RNA this is not what we want ultimately we don't want a DNA RNA hybrid here we're trying to replicate DNA when we're finished we want all of this to be DNA and we want all of the other strand to be DNA and we'll get there by the time we're done but again the reason we're doing this the reason we're mucking around with RNA in the first place is because of this limitation of the enzyme we actually want to use DNA polymerase cannot get started but primase can it's going to make this short RNA primer complementary to the DNA again you know T in the DNA a in the RNA C in the DNA G in the RNA and and so on so primase creates a short RNA primer that is complimentary to the DNA once this little primer is in place now we can proceed step number four DNA polymerase specifically DNA polymerase 3 so there's going to be another one we'll we'll bring up but the the one that's going to do the the biggest part of all this is called Paul 3 and we can see it here uh yeah it's extending this primer again it couldn't get started but once there's a a few nucleotides for it to build off of it's going to take it and just go to town building DNA pairing A's with T's and c's with G's uh creating double stranded DNA building this strand complimentary to this strand here so DNA Paul 3 extends ends the RNA primer building DNA complimentary to the single stranded template and yep we can see it here DNA Paul 3 again this sort of yellowish orange rectangle but yeah it's constructing DNA uh there's another protein associated with DNA polymerase because this this could be a pretty long job for DNA polymerase to build all this DNA we want to make sure it doesn't quit before it's you know reached the end uh so is a protein called the sliding clamp that's associated with DNA Paul 3 as the word clamp implies it's just going to hold it in place slide along with it make sure it doesn't fall off prematurely so again I'm not giving this its own little number here it's associated with DNA Paul 3 the sliding clamp protein holds it meaning DNA Paul 3 in place okay now is the hard part so now is the reason why I introduced the five Prime 3 Prime stuff at the end of the last recorded lecture this DNA polymerase 3 enzyme can only build DNA in the five Prime to thre Prime Direction remember every strand of DNA has a five Prime end and a thre Prime end and this this enzyme that's building DNA that's doing the the big important job of this entire process can only build in a certain direction if we go back to our figure showing DNA polymerase we could see that yeah I I've I've labeled this so uh the way this you know particular figure is drawn it looks like the template up here is three Prime on the left and five Prime on the right that means the DNA strand complementary to this template has to be the opposite of that remember these two strands are anti-parallel they run in opposite directions so if this strand up here is three Prime on the left five Prime on the right the Strand complementary to it has to be five Prime on the left and three Prime on the right and you'll notice the arrows here this DNA polymerase is building five Prime to three prime I suppose the three prime is not labeled here but we know that if this end of this particular strand is five Prime the other end has to be thre Prime Building five Prime to three prime let's look at the other strand again it's anti-parallel to the first original strand it has five Prime on the left three Prime on the right that means whatever strand we build has to be the opposite the whatever strand we're building has to be three Prime on the left and five Prime on the right because this enzyme can only build five Prime to three prime we see it extending this RNA primer and building from the five Prime end toward the three prime end again sorry the three prime isn't labeled here but we know that if this end is five Prime the other end has got to be three prime so again a small little statement with a profound impact on this entire process this enzyme is only capable of building in the 5 Prime to 3 Prime Direction now what you may have noticed here is that these two DNA Paul 3s they're both building five Prime to three prime but their arrows are pointing in opposite directions because the two templates are anti-parallel the two strands that we are freshly building are also going to be anti-parallel to one another one is going to point to the right one is going to point to the left these two strands of DNA are replicated in opposite direction so here's my summary of this and again some of these statements if you just take the statement in a vacuum it's like what the heck this is a very visual process as you're studying all of this I highly recommend this is true for every chapter in this quarter but especially true for this uh that that you have some sort of visual to go with these just statements about things in fact you could draw a lot of this yourself uh of just drawing a line for DNA another line for DNA always labeled the five Prime three prime stuff like this this is a great way to study to try to redraw these figures that will really help you appreciate these just statements so because the two strands of DNA are anti-parallel as defined in at the end of the last recorded lecture they are replicated in opposite direction this means we have two different directions of replication and we've got two different names for these directions so this DNA polymerase is building if we Follow the arrow here five Prime to 3 Prime toward the site of unwinding now DNA helicase uh and Topo isomerase they're not shown here uh but we know that they here we know that this double helix is still being unzipped it's still in the process of being unzipped we're going to replicate this entire chromosome we're not just going to stop when we get here DNA polymerase is all these things are kind of going on simultaneously so DNA helicase is going to continually unzip this and this DNA polymerase uh is you know going to build towards it towards that site of unwinding this is really convenient because this DNA polymerase that's building towards the side of unzipping or towards the side of unwinding gets to just keep going it's just going to keep building DNA following behind helicase which is just going to keep unzipping and it's going to make one big long piece of DNA that is synthesized continuously so this strand up here the one that's moving toward the side of unwinding is called the leading Strand and it builds DNA continuously because again it's just going to keep going there's going to be more unzipped as it makes its way down one big long piece of DNA in summary still talking about DNA Paul 3 there is a leading strand the leading strand is made five Prime to three prime toward the site of unwinding and again it is synthesized continuously no breaks no starts and stops it's just going to keep going as heel case keeps going it's the other strand that's going to be a pain the other strand goes in the other direction again they're replicated or they're synthesized they're created they're built in opposite directions to one another this other strand is going to be built five Prime to three prime but it's towards the left in this figure it is away from hila case it is away from the site of unwinding so we know what's going to happen as we build DNA it's going to build DNA A's to T's C's to G's build build build build build and then it's going to stop it's going to reach the end of the chromosomes so what what what what could we possibly do well uh once it stops it's going to have to start again so this strand I'll get to that in a second this this strand that goes away from the site of unwinding is called the lagging strand so the lagging strand of DNA is built five Prime to three prime away from again I'm underlining these important phrases away from the sight of unwinding it starts as close to the sight of unwinding as possible again we see this here it's started as as close to it can is where helicases is unzipping things it starts as close to that side of unwinding as possible and then it's going to end when it hits the end of DNA or when it hits the previous fragment wait a second fragment well okay let's let's look at this so This lagging strand is going to have to be constantly starting and stopping let's turn back to this big figure and again this was too complicated and too busy when we first started today but we recognize most of this stuff by now there's helicase there's Topo isomerase are single strand binding proteins uh yep there's primase building an RNA primer it looks like the way this figure has been drawn the leading strand is on the bottom and the lag strand is on top again which one is on top and which one is on the bottom just depends on how you draw your original DNA double helix whether you draw three prime to five Prime five Prime to three prime or whether you put the five Prime to three Prime on top and the three prime to five Prime on bottom leading strand lagging strand one is not always on top and on bottom which one is leading is the one that goes five Prime to three prime toward the side of unwinding uh and the lagging is the one that goes 5 to three away from the site of unwinding so yeah it's a little flip-flo from this but it's nice to see different figures showing this in different ways doesn't always have to be like this so yeah here's our leading strand DNA Paul 3 it's sliding clamp moving towards the side of unwinding as helicase continues to unzip it's just going to experience continuous DNA replication what about that lagging strand well here is DNA here's a you know a primer and here is the DNA that has you know ended when it hit the the end of the chromosome uh here is a fresh primer and it looks like DNA polymerase 3 built and then stopped because it ran into the previous fragment here's another primer and there's DNA polymerase 3 building five Prime to 3 Prime and it's going to stop when it hits this fragment and here's another primer being made by primas again as close to the sight of unwinding as possible you know what's going to happen even though it's not pictured here primase is going to build this primer then DNA polymerase 3 is going to extend this primer and it's going to stop when it hits the previous fragment so this entire lagging strand is starting and stopping starting and stopping starting and stopping starting and stopping this whole process in the lagging strand is called discontinuous DNA replication as opposed to continuous which is how we describe the leading strand this is also going to result in a bunch of these individual fragments where we started and stop started and stop start and stop start and stop these things are named after the Japanese um actually research team I think it was a husband and wife team not just one person but they had the same last name uh okazaki fragments named after the okazaki these little fragments each one has its own primer uh and then some DNA primer then some DNA primer then some DNA primer then some DNA so let summarize this um in the lagging strand new fragments are going to start as DNA unwinds more uh this results in many short fragments of DNA each one has its own RNA primer that's going to be a problem we'll deal with it soon these fragments are called okazaki fragments because this overall process is starting and stopping starting and stopping starting and stopping uh we say that the lag lagging strand is synthesized discontinuously um so again the lagging strand is the one that's difficult to to understand the leading strand is in some ways pretty pretty easy just going towards the side of unwinding if I may go back to this simplified thing uh but yeah the lagging strand is going to start and end and then as this unzips more it'll start again and end then it'll start again and end you're going to have thousands and thousands and thousands of these okazaki fragments um in the lagging strand importantly there are no okazaki fragments in the leading strand it synthesized continuously it's going to be one big long piece of DNA these little fragments are only present in the lagging strand and we need to deal with them so every one of these fragments has a piece of RNA an RNA primer here we don't want that we don't want any of this RNA we have to we have to get rid of it we needed the RNA as a way for DNA polymerase to get started there is a reason why we needed this but once we've gotten things going once we built a bunch of DNA our next order of business is try to clean things up and get rid of these so the RNA primers that we have constructed especially the ones present on every single okazaki fragment need to be replaced with DNA to do this we are going to bring in an enzyme called DNA polymerase one it's all Roman numerals by convention if you're curious there is a DNA polymerase 2 it's um beyond the scope of an intro course so in our in our intro biology we're only going to talk about Paul one and Paul three Paul 3 um was responsible for building all this DNA all this DNA Paul one is just going to replace RNA with DNA so let's take a look at this we have Paul one labeled here and we can kind of see what it's doing Paul one is coming in and removing the RNA primer here and replacing it with DNA and you might wonder why didn't we do this in the first place why didn't we just replace it with DNA well DNA Paul one just like DNA Paul 3 cannot get started it builds DNA but it can't put down the first DNA nucleotide it needs something to build off of well in this case what it's build off of is the next fragment so yeah all these okazaki fragments are you know next to one another so we're able to remove this RNA by building off of the FRA the next fragment and you know it's going to come over here uh later and it's going to remove this RNA and it's going to build off of the DNA from this fragment and when it removes this RNA primer hey the DNA hasn't even been built yet but by the time DNA polymerase 3 comes comes in and builds this DNA DNA polymerase 1 is going to be able to remove this orange primer by extending the DNA from the previous fragment so DNA Paul one does these two things it removes RNA primers very important get rid of that RNA and it uses DNA from the next fragment in line to build off of to fill in that Gap with DNA so by the time we're done all this RNA you know all these all these primers they will have been removed we needed them only temporarily to get things started but we're going to you know remove the primer fill it with DNA remove the primer fill in it with DNA remove the primer filling it with DNA by the time we're done there shouldn't be any of the RNA in the middle of this fragment there is one last little problem here uh and that is when RNA Prime when um I'm sorry when DNA polymerase 1 removes this primer and fills it in with DNA there is going to be a tiny little Gap all the nucleotides are going to be there DNA polymerase 1 will have you know paired all the A's with T's and c's with G's all the nucleotides are there there's just one teeny tiny missing phospho diester bond between the nucleotides so we've done our DNA Paul one we filled in this Gap there's now double stranded DNA where the RNA primer was but there is a missing phosphodiester bond between these two fragments this was one fragment this was the other fragment they're next to one another all the nucleotides are there one little missing Gap the enzyme that is going to seal that Gap together and again it's hard to see what it's doing but really it's just coming in here and filling in this Gap is something called ligase or DNA ligase and again tiny little blue circle tiny little job but it's an important one filling in this missing Gap so DNA liase seals together the now all DNA okazaki fragments all the RNA has been replaced with DNA all these okazaki fragments are now completely DNA and ligase is going to seal them all together so this means even though the lagging strand is synthesized discontinuously ly even though the lagging strand is start and stop and start and stop and start and stop and start and stop and it has all these individual fragments every single one has its own RNA primer because of the work of DNA Paul one and DNA liase by the time we are finished the lagging strand is going to be one big long piece of DNA it's going to look just like this nice neat leading strand strand uh there so yeah their the end result is going to be the same but the process is going to be different the leading strand is built in one go uh toward the site of unwinding see a little arrow here toward the site of unwinding the lagging strand is going to be built discontinuously starting and stopping starting and stopping starting and stopping but by the time we're done it's going to be one big long piece of DNA so yeah that's the action DNA ligase sealing together these now all DNA okazaki fragments now there is a table here from the textbook uh that looks like a bunch of information to memorize but if I can be honest with you we we talked about all this already so I I like this table because it's kind of just a summary of everything we've talked about so far so yeah you could you could print this out you could write this up you could study this whatever this is no new information this is just a single table that shows kind of a summary of all the enzymes we have talked about so far uh enzymes and proteins um it is worth noting a couple of things I don't know if I've mentioned this before almost all enzymes in biology and with Ace so if it's catalyzing a chemical reaction like you know connecting uh you know making phosphodiester bonds like ligase or creating RNA like primase or breaking hydrogen bonds like helicase or you know polymerase building DNA uh yeah if it if it ends in Ace you know that it's an enzyme it's making a chemical reaction happen um the and yeah so the sliding clamp is just holding things in place it's not catalyzing a chemical reaction the single strand binding proteins are just stabilizing DNA they're just binding the DNA they're not making a chemical reaction happen but yeah and and this will continue on in later chapters you know it's an enzyme if it end is an ace anyway the one other thing I want to point out here is uh just a simple way to remember the difference between Paul one and Paul three uh my kind of dumb way of remembering which one is which is uh three is a bigger number than one and three is doing a bigger job than Paul one DNA Paul 3 does all of the leading strand does all these the initial DNA in the lagging strand big job here Paul one just fills in these little gaps where the primer was Paul one is just building you know these little pieces of DNA to replace the RNA primers here so uh three is a bigger number oh I'm sorry I skipped to the wrong table here uh three is a bigger number than one three is doing a bigger DNA replication job than DNA polymerase 1 just my little way of remembering the difference as promised let us now talk about the differences between procaryotes and ukar again everything we talked about just now with DNA replication applied to to to both of these things but how is DNA replication different between these two types of cells well again fundamentally it's the same so the eukaryotic replication is fundamentally the same as procaryotes is uh except for oh the following okay this this is a big table these are differences between procaryotes and ukar lucky for us I don't don't want to cover every single one of these differences I just want to cover uh a small number of these um so you just need to know what I've written down in these slides uh just know you know there are a bunch of differences I'm highlighting a couple of the big ones so it's the same but UK carotic replication is slow lower and that should make sense the eukaryotic chromosome is uh more complicated it's got histone proteins and chromatin it just takes a longer time to go through the process of replication in a eukaryotic chromosome it's slower it's also a lot bigger you know the procaryotic chromosome the sort of closed circle is a lot smaller than the UK carotic chromosome which is this big long line so if we want to finish replication in a timely manner we're not just going to have a single replication fork if we're replicating a eukaryotic chromosome we're going to have a bunch of replication forks we're going to have multiple what are called origins of replication all along this big long chromosome so here's a replication fork with leading strand lagging strand here's a replication fork here's a replication fork here's one here's one here's one and usually even more so yeah ukar are going to have multiple origins of replic along their chromosomes which again makes sense they're bigger the other difference has to do with that fact that that uh that it's linear so procaryotic chromosomes are circles and eukariotic chromosomes are are lines or sausages so there's an issue here with the primers and the ends of these chromosomes so this is a a uniquely UK carote problem the five Prime ends of our linear chromosomes cannot replace the RNA primer with DNA remember that replacing the RNA primer with DNA was the job of of DNA polymerase one and I told you that it you know removed the RNA primer and it fills in this Gap using the next fragment's DNA but what if there is no next fragment what about the very first primer here on the leading strand the very edge of the chromosome and what about the very last primer here on the lagging strand how are we going to replace those let's think about this and look closely at the labels here there was an RNA primer here you could remove it but how are you going to fill this in we can't build DNA from three prime to five Prime so we can't like build from right to left to fill in this Gap and we can't extend a previous fragment because there is no previous fragment just you can't do anything here the same thing is true over here if you wanted to build five Prime to three prime you would need something to build off of and there is nothing to build off of this is the end of the line and we can't go three prime to five Prime to fill this in so there's there's nothing we can do and again kiotes don't have this problem it's a big old Circle everything is connected to everything else you could always fill in every single RNA primer you can always fill in everything because it's all connected to itself so this is a good visual of what I mean when I say that we cannot replace the RNA primer with DNA at the five Prime ends so these ends here we we just we can't replicate this little piece of DNA here there was a primer it's removed but we can't fill it in with DNA so what does that mean for our chromosomes how do how do we replicate this well the answer is we don't we can't this little portion of DNA at both ends of the chromosomes ends up not getting replicated we are going to lose this information our chromosomes are going to get a little bit shorter every time we go through the process of DNA re replication because we can't replicate these ends in summary nothing to build off of DNA polymerase definitely cannot build five Prime to three prime so this problem leads to shortening of chromosomes which is in all caps bad DNA is really important it is the instructions for how our cell is supposed to do everything even if it's just you know a handful of nucleotides on on each end losing any information is like you know taking the blueprint for a very important building and just snipping off little corners of it you snip the corner a little bit more and you snip The Edge a little bit more eventually it's going to have some catastrophic consequences on the important stuff uh in that blueprint so yeah losing losing DNA shortening of chromosomes is really bad and again this is a problem that only happens in ukar Pro carots don't have this problem so how do we solve this there has to be a solution to this we can't just lose information every time we do DNA replication well there is a solution to this and it's kind of tricky but it's also kind of clever so the solution to this problem is to use an enzyme called tarase to build structures called tilo mirors so tiir are defined in the key terms sorry I'm Shuffling through my paper um oh tiir are not in the key terms I'm sorry I have them here tiir are a large region of repetitive DNA they don't contain genes they are built and maintained by Tas so if we want to visualize these tiir uh there's the cell nucleus chromosomes these little red caps here the ends of the linear chromosomes these are the tiir and if we were to zoom in on the DNA here it would be repetitive sequences g g caaa g CCAA g g CCAA G CCA AA just the same sequence over and over and over and over again uh exactly what the sequence is you don't need to memorize and it varies depending on species to species but the the term repetitive is important here it's the the same thing repeated over and over and over again these tiir regions do not contain any genes the GGG CCC AAA whatever these repeats are are not important information they're not the instructions for anything these Tere structures are built and maintained by enzymes called tarase again if it ends in a it's an enzyme so don't get these two things mixed up tiir is the name of the structure the region toer Ace is the name of the enzyme that builds these things so how how does Tas build you know a region of repetitive DNA well let's take a look at this enzyme again again big complicated cool enzyme reduced to you know sort of a tan oval here but anyway tarase is a protein it is an enzyme but you can see there's a sequence of RNA oh you know it's RNA because there's uracils uh there's a sequence of RNA that is part of this DNA enzyme so it what it does is it binds to these repeats and look at that G's to C's A's to T's A's to U's G's to C's it binds to the overhang here on the tiir and T tarase has DNA replication functionality as an enzyme it is able to build DNA what it does is it builds DNA complementary to its own RNA template so half of its RNA template is binding to the overhang the other half of its built-in RNA template is telling it to build a a g across from this c a t across from this a a t across from this a an a across from this u a g across from this C it builds DNA Now it only Built a small amount of DNA but it's going to slide over again overhang is complementary to its template and then it's going to do it again build more DNA and then not picture it will slide over again and you know add on some more nucleotides add more DNA and you'll notice it's repeating the same sequence over and over and over again that's how we get the tiir it's you know according to whatever this RNA template in the tarase is now we can see primas coming in building an RNA primer DNA polymerase building DNA complimentary to all of these repeats and we're building fresh DNA what about this overhang here well here's the interesting thing about taras and tiir we still lose DNA every time we replicate this overhang right here this g g g TT a a whatever there's no way to replicate this there's there's no way to copy this over you can see polymerase uh right here building DNA there's no way to fill in this Gap we are going to lose this information this is going to be shortened there's no way to replicate this like I said back here we're going to lose the ends of our chromosomes but we haven't lost anything important all we've lost is you know one of these repeats as long as tarase keeps coming in and adding more repeats to the ends of our chromosomes the fact that we lose a little bit when we do DNA replication doesn't really matter the DNA that we're losing is never going to be important genetic blueprint information it's only ever going to be pointless repeats that don't code for anything that you could just build more of using the tarase enzyme I told you this was kind of a weird solution uh to this problem but here's my way to describe this so uh again tiir built and maintained by taras it has a built-in piece of RNA which it uses as a template to build DNA repeats again construct some DNA construct some DNA construct some DNA the important phrase here is that there's going to be no net loss because of the continued function of toer Ace building these tiir the chromosome ends will shorten every time there's a DNA replication but they are rebuilt by taras so we're going to end up having no net loss so yep again this is unique to ukar procaryotes don't have to deal with this but yes we lose some DNA every time we do DNA replication but because of Tas there's not going to be a net loss in this process oh okay that finally takes us to the end of DNA replication and you know the toras which kind of has to do with with DNA replication again I apologize for the accidental yellow here uh tiir is not in the key terms just to be perfectly clear it's defined right here large region doesn't contain genes built and maintained by Tas sorry that should not be yellow this brings us to the end of DNA replication we are not finished with the chapter on DNA structure and function but this is where I have to cut things off for this particular recorded lecture this is also the cut off for exam number three so part of this chapter um about DNA replication and you know the background about nucleotide structure part of this chapter is going to be part of exam number three but the stuff we haven't covered yet where I'll pick up in the next recorded lecture uh the other part of this chapter is going to be on exam number four so the the exam 3 cut off is right here at the end of this recorded lecture we'll pick up this chapter with a slightly different topic in the next one