in this video we're going to walk through the steps of DNA replication we're going to kind of break down the process into sort of three discrete steps we're going to identify what facilitates that steps which kind of spoil alert there are a whole bunch of enzymes that are going to kind of be at play throughout various points of replicating the DNA um and we're going to kind of identify what happens in that step on the picture that is shown below okay so let's start first with kind of step one and step one in this picture is what's being shown in this segment okay so we're going to kind of identify this as step one and in Step One the thing that we're accomplishing um is we are separating are strands of DNA okay so keep in mind that in order to use the DNA as a template we need to expose the nitrogenous bases in the parent strand so that each strand can become a template for a new daughter DNA molecule so we need to expose the nitrogenous bases on both strands of DNA to act as a template for our new D for our new d a this separation is going to involve the breaking of those hydrogen bonds that are holding our two strands together right it's not necessarily apparent in this picture but kind of remember when we've looked at images before that we have two sides to our DNA extending from each side is a nitrogenous base complements go together and then then those nitrogenous bases are held together with a hydrogen bond and so if we want to separate our strands of DNA then what we need to do is we need to break these hydrogen bonds so we're going to do this with an enzyme called helicase so helicase which right as the suffix would suggest this is an enzyme is going to break the hydrogen bonds holding the nitrogenous bases together so in essence helicase is kind of unzipping the DNA exposing the two sides now there is also an enzyme here called topis ceras um you don't need to remember that name basically what it's doing is it's kind of stabilizing the DNA it's helping to untwist it and keeping it from um keeping it stable so that it can be opened by the helicase but really the only enzyme that you need to kind of focus here we we'll sort of just cross out its name is you need to remember um the helicase which is the enzyme that's breaking those hydrogen bonds and exposing um the nitrogenous bases now this picture is just showing one little segment of DNA and we're kind of under the impression that you have like one helicase enzyme that's kind of unzipping from one side to the next but that's not a very accurate depiction so let's kind of look down here for a minute you also have this in your template right ultimately what we do is we create multiple locations where the DNA gets unzipped so kind of Imagine That in each one of these pictures right we have that that DNA helicase that's unzipping in many different directions and we have different areas where the separation is occurring and then we separate out from that location the purpose of this um and by the way where the separation is happening is called a replication fork and the reason why we have multiple replication forks is just to basically speed up the process at which um we're able to replicate our DNA right if we start replicating it in multiple locations then kind of extend towards each other then it's going to happen a lot faster than if we try to kind of start at one end and just just work our way to the other so that's Step One is separating the DNA breaking these hydrogen bonds so that we can expose the DNA to sort of act as a template um for our new strands of DNA okay so that's let's go ahead and kind of identify what we're going to say is step two step two in this picture is Illustrated on two different sides of the DNA strand we're going to kind of focus on this piece of the picture and in step two what we're doing is we are copying the parent DNA into the daughter DNA now there's going to be a couple of different aspects that go into this okay when we expose our DNA our original Parent DNA which in this picture is kind of this darker gray shading before we can start adding in the actual DNA nitrogenous bases we have to First add in a little tiny segment of RNA a little tiny primer and the reason why is that we need this primer to just attach our other DNA to so kind of step one of step two is an enzyme called primase okay you can see that right here primas is sort of this purplish looking um entity in the bottom picture an enzyme called primase is going to attach a small RNA primer so it's just a little tiny segment of RNA nitrogenous bases which in this picture the RNA primers are these little red segments so we're going to attach a little small RNA primer to the parent DNA and this is what we're going to attach our newly copied DNA to kind of acts like an anchor um onto which we're going to put our newly synthesized DNA once we've got that little primer in then we are ready to go to actually add in the new nitrogenous bases so from here an enzyme called DNA polymerase will match the nitrogenous bases in the parent DNA to complimentary bases is in the daughter DNA okay so let's kind of get a like a picture of that blown up a little bit we're going to kind of look at sort of a segment here of like what's actually happening kind of close up so let's say that we have our parent DNA strand and let's say that we've already we've already added in our here's our little here's our nitrogenous base sequence for this I'm just going to make up some letters a t g c c t Okay so we're looking at a small chunk of the parent DNA that has been exposed by the helicase first thing that's going to happen is primace is going to come in and is going to add a little tiny RNA primer so right there is no thyine in RNA so it would be UAC this is our little RNA chunk our primer which will eventually be removed kind of at the end step and then to this primer we are going to attach The New Daughter DNA and we're going to do that by bringing in complimentary bases to what's in the parent DNA so the complement to cytosine is guanine so we would kind of bring in our DNA here that matches that another guanine now we'd have an adenine let's extend our parent DNA strand to include just a couple more nitrogenous bases let's say that this is a and then let's throw in another a so this is DNA there is thymine and DNA so a t would go across from those at right and these would be attached to a phosphate backbone so in blue here is our daughter DNA strand the newly synthesized DNA basically what DNA helic uh DNA polymerase is doing is it's just using the parent DNA and bringing in the correct complement to go across from it thus creating the new half to the DNA and the same thing is happening on the other exposed side of DNA as well now kind of one factor to keep in mind here is that when we replicate our DNA we always have to replicate it in a certain direction the daughter DNA has to be built from the five Prime side of the DNA to the thre Prime side so DNA replication basically always has to go in this direction we're not going to get into what the five Prime and the three prime mean um it has to do with the structure of the sugars that are in the backbone in the way that they're oriented um not super critical for this level of biology but you do have to know that when we go to build our new DNA that it has to be from the five Prime side to the three prime side of the new DNA and the reason why that's important is because when we look at our DNA it is oriented in opposite directions so if you look at this top strand in the picture right if we're looking up here it is the way that I've drawn it in my image below five Prime is on this side three prime is across from it so you can see that in this top picture basically the DNA is being synthesized in this direction right this RNA polymerase is moving from right to left in the picture and is adding in new nitrogenous bases as it goes but the direction of the DNA is opposite on the other strand so if we look at the bottom strand down here five Prime sorry the three Prime is over here and the five Prime is over here so the DNA is actually being replicated in the opposite direction on the bottom strand so because we always need to synthesize our DNA from 5 Prime to 3 Prime what this means when we go to copy the two DNA strands basically we're going to be copying them in opposite directions of each other this is going to create a leading and a lagging strand so let's just kind of do a compare and contrast between the leading and the lagging strand if we look at this picture up here I'm going to kind of highlight it or Circle it in yellow the top strand is the leading Strand and the bottom strand is the lagging strand in this picture okay and let's go ahead and kind of just do a comparison down here the leading strand On Any Given DNA strand is going to be the Strand where DNA polymerase copies in the direction that DNA helicase is moving and the lagging is going to be the opposite it's going to be the Strand where DNA polymerase copies in the opposite direction that DNA helicase is moving all right so let's go back and look at the picture and we can see kind of that in illustration form okay if we look at this picture the black line that I've drawn represents the direction that DNA polymerase is moving right and down here again the black line is showing that the direction that the DNA polymerase is copying we can see that they are opposite of each other now if we look at the DNA helila case go ahead and look at that green the DNA helicase is moving this direction right the DNA helicase is moving from right to left in the picture so we can see the leading strand is the one where the direction that the DNA helic case is moving is the same direction that that the DNA polymerase is moving okay basically the DNA is being unzipped on the leading strand the same direction that it's being copied okay on the lagging strand the DNA is being copied in the opposite direction that the DNA is being unzipped as a result the leading strand the reason why it's called the leading is because it's going to be faster in terms of copying it on the flip side the lagging strand is going to be slower to copy here's kind of a good analogy for this let's imagine that you are copying a cookbook right we've said that a cookbook is kind of like our analogy to a chromosome of our DNA and let's say that it's one of those books where it's like a it's a three- ring binder and so you can open up the binding to take the pages out and photocopy them okay the leading strand would be like if you took the pages out of the binder from page one moving your way to page 10 and you are copying them in the order that you are taking them out of the book so like you remove page one you put page one into the copy machine and it spits out a copy you put in you take page two out of your three- ring binder you put it into the copy or it spits out its copy right the order that you are removing the pages from The cookbook is the same order that you are putting them into the copier because perhaps it's important for you to always copy in chronological page order so it's going to be very fast because literally as you take a page out of the book you put it into the copier and you get your copy the lagging strand would be like if you started taking the pages out of the um cookbook backwards but you still were intent on copying them in chronological order so for instance let's say that you have 50 pages in your cookbook so the first thing you do is you take out page 50 and then you take out page 49 and then you take out page 48 then you take out page 46 after you've taken out all four pages then you feed them into your copier in chronological order so you put in page 46 then page 47 then page 48 then page 49 then page 50 you got those copied then you keep taking more pages out of your cookbook right now you take out page 45 44 43 42 and once you have like five or six of them then you feed them into the copier in chronological order the order that you are taking the pages out of the book is opposite of the order that you are copying them right you're taking them out of the book backwards from the largest page to the smallest page but you're intent on copying them in chronological order so that's going to be a lot slower for you to ultimately copy you'd be copying the Mater material in kind of like small chunks cuz every time you remove five or six pages then you run it through the copier in chronological order whereas if you were to start at the beginning of the cookbook you are literally copying the book as you remove it so it ends up being a lot faster that's what the leading strand is like it's faster to copy basically the copying is more continuous you're basically copying in sort of one continuous strand as a result you need very few RNA primers are needed the RNA primer is kind of almost like a page that you have to put at the beginning of whatever you photocopy so you're not going to need very many RNA primers if you're copying the book in the order like if you're taking it out of the book in the order that you're copying it whereas you're going to need a lot more and then kind of the the last sort of piece and so continued in copying in one continuous strands on the lagging strand you end up copying kind of a lot of small fragment the last difference is that for the leading strand you will be able to copy all the DNA on the lagging strand you will not be able to copy over the last bit or the last several nitrogenous bases on the lagging strand so you end up not copying the entirety of your lagging strand although you are capable of copying the entirety of your leading Strand and that really has to do with the placement of those RNA primers if that's something you find really interesting I'd be happy to kind of sit down and sort of map out why you cannot copy the entirety of the lagging strand but for the sake of this class we're just going to kind of leave it at because of the fact that you have these RNA primers you can't copy the very last little segment all right so the leading strand is the Strand where the DNA polymerase and the DNA helicase are kind of doing their jobs in the same direction you're unzipping the DNA in the same direction that you're copying the DNA so it ends up being a lot faster you kind of you copy the DNA into sort of one nice continuous piece you only need like one primer at the very beginning and you can copy the DNA all the way to the end with the lagging strand basically your DNA polymerase is copying the DNA in the opposite direction that the DNA helicate case is opening the DNA so it's going to be a lot slower process because you have to keep going kind of upstream and placing in an R primer and then copying towards the three prime side of the DNA so you end up copying your DNA into all of these little tiny chunks in fragments meaning you need a whole lot more of these RNA primers and at the very end it's going to be impossible for you to copy that last little segment of DNA now which side of the DNA is the leading and the lagging strand will fluctuate um each time the DNA gets copied over so that they end up getting deleted at even rates since you can't copy the very last segment okay so that kind of takes down the copying the parent DNA down step right basically now that we have these exposed nitrogenous bases DNA polymerase is going to come in and it's going to add the complimentary nitrogenous base es to the parent DNA and attached them to a little RNA primer that an enzyme called primase added in okay so then that leads us to our very final step um which we're going to do in let's do purple as our color okay our last step step three is going to be focusing on sort of this piece of the picture and step three what we're going to do is we're just going to kind of put some finishing touches on our copy DNA so we're going to call this polishing the copied DNA okay so the first thing that we need to do before we can say that like we're completely done um is we need to go in and we need to remove all of those RNA primers right so we need to remove the RNA primers those little chunks that are pictured in red right we needed them to kind of act as an anchor point for making our new DNA but we don't want to keep them in there because the whole point is to have two identical copies of DNA at the end not mostly DNA with little tiny chunks of RNA in it so we need to remove these RNA primers and replace them with DNA if we can this is done by DNA polymerase now if you blow up the picture that we are looking at in this illustration you'll notice that there's DNA polymerase 1 and DNA polymerase 3 like in essence there's different versions of DNA polymerase that do different things we're going to we're not going to worry about that differentiation okay um we're only talking about DNA polymerase as a whole one of DNA Ply's job is to go in and remove the RNA primers and add in DNA you can see that taking place right here right this DNA polymerase is taking out that little RNA primer and it's now placed in the correct chunk of DNA in its place another thing that needs to happen is that we need to merge the DNA fragments on the lagging strand of DNA so remember on the lagging strand of DNA we ended up copying the DNA into all of these little tiny chunks right kind of like copying five pages out of your Co cookbook at a time well we need to kind of glue those little fragments together and we're going to do that with an enzyme called DNA ligase DNA ligase is an enzyme that kind of comes in and glues the fragments together so that you end up with sort of continuous strands of DNA and not all of these little tiny chunks and then the last thing that we need to do is we need to proof read we need to make sure the DNA was copied correctly this is also done by DNA polymerase so if a nitrogenous base was mistakenly put into a particular location on our uh daughter DNA then DNA polymerase will kind of go through and sort of check what was added in and make adjustments if necessary so you can kind of think of this as sort of like a process of proofreading our copied versions to make sure they do in fact reflect our original copy of DNA all right so quick recap we've got essentially three steps to the replication of DNA step one we have to expose the side sides of the DNA so they can act as templates for the daughter DNA this is done by an enzyme called DNA helicase it's going to go through and break those hydrogen bonds in essence unzipping the DNA and exposing the sides for copying in step two we need to physically copy the DNA right so we start by having an enzyme called primase put down a little tiny RNA primer which we can see that taking place here the purple entity is the primase and it's adding in the primer once that primer has been put in then DNA polymerase which is this kind of teal colored looking enzyme right here it's going to go through and add in the newly synthesized DNA by adding in the complements so if there is an addine in the parent DNA then the corresponding thyine is put into the daughter DNA right if there's a g then that's accompanied by a c t accompanied by an a right the complimentary base pairs are brought in by the DNA polymerase and placed across the parent DNA which is acting as a template now ultimately the DNA always needs to be copied from five Prime to 3 Prime for your new DNA and so because the DNA are structured in opposite directions that means that your copying is going to happen in two different directions one side of the DNA is going to be a bit easier to copy because the coping is going to happen in the same direction that the DNA is being unzipped by the DNA helicase and that's called the leading strand you end up copying your DNA into kind of one nice continuous strand you only need sort of one RNA primer to act as your Anchor Point and then as you unzip the DNA you add in your new nitrogenous bases the other strand is a little bit more cumbersome basically you are opening the DNA in the opposite direction that you're trying to copy it so what happens is you put down an RNA primer and you build in the direction you're supposed to until more DNA gets unzipped once more DNA is unzipped you put down another primer and then you build toward the segment that you have already been working on in essence you're going to end up with a more fragmented piece of daughter DNA with a lot of primers that you're then going to have to go in and get rid of during step three which is where we end up polishing sort of our copy DNA all those primers have got to come out and be replaced with DNA which DNA polymerase does DNA ligase which is kind of this yellowish looking enzyme right here that's going to go in and is going to glue together the fragments so that you have nice continuous strands and then DNA polymerase is also going to kind of be proofreading It Go as it goes to ensure that the correct nitrogenous bases have been placed in all of the various locations and those are the steps that we undergo in order to make an identical copy of DNA that we already have