hey biology students uh we're going to take a couple minutes here and go through a uh a summary of DNA replication that you can have 247 access to as opposed to just the time we get to spend covering it in class I need to start off by saying that this image is not mine I did not create it but as I made this the PowerPoint uh that I this is on two years ago I don't remember where I got it from and I couldn't find it so hopefully I'm not viting violating any copyright laws but this is not my work in image however the explanation that follows uh is is original and it is mine so here you can see um and for those of you that are not in my class I'm assuming that you already know a little bit about DNA replication so we're going to go walk through this first by following what happens in the leading strand shown here on top and then the lagging strand on bottom so let's start off with noting the template orientation template strand orientation in the leading strand notice the template strand is traveling in the three to five direction as we move right to left on your screen in this image the overall direction of replication as indicated down here on the bottom is actually going right to left okay so when I say the template strand has 3 to five orientation that is this dark blue strand on top so this is this is the leading strand now because as we know from class the enzyme DNA polymerase can only add sequential nucleotides in the five Prime to three prime Direction so since the template strand is going 3 to 5 that allows the syn synthesiz strand to go 5 to three in other words the DNA polymerase can just Cruise right along this template strand adding individual nucleotides as it goes so here these purple ovals are DNA it's the DNA helicase complex which is going to unzip DNA okay so as the DNA gets unzipped it presents one of the strands one of the template strands in order for it to be replicated right behind the helicase complex we have these little red semi circles which are single strand binding proteins they prevent these two template strands of DNA from reannealing so if left to its own devices uh this DNA strand would probably come back together and that would be really bad we wouldn't be able to replicate the DNA so these little red semicircles here uh as shown in the picture are going to hold the two strands apart from one another so now on the leading strand somewhere off to the right of the screen where we can't see um an RNA primer would have been added by the enzyme RNA primer maybe somewhere over here and as soon as that RNA primer is added DNA polymerase now has a binding site where it can attach to the DNA and then add nucleotides in the five Prime to three prime Direction and recall that DNA polymerase needs that RNA primer because it is a little bit deficient in its uh not a little bit it is deficient in its ability to synthesize DNA if it doesn't have a double stranded area of DNA for it to originally bind to and so that's the reason why we have the RNA primase and the RNA primer and that pretty much covers the leading strand once the RNA primer is laid down DNA polymerase can just follow the helicase complex and as the DNA gets unzipped polymerase follows behind adding uh sequential nucleotides in the five Prime to three prime Direction and now for the lagging straand it's a little bit more difficult as you can see in the picture these lagging straand is actually added in fragments and so as the DNA gets unzipped the template strand is actually traveling in the five Prime to three prime Direction moving right to left which means the DNA polymerase cannot add new bases in the same direction it can't add bases traveling to the left on your screen it's going to have to travel from left to right okay because when it travels from left to right it's able to add new nucleotides in the synthesized strand in the five Prime to three prime Direction and so as DNA helicase unzips a section of DNA right behind where it's unzipped in this area RNA primas can come in and add an RNA primer and then DNA polymerase binds right here and can synthesize a small segment of DNA up until it bumps into another one of these fragments okay and so people always ask well why is it in these fragments why can't it go in one piece and the reason why DNA polymerase is not going to synthesize this lagging strand in one piece is because if we waited for the helicase to unzip the entire DNA strand then there'd be a possibility for the DNA strand to fold back on itself get twisted up um a a bunch of things that could be very complicated could happen that prevent us from synthesizing a new strand of DNA and so DNA instead on the lagging strand gets added in these fragments called okazaki fragments and so DNA polymerase will add new bases in the five Prime to three prime Direction until it bumps into an RNA primer in which case it will detach and stop and then later processes will connect those pieces so hopefully this is helpful and good luck on your test