 All right, does this look familiar? Well, I guess we haven't done one of these yet, but this is an animation that we're gonna look at. And in normal style, there's like 100 pages of this and we're gonna walk through the whole process of DNA replication. Let's get started. See how good it's working, how it's getting started. Okay, let's try it. In eukaryotes, where does DNA replication take place? So let's orient ourselves to where we are, where this is all happening. What do you think? This is happening in the nucleus home kids, which is, again, like, this is significant. We have to have all those enzymes present in the nucleus so that we can pull this off. If you look at the structure of DNA, what I'm showing you here is one nucleotide. And this nucleotide, you can see I've labeled the deoxyribose sugar. I've got the phosphate and I've got the nitrogen base. Very diagrammatic, but I did it this way so that we could watch the parts and pieces happening. So let's build a strand of DNA. Looks like we are, first of all, gonna look at our four nitrogen bases. So we have adenine and thymine. And look at the difference between adenine and thymine. Adenine is a purine or pyrimidine. Big base, little word, it's a purine. And thymine is the small base, big word, pyrimidine, and they connect to each other. Here's cytosine. The way that I've done this lets you see that the light colors, the light colors form a bond with each other. So the A and the T are gonna form a bond, but the like sizes are illustrating the similar, who's a pyrimidine and who's a purine. So we have cytosine here and it's a pyrimidine. And guanine is a purine. How is RNA different? So just for the fun of it, let's talk through how RNA is different from DNA. We're just gonna have a different sugar and that's it, is that it? No, it's not it because we also have uracil that is different. But other than that, our bases are the same. DNA likes to hang out in the double helix. RNA is like, dude, I like to make different shapes with my cool molecules. And we will see some of those really cool shapes. Okay, we're gonna make one complete molecule of DNA. Here's the question. Predict what one molecule will look like if it has nine nucleotides per side. So if you think about using these nucleotide building blocks, predict what that nucleotide is gonna, what the whole molecule is gonna look like if we pull together nine nucleotides to build this thing. Okay, I'm gonna start us off with the five, I'm gonna label our five prime and three prime edges so that you can also see this. So predict where am I gonna put the five prime and where am I gonna put the three prime on this particular nucleotide? Did you put the five prime on the phosphate end? Nice work. And the three prime on the sugar end. I've got the bond there because it's gonna, like we need that bond to connect with the next one and that's our three prime sugar. All right, let's see. We're gonna add somebody else. Check it out. Look at how the three prime end moved. We formed a chemical bond. In fact, can you see this mouse? Yes, we formed a chemical bond between the phosphate of the second nucleotide and the sugar of the first nucleotide. And our five prime end got farther away from the three prime end. We're adding more nucleotides. The whole thing is getting longer and you'll notice that nine of them. There, we have nine of them. Now we're gonna build the other side of our single molecule and you can, we're good, right? We've got our five prime end labeled and our three prime end labeled. What's gonna be true of the other side? And just for fun of it, do you agree that we just built that molecule in the five prime to three prime direction? Let's build the other side in the five prime to three prime direction. So predict where are we going to have our first molecule and what's it gonna be? Where's our first nucleotide gonna go and what is the base gonna be? Is that what you guessed? Check it out, it's upside down because it is, it's upside down. But do you see how we have a guanine forming a bond with the cytosine? And that little you can see right here, I've made a little hydrogen bond connecting them. So there's an actual thing there to say, yes, this is the bond that forms between our nitrogen bases. Okay, predict who's the next base that we're gonna add? Oops, you have to go the other way. This is the way that we have to go to find out what the next one is and look what direction we're building. We're building from the five prime to the three prime end. We keep adding on to that three prime end and the three prime end gets farther and farther away from the five prime end. We're building in the opposite direction. We've completed our molecule. That whole thing, all we did just now is review DNA structure. We did not replicate a molecule. We didn't do anything new. But you can see really clearly the way we've got anti-parallel strands and that the three prime and five prime can help us understand the directionality of these molecules. Okay, are you ready to go through some replication? We're gonna replicate in the downward direction. So I want you to predict what the two copies are gonna look like. Might be worth drawing it all out and visualizing what you're gonna end up with at the end of this. What's our first enzyme that's gonna be involved? Who would you guess we would need to have involved here? We gotta get Helicase involved because Helicase is the guy who's gonna split, break all those hydrogen bonds down the middle and separate the two strands of DNA. Get to work, Helicase. Oh look, I'm like nothing's happening. Did you see it before I saw it? The hydrogen bonds are being broken and now they're all broken. Thank you very much, Helicase. And now we separate the two molecules. The two strands are being separated. We separated the strands just to give room for the two new strands to be made. And I only did nine, nine nucleotides because you can imagine how long it takes to do these things. So I imagine that above this is our double Helix and this is our replication zone where we're gonna do all the DNA replication but there's a double Helix up there and Topo isomerase is up there making sure we don't super coil ourselves. Okay, who's next? What enzyme are you gonna predict is gonna be the next person to be involved here? What do you think? I know what I think. I think we need primase. That's exactly what I was gonna say. It's a good thing. Can you tell that I haven't looked at this yet before I'm just recording it for fun. So primase comes in and what did we say primase does? Primase lays down an RNA primer and DNA polymerase is gonna build the new molecule in the five prime to three prime direction. So predict where, predict where you're gonna lay down your primer. And which side are you gonna lay it down? Oh, because we were making a copy down. So which side are we gonna do? Since DNA polymerase has to move in the, five has to build the new molecule in the five prime to three prime direction. We gotta say, if we have to build down we're gonna start with this strand on the right. Goodbye primase, who's coming in next? I'm saying DNA polymerase next. Yes, I got it right. DNA polymerase is here. We've got our RNA primer laid down and now we can go to town, predict the nucleotides we're gonna lay down. What do you notice about these nucleotides as we build them together? It's almost exactly a perfect copy except for what? That primer. I believe that I looked it up and DNA polymerase removes the primer in this process. There's actually multiple forms of DNA polymerase, but I think that's the one who's responsible for removing that primer, which is something that we need to do. Let's see what happens next. I love this. I guess it makes sense that I predict my own brains thing because we'll see what I said here. I did say DNA polymerase replaces the primer, so let's do that. Done, we've replaced the primer and now if you look carefully at that molecule, it's identical to what we started with. I think we're going to make the next side, but what might be tricky? If we wanna make a copy down, we can't make a copy down because if we made our copy down, we'd be building in the three prime to five prime direction, so we can't do that. So we have to build from the bottom up. And that's trickier because we're opening up here. We're unraveling up here and dude, yes, the other side just can zip along. This side we're gonna have to start at the bottom and go up. Lay down a primer. DNA polymerase comes in and builds a strand. Primase is gonna come back and lay down another primer. This is the lagging strand and it's because we have to build, we have to go backwards, which is a bummer. DNA polymerase comes down once we've got the primer. Now, I don't think that this is very clear. I'm gonna show, oops, okay. I'm gonna show you this right here. I don't think that that bond should be there because this is a fragment. We're not, this DNA nucleotide isn't going to form a bond right, oh, nope, that ligase is coming in later because we have to have DNA polymerase replace those primers. That's what happens first, watch. Replace a primer, replace a primer. Now, this is what I was trying to tell you that once DNA polymerase replaces the primer, that's where we get these fragments that aren't there on the other side because DNA polymerase just gets to do its job and there aren't as many primers to fix. So, who comes in and fixes that ligase? Watch that point, watch right here. Ligase is gonna come in and build, look at how lovely that is. Yes, right there, ligase builds, forms a bond between those Okazaki fragments on the lagging strand of DNA. All right, two identical molecules. Started semi-conservative because we started with the outside ends of our old DNA, the inside sides are new DNA. Dude, what's this? I think that we already did our, and I think, what is that? I think we're done. Yeah, those are my snacks at the end, my things that I didn't want to delete. This is where I should have had a slide that said the end because we stopped and we're done. We have one more clip in this lecture and we're gonna talk really briefly about we're gonna remind ourselves of the big picture again.