 I have an animation to help you visualize this process of DNA replication. Remember that DNA is a double-stranded molecule made up of nucleotides. These are the nucleotides that I am showing you. First of all, we have this dark pink pentagon is pentagon. Oh, hell, I have no idea. Whatever that shape is right there, the pento, whatever. I called it a pentagram. Dude, that's going to really irritate me. I know this is a circle, and I know this is a triangle. And I don't know. I can't, somebody help me. I'm sure you will tell me what that shape is. That's representing deoxyribose. So what kind of nucleotides are these? These are DNA nucleotides. Another hint that they're DNA is that this is thymine. This is adenine, guanine, and cytosine. These are my nitrogen bases. And what do you think the bright yellow circle is? Those are my phosphate molecules. Okay, so let's put these guys together and make a double-strand of DNA. Ah, check it out. This is an actual double-strand. If you look at it, you can see that indeed it is so. If they all match up, the little white pieces here represent hydrogen bonds between my nitrogen bases. And I'm going to show you that we actually have some enzymes that are involved in this process. I'm not going to go through the enzymes until we actually see what they're going to do. So are you ready to see how this works? The, oh, wait a second. Apparently, I want to tell you, first of all, that the enzymes involved are nothing more than proteins. Just in case you were wondering, do they really look like this? No, they don't. They really look like this. How crazy is that that they actually know what DNA polymerase looks like? DNA polymerase is my key player in DNA replication. We're just making a copy of the DNA molecule. So we're going to pretend like DNA polymerase looks like that, but don't, even though it's a wendified animation, don't be confused because this is just chemistry. I just like making it fun. All right, shall we see who gets started first? First of all, who's this guy? Helicase. Helicase is going to come in to a DNA molecule and start the whole process by breaking, look at what's happening, breaking the hydrogen bonds between the nitrogen bases and basically splitting open the DNA. That's the job of Helicase. Awesome. Helicase is done. Who's going to come in next? Next, we have our friend Primase. Primase, all these guys are enzymes. And Primase looks way cooler than Helicase. I mean, I'm a fan of that color hair. Primase comes in and is actually going to lay down an RNA primer. Primase makes a primer. The main dog, who is the main dog? DNA polymerase. DNA polymerase can't do its job unless it has an RNA nucleotide to attach to. So check this out. If you look, the RNA nucleotide has ribose sugar. It's a lighter pink. And this RNA nucleotide actually has a uracil base attached to it. This one has adenine, so it could be DNA. But the ribose sugar lets you know that that's actually RNA. So Helicase splits it apart. Primase comes in and lays down primers. And those primers are necessary for my next person, who is DNA polymerase? Did you see DNA polymerase is headed out? DNA polymerase is going to get some, what are these things called, DNA nucleotides. DNA polymerase is going to attach DNA, matching DNA nucleotides to the split apart template. And look, it brought in a thymine nucleotide to bond to the adenine nucleotide. And let's see what happens next. It goes out to get another one. Here, let's add another one. And another one. And another one. And I'm not actually sure if this is technically correct that all of these different enzymes come in and do this job. But they do, they all, DNA polymerase is the person responsible for adding in the nucleotides person, the enzyme responsible for adding in the nucleotides in the appropriate location. Now, that was pretty fast, wasn't it? You tell me, what direction did the new strand get built in? This is important. Do you agree that this is a phosphate end? So this is a five prime end. I'd write it on here if I could, but I can't. And here's a sugar end. This is my three prime end. Do you agree that DNA polymerase built the new strand in the five prime to three prime direction? Yes, you do agree with that. I'm going to even write that down for you because I don't want you to forget that part. DNA polymerase builds a new strand in the five prime to three prime direction. This new strand is built from five prime to three prime and I'm not the other way around. Now, this is where your mind is going to get blown because watch what's going to happen on my other side. Wait a minute, I'm not blowing your mind yet. First of all, I'm going to bring in a DNA polymerase to replace the primer. Now, what do you have? What? You have an entire new molecule that's identical to what you started with. DNA polymerase comes in, brings in all the nucleotides, they match perfectly and then it replaces the primer and done. You've got one half, you've got one brand new molecule. You still have this other side that's kind of doing its thing over here and it's going to be different on the other side. So watch. First of all, I'm showing you up here in the corner is DNA polymerase that's responsible for what's happening. What direction could DNA polymerase work? Five prime to three prime. Five prime to three prime. DNA polymerase is going to have to build backwards on this strand of DNA. On the first strand, it's called the leading strand. Dude, boom, it happens fast, super quick, done. Five prime to three prime, you just go. On the lagging strand, it's not the same. On the lagging strand, you have to build from five prime to three prime just like normal, oops, other way. But now, if DNA polymerase is working this direction, now it's going to have to come up, lay down a new primer and go back the other direction again. You basically make fragments and what do you know? They're called okazaki fragments on the lagging strand. So watch, here comes primase. Primase is going to drop in another primer. Which one's faster, the leading strand or the lagging strand? No question, the lagging strand is going to be much slower. Primase dropped the primer, DNA polymerase can come in and make another okazaki fragment. Now, you'll notice that on the leading strand, we just formed chemical bonds, it was super easy. But notice on the lagging strand, we're going to have these breaks in our molecule because this whole thing, this is a fragment. This is an okazaki fragment, this is an okazaki fragment. So we have to do something about that. We're going to do ligase to the rescue. But also, we're going to have to replace our primers. And here's my little DNA polymerase replacing the primers. Right, what, let me see, there it goes. There's my primer got replaced. And then what do you think ligase is going to do? Oops, I think ligase already did its job. Ha, it did. Look, we replaced this primer. We have two DNA nucleotides together. And now watch, this is what ligase does. What just happened? Ligase ligated the two okazaki strands together. And now, finally, after all that, we have two molecules of DNA that are identical to what you started with. So if we were to look at them, this was my original one side. This was my original other side. And now you can see that once we feed these in. And the interesting thing is that Watson and Crick, when they discovered the structure of DNA, they totally speculated that they guessed that this would be a possibility that you would have, that this could lead to an explanation for how DNA replicated itself. All right, dog pounds. In the next two lectures, we're going to talk about DNA function. How is it that DNA actually makes or codes for the building of protein?