Hi, in this video, we're going to be talking about helical formations of DNA. So, one of the first helical formations that I want to talk to you about, which is really important, and you may not have heard of before, is supercoiling. Supercoiling is a property of helical DNA. And so it's kind of hard to imagine, but supercoiling is DNA that has twisted upon itself. And so, if you can imagine, if you have two ropes or two pieces of hair or two shoelaces, and you make them into a helix. So, twist them together; when you keep twisting them, eventually, they fold up on each other. And so that's the best way to describe this. And so in DNA, like in ropes or pairs of shoestrings, this also can occur, and that can occur in circular or linear DNA. But you can imagine this is really bad. So, if you're twisting a rope, and it twisted upon itself, that's not necessarily a very useful rope. And then the same with DNA, it really prevents a lot of things from happening that should happen with DNA, like replication or anything gene expression. And so, we need enzymes and things that can break that apart. These enzymes are called topoisomerases. It's kind of a mouthful, and they are enzymes that convert DNA between a supercoiled and relaxed state. Now there are two of them, they're type I and type II. And they work in reducing supercoiling by two different mechanisms. So type I does a single strand break, and type II does double strand breaks, and they release breaks. They put breaks into DNA to release tension. So you can see here, this is going to be supercoiled DNA. And this is circular DNA. And so you can see here that if there's a nick for sort of a break here, there's another one here, it releases this tension, and eventually, there's another one here. And so you get this circular DNA molecule which is what it's supposed to look like, and this sort of relaxed state instead of in a supercoiled form.
Now, there's another way that we can sort of mess with the DNA helix, and that's through DNA denaturing, which is going to be separating or renaturing, which is rejoining strands of DNA. Now, this occurs in cells, especially during things like replication or gene expression. But we can also do it in laboratories. And how this happens is to denature DNA, we can denature it by breaking hydrogen bonds. And so, how do we break hydrogen bonds? Well, you sort of break them in ways that you would break other bonds. You can increase the heat, you can change the pH, you can expose them to UV light, and all of these things will break hydrogen bonds and sort of separate the two DNA strands. Now, in the laboratory, we, of course, need to be able to quantify this. You know, how much heat or how much pH, and so the way we do heat is we actually say the DNA melting temperature. And that's a temperature that separates the DNA strands. But, of course, like other chemical bonds, this is going to depend on the number of hydrogen bonds present. And so, if you remember, the GC pairs here have an extra hydrogen bond. So they bond with three hydrogen bonds instead of the AT, which uses two hydrogen bonds. And so, in strands of DNA that have a really high GC content, which means that they have a lot of GC base pairs, that's, of course, going to raise the temperature needed to break all of those bonds and to separate the strands. So here we have the DNA double helix; you can see that it's separated through here. And so, it's denaturing, and eventually, things can come back in here and renature it, which means putting it back together; that's what this looks like. So now, let's move on.
