So let's talk about homologous recombination, which is what this video is going to be about. And homologous recombination is defined by an exchange of genetic material at equivalent positions, meaning at the same genes, the same position, along 2 homologous chromosomes. Now, it can be initiated, if I can spell, started, by single strand breaks. You may also see these as single strand nicks, same thing, or double stranded breaks. We're going to talk about each one individually. So first, let's talk about the single strand breaks or nicks. So what happens is, first, is that all of this is the word form of this example here. So I'm going to walk you through the example, and if you need to go back and read it, this is exactly what I'm saying. So, here we have 2 homologous chromosomes, 1 and 2. The reason there are 4 lines drawn here is because each homologous chromosome is a double helix. Right? So there are 2 strands of DNA. So I'm not drawing it as a double helix because it's really confusing if it is, but just know that these are 2 double helixes because they're 2 homologous chromosomes. Now, instead, we're going to talk about the single strand nick first, so that's what I used to see here. There's been some kind of nick or break in one strand of each of the homologous chromosome double helices, and this strand leaves an opening so that the blue strand can invade the red, and the red chromosome can invade the blue chromosome. So when that happens, it creates this structure, which is called a cross bridge structure, here. And, it's actually spelled with one I. It's just a typo here with 2 i's. And, this cross-bridge structure can move up and down the chromosome in a process called branch migration. Let me back up. Is this crossbred moving? Now, when the blue, it makes the red because they're homologous chromosomes and because this happened at the equivalent position, this means that these two strands are now complementary. So hydrogen bonds begin forming between the blue and the red strands on both of these regions where it's invaded. Now when this happens, it's often drawn like this weird x structure here, and this is because, it's easy to see how the chromosomes are made, but not so easy to refer from here. But essentially, what you're seeing is this. So you're seeing, this x is here. This cross bridge is here. And so what happens is that at this cross bridge, there is an enzyme that comes in and makes another nick, and this nick occurs here. So what you get is you get one set of homologous chromosomes, which are now here, and you get one set which is over here. Now these are not drawn equally, that's just because of me, not because they aren't actually equal. But essentially, these 2 homologous chromosomes now contain a mixture of red and blue DNA, and you can see them down here. So you have the first longest chromosome and the second, and now they have mixtures of red and blue DNA on both of the DNA strands even though there was just one invasion that occurred. So don't get confused by this image. It's just it's not that the chromosomes actually make this sort of x image, it's just drawn that way to show you where the nick is occurring and how these chromosomes are being made in the end. So that is the single strand breaks.
Let's now turn the page.