Recombination - Video Tutorials & Practice Problems

Recombination after Single Strand Breaks

Hi in this video we're gonna be talking about recombination. 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 and equivalent positions, meaning that the same genes the same position along two homologous chromosomes. Now it can be initiated. Second spell started by single strand breaks. You may also see these as single strand next same thing or double stranded breaks. We're gonna 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 gonna walk you through the example and if you need to go back and read it, this is exactly what I'm saying. So um here we have two homologous chromosomes, one and two. The reason there's four lines drawn here is because each homologous chromosome, the double helix, right? So there's two strands of D. N. A. So I'm not drawing it as a double helix because it's really confusing if it is but just know that these are double. These are two double helix is because they're two homologous chromosomes. Now I said we're going to talk about the single strand neck first. So that's what I you see here there's been some kind of nick or a break in one strand of each of the homologous chromosomes double helix is. 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 was just a typo here with the two eyes. And um the this cross bridge structure can move up and down the chromosome. And in a process called branch migration, let me back up. Is this crossbred moving now when the blue invades 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 um 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 um this X. Is here this cross bridges here. And so what happens is that at this cross bridge there is a um 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 is 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. Um But essentially these two homologous chromosomes now contain a mixture of red and blue D. N. A. And you can see them down here. So you have the first homologous chromosome in the second and now they have mixtures of red and blue D. N. A. On both of the D. N. A. 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.

Recombination after Double Strand Breaks

Okay so the double strand breaks, like the single strand breaks, follow certain steps. Again this image goes along with these written steps here. So I'm going to just go through the image. So what happens here is you have two homologous chromosomes. Remember there's two D. N. A strands because each chromosome has two D. N. A. Strands that make up a double helix. So what happens is you get a double strand break usually just in one of them not in both. So a double strand break and a protein comes in called wreck A. And this creates sort of gets rid of these nucleotides here to create what's called a three prime overhang. Let me back out of the way. So you make sure you can see everything now when this three prime overhang is here. What happens is again a an invasion occurs where the blue invades the red and the red invades the blue. Um in order to be able to make sure that it has complementary nucleotides. Because if there's a double strand break that means that you're missing nucleotides. And therefore you need to get those nucleotides from the blue strand because the blue strand will have them because they're homologous chromosomes. So they have the same information on them. So um this blue strand here goes up to have give a template for this top red strand that's now broken and this red strand invades the blue strand so that it can get a complimentary D. N. A sequence to repair that broken strand as well. Now this strand here when it's a double strand break is called a holiday junction. And this holiday junction can also move up and down the chromosome. So here's an example. There's two Holliday junctions forming here where this strand because it's not broken, it's um it's one continuous piece invading that red strand where the red string is broken. So you have these two that are forming here. But essentially the blue strand in both cases up here and down here is acting as a template to replace that information. So what happens is when these reform you get red blue and red. So you have this red strand here. This blue strand here and this red strand here and you have red red. And then the place where this was broken used the blue strand for as a template. So now you have the blue strand genetic information here. The same thing happens here. So you get one entire blue strand which is down here and you get blue blue. But then you get red where this red strand has invaded. But because it used the blue strand as a template, you see this blue line here. Now we call this a combination right? Because it's occurring at equivalent genetic locations and positions. But you can see that there's a combination. Now there can be slight differences between homologous chromosomes and we actually know this in a lot of ways right? There can be a little differences. So if you have two different alleles. Well if you do recombination that can result in a your homologous chromosome having different alleles than it did before. It can also um dominant or recessive alleles. It can also have different point mutations or something a slight a genetic change that gets transferred to that other strand when it didn't have it before because of those D. N. A. Double stranded breaks. So that's your combination. Let's not turn the page.

Gene Conversion

Okay, so now let's talk about gene conversion. Now gene conversion is the non reciprocal genetic exchange between two closely linked genes. So we talked about recombination. We were talking about the same genetic low key low side. We're talking about the same genes of the same androgenic regions that equip different positions was the term I used. This is actually non reciprocal genetic exchange. So these are genes that may be closely linked, meaning that they're close together on the chromosomes but that they're not the same genes, it's not the same position. Non reciprocal, not the same. And so this is caused due to some kind of mismatch of base pairs during the duplex formation. So what this happens? So what happens is if you have some kind of break and you have another chromosome down here, if this chromosome this section here comes up and matches with this section due to either a closely related sequence or just a mismatch of D. N. A. Instead of invading this region. So this region would be recombination, right? Because it would be the same region. But this one it can actually be gene conversion because it's actually a non uh not the same region. And what can happen is it has the ability to convert one genetic allele into another allele. And so when you see this in real life, what happens is that if you have an a a genotype, So hetero ziggy's genotype and you put that genotype into gametes. Half of the gametes will get a dominant allele and half will get a recessive allele gene conversion will actually cause you to get 3/4 of the gametes with the dominant allele and 1/4 to get the recessive allele. Or it can be vice versa depending on how the convergence happened. So what happens here is you have two homologous chromosomes and some kind of conversion happens where it's it's not genetically precise and so you actually get the blue alleles transferring entirely to the red chromosome even though it's not at the same position. And so if you have have a A. Here and you have a um a here you can see how these gene conversions ended up resulting in um improper alil ratios. So that is gene conversion. So with that let's now move on.

Homologous recombination is defined as what?

• A. Exchange of alleles from two different genes
• B. Exchange of regulatory regions from two different genes
• C. Exchange that occurs at equivalent positions in two non-homologous chromosomes
• D. Exchange that occurs at equivalent positions in two homologous chromosomes

Homologous recombination repairs double-strand breaks only

• A. True
• B. False

Gene conversion can cause a Aa genotype to be sorted into gametes in which way?

• A. All gametes get A allele
• B. All gametes get a allele
• C. ½ gametes get A and ½ gametes get a
• D. ¾ gametes get a and ¼ gametes get A