Recombination - Online Tutor, Practice Problems & Exam Prep

Homologous recombination involves the exchange of genetic material between homologous chromosomes at equivalent positions, initiated by single or double strand breaks. Single strand breaks create a cross bridge structure allowing DNA strands to invade each other, while double strand breaks form a Holliday Junction for template repair. Gene conversion, a nonreciprocal exchange, can alter allele ratios, leading to unexpected genetic outcomes. Understanding these processes is crucial for grasping genetic variation and inheritance mechanisms.

Recombination after Single Strand Breaks

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.

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 2 homologous chromosomes. Remember, there are 2 DNA strands, because each chromosome has 2 DNA strands that make up a double helix. So what happens is you get a double strand break, usually in just one of them, not in both.

So a double strand break, and a protein comes in called RecA, and this creates, sort of gets rid of these nucleotides here to create what's called a 3prime overhang. Let me back out of the way so you make sure you can see everything. Now, when this 3prime overhang is here, what happens is again an invasion occurs where the blue invades the red and the red invades the blue, in order 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, this blue strand here goes up to provide 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 complementary DNA sequence, to repair that broken strand, as well. Now this strand here, when it's a double strand break, it's called a Holiday Junction, and this Holiday Junction can also move up and down the chromosome. So here's an example of there being 2 Holiday Junctions forming here where this strand, because it's not broken, is one continuous piece invading that red strand, where the red strand is broken.

So you have these, like, 2 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 uses the blue strand 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 recombination. Right? Because it's occurring at equivalent genetic locations at positions, but you can see that there's a combination.

Now there can be slight differences between homozygous chromosomes, and we actually know this in a lot of ways. Right? There can be allelic differences. So if you have 2 different alleles, well, if you do recombination, that can result in your homologous chromosome having different alleles than it did before. It can also dominate or show recessive alleles. It can also have different point mutations or something, a slight genetic change that gets transferred to that other strand when it didn't have it before because of those DNA double-strand breaks. So that's recombination.

Let's now turn the page.

Gene Conversion

Okay. So now let's talk about gene conversion. Gene conversion is the nonreciprocal genetic exchange between 2 closely linked genes. When we talked about recombination, we were discussing the same genetic loci, the same genes at the endrogenic regions at equivalent positions, which is the term I use. This is actually a nonreciprocal genetic exchange. These are genes that may be closely linked, meaning that they're close together on the chromosomes, but they are not the same genes. It's not the same position. It is nonreciprocal. Not the same. This is caused due to some kind of mismatch of base pairs during the duplex formation. What happens is 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 either due to a closely related sequence or there's a mismatch of DNA 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 not the same region. What can happen is it has the ability to convert one genetic allele into another allele.

When you see this in real life, what happens is that if you have an AA genotype, so a heterozygous genotype, and you place 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 fourths of the gametes with the dominant allele and 1 fourth to get the recessive allele, or it can be vice versa depending on how the conversions happen. What happens here is you have 2 homologous chromosomes and some kind of conversion happens where it's not genetically precise. Therefore, you actually get the blue alleles transferring entirely to the red chromosome, even though it's not at the same position. If you have AA here and AA here, you can see how these gene conversions end up resulting in improper allele ratios. 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

Here’s what students ask on this topic:

What is homologous recombination and how does it occur?

Homologous recombination is the exchange of genetic material between homologous chromosomes at equivalent positions. It can be initiated by single or double strand breaks. In single strand breaks, a nick in one strand allows the DNA to invade the homologous chromosome, forming a cross bridge structure. This structure can move along the chromosome in a process called branch migration. In double strand breaks, a protein called RecA creates a 3' overhang, leading to strand invasion and the formation of a Holliday Junction. This junction can also migrate, facilitating the repair of the broken DNA strands using the homologous chromosome as a template.

What is the difference between single strand breaks and double strand breaks in homologous recombination?

Single strand breaks involve a nick in one strand of the DNA double helix, allowing the DNA strand to invade the homologous chromosome and form a cross bridge structure. This structure can move along the chromosome through branch migration. Double strand breaks, on the other hand, involve breaks in both strands of the DNA double helix. A protein called RecA creates a 3' overhang, leading to strand invasion and the formation of a Holliday Junction. This junction can migrate along the chromosome, facilitating the repair of the broken DNA strands using the homologous chromosome as a template.

What is gene conversion and how does it differ from homologous recombination?

Gene conversion is a nonreciprocal genetic exchange between two closely linked genes, meaning they are close together on the chromosome but not at the same position. It occurs due to a mismatch of base pairs during duplex formation, leading to the conversion of one genetic allele into another. Unlike homologous recombination, which involves the exchange of genetic material at equivalent positions on homologous chromosomes, gene conversion can result in unexpected allele ratios. For example, instead of a 1:1 ratio of dominant to recessive alleles, gene conversion can lead to a 3:1 or 1:3 ratio.

What is a Holliday Junction and what role does it play in homologous recombination?

A Holliday Junction is a cross-shaped structure that forms during homologous recombination when there is a double strand break. It occurs when the broken DNA strands invade the homologous chromosome and use it as a template for repair. The Holliday Junction can migrate along the chromosome, facilitating the exchange of genetic material between the homologous chromosomes. This structure is crucial for ensuring that the genetic information is accurately repaired and exchanged, contributing to genetic diversity and proper chromosome segregation during cell division.

How does branch migration occur during homologous recombination?

Branch migration occurs during homologous recombination when the cross bridge structure, formed after a single strand break, moves along the chromosome. This movement is facilitated by enzymes that help the DNA strands slide past each other, allowing the exchange of genetic material to extend along the chromosome. Branch migration ensures that the homologous chromosomes can accurately align and exchange genetic information over a larger region, contributing to genetic diversity and proper chromosome segregation during cell division.