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Chromosomal crossover in meiosis I

Meiosis is a process where germ cells divide to produce gametes, such as sperm and egg cells. In prophase I of meiosis, chromosomes condense and homologous recombination takes place, leading to genetic variation through chromosomal crossover. This forms a tetrad, which is made up of four chromatids (two sister chromatids per chromosome). Created by Sal Khan.

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Video transcript

- Let's now jump into understanding meiosis in some depth. So let's start with the germ cell. As we mentioned already, a germ cell is a cell that it can either go to mitosis to produce other germ cells or it can undergo meiosis in order to produce gametes. So this is a germ cell right over here. Let me draw the nuclear membrane. Let me draw the nucleus larger because that's where we care a lot about the chromosomes in it. And let me draw a centrosome which will play a role later on. I wanna do that in ... Let's see, I'll do that in this blue color. Each centromosome has two centrioles in it. I just wanna clarify some of the terminology. And in the mitosis videos, I focused on cells of an organism, I just kind of made it up, that had two chromosomes, that had a diploid number of two that had one homologous pair, that had one chromosome from each of its parents. For this video, I'm gonna focus on a species, not human beings, that would have 23 pairs or 46 chromosomes. I'm gonna focus on a species that has, that's diploid number is four. And so, let's say it has two chromosomes from the father. And let me do that. I'll do that in this orange color. Now, I'll do that in the chromatin, I'll kind of depict the chromatin state, it's kind of unwound. So maybe it has a long one from the father and it has a short one from the father. And then it has homologous chromosomes from the mother. So it would have the long one from the mother and it would have the short one from the mother just like that. And obviously this is a huge simplification but hopefully this discuss the point across. So here, it has a diploid number of chromosomes. So this is, let me write this down. This is diploid number is equal to, we have four chromosomes. And then this thing, this germ cell. Let me write this down. This is a germ cell right over here. It will go through interphase. So let me draw that. So it will go through interphase, in which it grows and it can replicate its DNA and its centrosome. And so, let me draw that. So after it goes through interface, I wanna use my space carefully because I have a lot of steps to go through. After it goes through interface, I am going to have in my nucleus here, my DNA will have replicated. So this long chromosome from my father, now all the DNA will have replicated so it may look something like that. And it's attached at a centromere, All these centro words, at a centromere right here. But I'm still trying to draw it in kind of the chromatin state. It's actually all spread out. It's not bunched up so that you can see it very clearly as these X's in a simple microscope. So it's just replicated. And after replicating, it is still one chromosome. It has twice the genetic material but it is still one chromosome. That one chromosome is now made up of two sister chromatids. we talked a lot about that in the mitosis video, but it doesn't hurt to reinforce because it can get a little bit confusing. And then you have that shorter chromosome from the father and then that also replicates into two sister chromatids attached at a centromere. So these are still two chromosomes from the father. It has twice the amount of DNA but it's containing the same information, just duplicate versions of that same information. And the same thing's gonna happen from the mother. You had that long chromosome from the mother, homologous to this right over here. It's going to replicate. So it's now going to be two sister chromatids. And then you have a short strand from the mother that was homologous to this one from your father. And that's also gonna replicate. And so, it's like that. And at the end of interface, it would actually all be spread out. Once again, it won't be bunched up into these clearly discernible X's. I drew them a little bit that way, otherwise, because you would have trouble seeing how that replicated. And we also have replicated our centrosome as we've gone through interphase. Now, we are ready. In fact, now we are ready for either mitosis or meiosis. But as I said, the focus of this video is going to be meiosis so let's do some meiosis. So the first phase, so the first several phases we call meiosis I. And the beginning of meiosis I is prophase I. So let's see what happens in prophase I. So prophase I. And so, let me draw the cell right over here. So prophase I. A couple of things happen. The nuclear membrane begins to dissolve. This is very similar to prophase when we're looking at mitosis. So the nuclear envelope begins to dissolve. These things start to maybe migrate a little bit. So these characters are trying to go at different ends. And the DNA starts to bunch up into kind of its condensed form. So now I can draw it. So now I can start to draw it as proper. So this is the one from the father right over here. And this is the one from the mother. And I'm drawing, I'm overlapping on purpose because something very interesting happens especially in meiosis. So it's the mother right over here. Let me see. Let's now do the centromere in blue now. That's the centromere. Now this is the shorter ones from the father. These are the shorter ones from the mother. And actually, let me just do draw them on opposite sides just to show that they don't have to, the ones from the father aren't always on the left hand side. So this is the shorter one from the father. They couldn't be all on the left hand side but doesn't this all they have to be. And this is the shorter one from the mother. And I will draw this overlapping although they could have. Shorter one from the mother. And once again, each of these, this is a homologous pair, that's a homologous pair over there. Now, the DNA has been replicated so in each of the chromosomes in a homologous pair, you have two sister chromatids. And so, in this entire homologous pair, you have four chromatids. And so, this is sometimes called a tetrad. So let me just give ourselves some terminology. So this right over here is called a tetrad or often called a tetrad. Now, the reason why I drew this overlapping is when we are in prophase I in meiosis I. Let me label this. This is prophase I. You can get some genetic recombination, some homologous recombination. Once again, this is homologous pair. One chromosome from the father that I've gotten from the father. The species or the cell got it from its father's cell and one from the mother. And they're homologous. They might contain different base pairs, different actual DNA, but they code for the same genes. Over simplification, but in a similar place on each of these it might code for eye color or I don't know, personality. Nothing is that simple in how tall you get and it's not that simple in DNA but just to give you an idea of how it is. And the reason why I overlapped them like this is to show how the recombination can occur. So actually, let me zoom in. So this is the one from the father. Once again, it's on the condensed form. This is one chromosome made up of two sister chromatids right over here. And I drew the centromere, not to be confused with centrosomes. That's where they are, those sister chromatids are attached. And then, I will draw the homologous chromosome from the mother. So the homologous chromosome from the mother just like that. Homologous chromosome from the mother. And the recombination can occur at a point right over here. So after you're done with the recombination, this side might look something more like this. So let me draw it like this. So, they essentially break up and they swap those little sections. There's one way to think about it. So this one, we'll now have a little piece from the mother. It might code for similar genes. But now it contains the mother's genetic information. And then this one over here will now have the piece. And you could say even homologous piece from the father. Let me do these two centromeres. And this is really interesting. All the time, there couldn't be recombination and often times it can lead to kind of non-optimal things, nonsense code and DNA. It might lead to a nonfunctional organism. But this happens fairly common in the meiosis and it's a way, once again, to get more variation. We've talked about sexual reproduction before. And sexual reproduction introduces variation into a population. And this, obviously, when different sperms find different eggs that introduces variation. But then, even amongst homologous pairs you can actually have exchange between this chromosome. And that's interesting because as we mentioned, each of these chromosomes, they code for a bunch of different genes. And a gene is kinda looking code for a specific or a set of proteins. So this right over here, and this is what I'm about to say is gonna be huge over simplification. Maybe right over here you coded for eye color or it was related to, or it helps code for eye color. And then you got that from your dad. And here, it helped code for eye color. And you got that from your mom. Your mom might have trended you towards a lighter eye color and your dad might have trended you towards a darker eye color. But now, the one from your mom is on this chromosome, this gene, and then the one or they've both the same gene. They're just different allele. They're coding for different variance of that gene. And then the allele from your dad is over here. And once again, some people get confused with genes and chromosomes and all of these. Each of these chromosomes contain a bunch of genes. These are very long DNA molecules. This code for a bunch of different genes. So gene will be a little section of here that could code for a particular protein. So that's what happens in prophase I. In prophase I, you have this condensation of your chromosomes, of your homologous pairs. You can have this recombination. And it's really interesting, this recombination doesn't tend to happen at just random points that would kind of break the genetic information. It tends to happen at fairly clean points. And the places where this breakup is happening, these are called the plural, if you just talk about one point, it's a chiasma, or if you're talking about the plural, it's chiasmata. Sounds like it could be a horror movie. So, chiasma. Chiasma. And the fact that they happen, they tend to happen fairly cleanly, this is once again, kind of the beauty of the universe or at least of biology is that through billions of years of evolution, these things have kind of optimized for more variation and to happen in fairly clean ways. So I'm gonna leave this video right there. I know I just got to prophase I. But this was a really, really important idea of this homologous recombination or this chromosomal crossover that we see right over here. And then from there, we can continue through the rest of meiosis I and then meiosis II.