We've gone over the general idea behind mitosis and meiosis. It's a good idea in this video to go a little bit more in detail, I've already done a video on mitosis, and in this one we'll go into the details of meiosis. And just as a review, mitosis, you start with a diploid cell and you end up with two diploid cells.
It's essentially, it just duplicates itself. And formally, mitosis is really the process of the duplication of the nucleus, but it normally ends up with two entire cells. Cytokinesis takes place. So this is mitosis. And we have a video on it where we go into the phases of it, prophase, metaphase, anaphase, and telophase.
Mitosis. Meiosis, which, and this occur, mitosis occurs in pretty much all of our somatic cells, you know, as our skin cells replicate and our hair cells and our, you know, All the tissue in our body, as it duplicates itself, it goes through mitosis. Meiosis occurs in the germ cells, and it's used to essentially produce gametes to facilitate sexual reproduction. So if I start off with a diploid cell, and that's my diploid cell right there. This would be a germ cell.
It's not just any cell in the body. It's a germ cell. It could undergo mitosis to produce more germ cells, but we'll talk about how it produces the gametes.
It actually goes under two rounds. The first round is, and they're combined called meiosis, but the first round you could call it meiosis I. So I'll call that M1.
I'm not talking about the money supply here. And in the first round of meiosis, this diploid cell essentially splits into two haploid cells. So if you started off with 43 chromosomes, you formally have 23 chromosomes in each one.
Or you can almost view it, if you have 23 pairs here, Each has two chromosomes. Those pairs get split into this stage. And then in meiosis II, these things get split in a mechanism very similar to mitosis. So we'll see that when we actually go through the phases.
In fact, the prophase, metaphase, anaphase, telophase also exist in each of these phases of meiosis. So let me just draw the. end product. So the end product is you have four cells, and each of them are haploid.
And you could already see, this process right here, you essentially split up your chromosomes, because you end up with half in each one. But here, you start with N and you end up with two chromosomes that each have N. So it's very similar to this. You preserve the number of chromosomes. So let's delve into the details of how it all happens.
So all cells. spend most of their time in interphase. Interphase is just a time when the cell is living and transcribing and doing what it needs to do.
But just like in mitosis, one key thing does happen during the interphase. And actually it happens during the same thing, the S-phase of the interphase. So if that's my cell, that's my nucleus right here.
And I'm going to draw it as chromosomes, but you have to remember that when we're outside of Mitosis or meiosis, formally, the chromosomes are all unwound and they exist as chromatin, which we've talked about before. It's kind of the unwound state of the DNA. But I'm going to draw them wound up because I need to show you that they replicate.
Now I'm going to be a little careful here. In the mitosis video, I just had two chromosomes. They replicated and then they split apart.
When we talk about meiosis, we have to be careful to show the homologous pairs. So let's say that I have two homologous pairs. So let's say I have So this is the one, let me do it in appropriate colors.
So this is the one I got from my dad. This is the one I got from my mom. They're homologous. And let's say that I have another one that I got from my dad that is, oh, let me do it in blue.
Actually, maybe I should do all the ones from my dad in this color. Maybe it's a little bit longer. You get the idea. And then a homologous one for my mom that's also a little bit longer.
Now. During the S phase of the interphase, and this is just like what happens in mitosis. So this kind of, you can almost view it as it always happens during interphase. It doesn't happen in either meiosis or mitosis.
You have replication of your DNA. So each of these from the homologous pair, and remember, homologous pairs mean that they're not identical chromosomes, but they do code for the same genes. They might have different versions or different alleles for a gene or for a certain trait, but they code essentially for the same kind of stuff.
Now, replication of these, so each of these chromosomes in this pair replicate. So that one from my dad replicates like this. Replicates, and it's connected by a centromere. And the one from my mom replicates like that. And it's connected by a centromere like that.
And then the other one does as well. That's the longer one, actually. I should be a little bit more explicit in which one's shorter and longer. The one from my mom does the same thing.
the S phase of interphase. We haven't entered the actual cell division yet. And the same thing is true, and this is kind of a little bit of a sideshow, of the centrosomes.
And we saw in the mitosis video that these are involved in kind of eventually creating the microtubule structure and pulling everything apart. But you'll have one centrosome that's hanging out here, and then it facilitates its own replication. So then you have two centrosomes. So this is all occurring in the interphase. In the interphase, and particularly in the S part of the interphase, not the growth part.
But once that happens, we're ready for either mitosis or meiosis, but we're going to do meiosis now. This is a germ cell. So what happens is we enter into prophase I.
So if you remember, in my, let me write this down because I think it's important. In mitosis, you have prophase. Prophase, metaphase, metaphase, anaphase, and telephase. I keep writing phase down.
PMAT. In meiosis, you experience these in each stage. So you have prophase I, followed by metaphase I, followed by anaphase I, followed by telephase I.
Then, after you've done meiosis I, then it all happens again. You have prophase II. Followed by metaphase 2, followed by anaphase 2, followed by telophase.
So if you really just want to memorize the names, which you unfortunately have to do in this, especially if you're going to get tested on it, although it's not that important to kind of understand the concept of what's happening, you just have to remember prophase, metaphase, anaphase, telophase, and it'll really cover everything. You just have to memorize in meiosis, it's happening twice. And what's happening is a little bit different.
That's what I really want to focus on here. So let's enter prophase 1. of meiosis I. So let me call this prophase I.
So what's going to happen? So just like in prophase and mitosis, a couple of things start happening. Your nuclear envelope starts disappearing. Nuclear envelope starts to, the centromeres, sorry, not centromeres, I'm going to confuse you, the centrosomes, the centromeres are these things connecting these sister chromatids, but the centrosomes start facilitating the development of the spindles and they start pushing apart a little bit from the spindles. They start pushing apart and going to opposite sides of the chromosomes.
And this is the really important thing in prophase I. Well, and actually I'll make this point. Remember, an interface, even though I drew it this way, they don't exist in this state, the actual chromosomes. They exist more in a chromatin state. So if I were to really draw it, it would look like this.
The chromosomes, it would all be all over the place. And actually, it would be very difficult to actually see it in a microscope. It would just be a big mess of proteins and of histones, which are proteins, and the actual DNA. And that's what's actually referred to as the chromatin. Now, in prophase, that starts to form into the chromosomes.
It starts to have a little bit of structure. And this is completely analogous to what happens in prophase in mitosis. Now, the one interesting thing happens is that the homologous pairs line up. And actually, I drew it like that over here.
Maybe I should just cut and paste it. Let me just do that. If I just cut and paste that. Although I said that the nucleus is disappearing.
So let me get rid of the nucleus. I already said that. The nucleus is slowly disassembling. The proteins are coming apart during this prophase I. I won't draw the whole cell here, because what's interesting here is happening at the nuclear, or what once was the nucleus level.
So the interesting thing here that's different than from mitosis is that the homologous pairs line up next to each other. Not only do they line up. But they can actually share, they can actually have genetic recombination. So you have these points where analogous, or I guess you could say homologous points on two of these chromosomes will cross over each other. So let me draw that in detail.
So let me just focus on maybe these two right here. So this is, so I have One chromosome from my dad. And it's made up of two chromatids. So it's already replicated, but we only consider it one chromosome. And then I have one from my mom in green.
And I'm going to draw it like that. One from my mom in green. And it also has two chromatids. And sometimes this is called a tetrad, because it has four chromatids. in it, but it's in a pair of homologous chromosomes.
These are the centromeres, of course. What happens is you have crossing over. And it's actually a very surprisingly, I guess, organized process.
When I say organized, it crosses over at a point where, for the most part, you're exchanging similar genes. It's not like one is getting two versions of the gene, and the other is getting two versions of another gene. You're exchanging in a way that both chromosomes are still encoding for the different genes, but they're getting different versions of those genes, or different alleles, which are just versions of those genes.
So once this is done, you have the ones from my father are now not completely from my father, so it might look something like this. Let me see it look like this. The one from my father now has this little bit from my mother, and the one from my mother's chromosome is green. Little bit from my mother, and the one from my mother has a little bit from my father. And this is really amazing because it shows you that this is so favorable for creating variation in a population that it's really become a formal part of the meiosis process.
It happens so frequently. This isn't just some random fluke. And it happens in a reasonably organized way in that it actually happens at a point where it doesn't kind of create junk genes.
Because you can imagine, this cutoff point, which is called a chiasma. It could have happened in the middle of some gene. And it could have created all this random noise. And it could have broken down some protein development in the future, or who knows what.
But it doesn't happen that way. It happens in a reasonably organized way, which kind of speaks to the idea that it's part of the process. So in prophase I, you also have this happening. So once that happens, you could have this guy's got a little bit of that chromatid. And then.
This guy's got a little bit of that chromatid. So all of this stuff happens in prophase I. You have this crossing over, the nuclear envelope starts to disassemble, and then all of these guys align, and the chromatin starts forming into these more tightly-wind structures of chromosomes. And really, that's all, when we talk about even mitosis, that's where a lot of the action really took place. Once that happens, then we're ready to enter into the metaphase I.
So let's go down to metaphase I. So in metaphase one, let me just copy and paste what I've already done. The nuclear envelope is now gone. The centrosomes have gone to opposite sides of the of the cell itself. Maybe I should draw the entire cell now, now that there's no nucleus, let me erase the nucleus a little bit better than I've done.
And of course we have the spindle fibers that have been generated by now. With the help of the centrosomes. And some of them, we learned, this is exactly what happened in mitosis.
They attach to the kinetochores, which are attached to the centromeres. the centromeres of these chromosomes. Now what's interesting here is that they each attach. So this guy's going to attach to, and actually let me do something interesting here. Instead of doing it this way, because I want to show that all my dad's chromosomes don't go to one side and all my mom's chromosomes don't go to the other side.
So instead of drawing these two guys like this, let me see if I can flip them. Let me see. Let me just flip them the other way. Whether or not they're, which direction they're flipped is completely random, and that's what adds to the variation. As we said before, sexual reproduction is key to introducing variation into a population.
So that's the moms, and that's the dad. They don't have to. All of the ones from my dad might have ended up on one side. All of them from my mom might end up on one side.
Although when you're talking about 23 pairs, the probability becomes a lot, lot lower. So this is one from my dad. And of course it has some centromeres.
Let me draw that there. And so these microspindles, some of them attach to kinetochores, which are these protein structures on the centromeres. And this is just like metaphase. It's very similar to metaphase in mitosis.
This is called metaphase 1. And everything aligns. Now we're going to enter anaphase 1. Now anaphase 1 is interesting because, remember, in mitosis, in anaphase, the actual chromatids, the sister chromatids, separated from each other. That's not the case in anaphase 1 here in meiosis. So when we enter anaphase 1. You have just the homologous pairs separate. So the chromatids stay with their sister chromatids.
So on this side, you'll have these two go there. And let me just, while I have the green out, let me see if I can draw this respectably. I have the purple, it's a little bit shorter version here, he's got a little bit of a stub of green there.
This guy's got a little stub of purple there. And then they have this longer purple chromosome here. This is anaphase I. They're being pulled apart.
But they're being pulled apart, the homologous pair is being pulled apart, not the actual chromosomes, not the chromatids are being. So let me just draw this. So then you have your microtubules.
Some are connected to these kinetochores. You have your centromeres. And of course, all of this is occurring within the cell.
These are getting pulled apart. So it's analogous to anaphase and mitosis, but the key difference is you're pulling apart homologous pairs. You're not actually splitting the chromosomes into their constituent chromatids.
And that's key. And if you forget that, you can review the mitosis video. So this is anaphase I. This is anaphase I.
And then as you can imagine, telephase I is essentially once these guys are at their respective ends of the cell, it's getting tiring redrawing all of these, but I guess it gives you time to let it all sink in. So these guys are now at the left end of the cell. And now these guys. are now at the right end of the cell.
Now the microtubules start to disassemble. So maybe they're there a little bit, but they're disassembling. You still have your centromeres here.
And they're at opposite poles. And to some degree, the early part of telophase, they're still pushing the cell apart. At the same time, you have cytokinesis happening. So by the end of telophase I, so you have this, the actual cytoplasm is splitting.
During telophase, right there. And the nuclear envelope is forming. You can almost view it as the opposite of prophase. The nuclear envelope is forming, and by the end of telophase I, it will have completely divided.
So this is telophase I. Now notice, we started off with a diploid cell. It had it had two pairs of homologous chromosomes, but it had four chromosomes.
Now, each cell only has two chromosomes. Essentially, each cell got one of the pair of each of those homologous pairs, but it was done randomly, and that's where a lot of the variation is introduced. Now, once we're at this stage, each of these cells now undergo meiosis II, which is actually very similar to mitosis.
And sometimes there's actually an in-between stage called interphase II, where the cell kind of rests and whatever else, and actually the centromeres now have to duplicate again. So what is these two cells? I've drawn them separately. Let's see what happens next. So let's say that the centromere, actually I shouldn't have drawn the centromere inside the nucleus like that.
The centromere is going to be outside the nucleus, outside of the newly formed nucleus. There and there. And then it'll actually replicate itself at this point as well.
So now we have two cells. Let me just. cut and paste what I have before.
Let me see, I have this one, this chromosome right here. It's got this little green stub there. And then I have this longer, fully green chromosome there.
Now this guy, he's got this little purple stub here. Let me draw this whole purple chromosome there. And then this guy has one chromatid like that and one chromatid like that.
Now when we we enter prophase II, what do you think is going to happen? Well, just like before, you have your nuclear envelope that formed in telophase I. It's kind of a temporary thing.
It starts to disintegrate again. It starts to disintegrate again. And then you have your centromeres. They start pushing apart.
So now I had two centromeres. They replicated. And now they start pushing apart while they generate their little spindles. They push apart in opposite directions.
Now this is happening in two cells, of course. They go in opposite directions while they generate their spindle fibers. And let me make it very clear that this is two cells we're talking about.
That's one of them, and that's the second of them. Now it's going to enter metaphase II, which is analogous to metaphase I or metaphase in mitosis, where the chromosomes get lined up. And let me draw it this way.
So now the centromeres, they've migrated to the two poles. of the cell. So those are my centromeres. I have all of my spindle fibers.
Oh, sorry. Did I call those centromeres? The centrosomes.
I don't know how long I've been calling them centromeres. These are centrosomes. My brain keeps confusing. The centromeres, and maybe this will help you not do what I just did, the centromeres are the things that are connecting the two sister chromatids. Those are centromeres.
Centrosomes are the things that are pushing back, that generate the spindle fibers. And the chromosomes line up during metaphase. Metaphase always involves the lining up of chromosomes. So that one, let me just draw it.
So I have that and that. And this one's got a purple guy, too. This guy's got a purple guy, a long purple guy. And then there's a little stub for the other guy.
This guy's got a long green guy. And this guy's got a little green stub. And then this guy's got, this is the short green guy right there, and of course they're being aligned.
Some of these spindle fibers have been attached to the centromeres or the kinetochores that are on the centromeres that connect these two chromatids, these sister chromatids. And of course we don't have a nuclear membrane anymore, and these are of course two separate cells. And then you can guess what happens in anaphase too. It's just like anaphase in mitosis. Anaphase.
These things get pulled apart by the kinetochore microtubules, while the other microtubules keep growing and push these two things further apart. So let me show that. And the key here, this is the difference between anaphase 2 and anaphase 1. Anaphase 1, the homologous pairs were broken up, but the chromosomes themselves were not. Now in anaphase 2, we don't have homologous pairs.
We just have chromatid pairs, sister chromatids. Now those are separated. And it's just very similar to anaphase in mitosis. So now, this guy gets pulled in that direction, so it looks something like this. The drawing here is the hardest part of this video.
So that guy gets pulled there, that guy's getting pulled in that direction. He's got that little green stub on him. And then you have one green guy getting pulled in that direction, is the longer chromosome.
And then one of the other longer is getting pulled in that direction. And it's all by these microtubules connected at the kinetic core structures by a centrosome, is kind of the coordinating body. It's all being pulled apart. Anaphase has always involved the pulling part of the chromosomes, or pulling apart of something. Put it that way, and it's happening on this side of the cell as well.
Of course, this is all one cell. And just like in mitosis, as soon as the sister chromatids are split apart, they are now referred to as chromosomes, or sister chromosomes. And of course, this is happening twice. This is also happening in the other cell. This is also happening in the other cell.
The other cell's a little bit cleaner because it didn't have that crossover occur. So you have You have the longer purple one. He gets split up into two chromatids, which we are now calling chromosomes or sister chromosomes.
And then this guy up here, he gets split up into this short green, and then there's a, let me do it this way, this short green, and he's got a little purple stub on it right there. And of course, it's all being pulled away by the same idea, by the centrosomes. I want to make sure I get that word right.
I'm afraid whether I used centromeres for the whole first part of the video. But hopefully my confusion will help you from getting confused, because you'll realize that it's a pitfall to fall into. So that's anaphase. Everything is getting pulled apart.
And then you can imagine what telophase II is. In fact, I won't even redraw it. Telophase II, these things get pulled apart even more.
So this is telophase II. They get pulled apart even more. Even more, the cell elongates.
You start having this cleavage occur right here. So at the same time that telophase IIs, these get pulled apart, and you have the cytokinesis. The tubules start disintegrating, and then you have a nucleus that forms around these.
So what is the end result of all of these? Well, that guy is going to turn into a nucleus that has This purple dude with a little green stub, and then a long green guy. And then he's got his nuclear membrane.
And of course there's the entire cytoplasm and the rest of the cell. The other person that was his kind of partner in this meiosis too is going to have a short purple and a long green. In the long green, he has a nuclear membrane.
And of course, it has cytoplasm around it. And then on this side, you have something similar happening. See this first guy?
This first one right here is a two long purple ones get separated. So let me see. You have one long purple in that cell, then you have another long purple in this cell. And then in the top one, you have a short green one. And at the bottom one, you have a short green one that got little bit of a homologous part of one of my dad's chromosomes on it.
And of course, these also have nuclear membranes and of course, has a cytoplasm in the rest of the cell, which we'll learn more about all of the other things. So what we see here is that we went from a diploid, starting way, where did we start? We started up here with a diploid germ cell.
We went through two stages of division. The first stage. split up homologous pairs, but it started over with that crossing over, that genetic recombination, which is a key feature of meiosis, which adds a lot of variation to a species or to a gene pool.
And then the second phase separated the sister chromatids, just like what happens in mitosis, and we end up with four haploid cells, because they have half the contingency of chromosomes, and these are called gametes.