Subtitles section Play video Print subtitles 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. Just as a review,mitosis,you start with a diploid cell, and you end up with two diploid cells. 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. We have a video on it where we go into the phases of it: prophase,metaphase,anaphase and telophase. Mitosis occurs in pretty much all of our somatic cells as our skin cells replicate,and our hair cells and all the tissue in our body as it duplicates itself, it goes through mitosis. Meiosis occurs in the germ cells and it's used essentially to 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. They're combined,called meiosis, but the first round you could call it meiosis 1,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 have two chromosomes,those pairs get split into this stage. And then in meiosis 2, these things get split in a mechanism very similar to mitosis. 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. 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--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. 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 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, it 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 shorter one. Oh,that's the longer one,actually. That's the longer one. I should be a little bit more explicit in which one's shorter and longer. The one from my mom does the same thing. This is in 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 in 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, and particularly in the S part of the interphase,not the growth part. But once that's happens,we're ready-- in fact,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,metaphase, anaphase and telophase. I won't keep writing phase down.PMAT. In meiosis,you experience these in each stage, so you have to prophase I,followed by metaphase I, followed by anaphase I,followed by telophase I. Then after you've done meiosis 1,then it all happens again. You have prophase II,followed by metaphase II, followed by anaphase II,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 after memorize in meiosis,it's happening twice. And what's happening is a little bit different, and that's what I really want to focus on here. So let's enter prophase I 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. The centromeres--sorry,not centromeres. I'm getting confused now.The centrosomes. The centromeres are these things connecting these sister chromatids. 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. And actually,I'll make this point. Remember,in 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 it actually 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 that happens is that the homologous pairs line up.And actually, I drew it like that over here and 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, 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 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 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. I'm going to draw it like that. One from my mom in green,and it also has two chromatids. 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 a surprisingly organized process. When I say organized,it crosses over at a homologous point. 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 a gene and the other is getting two versions of another gene. You're changing in a way that both chromosomes are still coding 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, the ones from my father are now not completely from my father, so it might look something like this. Let me see,it'll look like this. The one from my father now has this little bit from my mother, and the one from my--oh,no,my mother's chromosome is green-- a 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 has 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. It actually happens at a point where it doesn't kind of create junk genes. Because you can imagine,this cut-off point,which is called a chiasma, it could have happened in the middle of some gene, and it could have created some 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 wound 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. In metaphase I--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 cell itself.