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  • 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.