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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.

  • Maybe I should draw the entire cell now that there's no nucleus.

  • Let me erase the nucleus a little bit better than I've done.

  • Let me erase all of that.

  • And,of course,we have the spindles fibers that have been

  • generated by now with the help of the centrosomes.

  • And some of them,as we learned,

  • this is exactly what happened in mitosis.

  • They attach to the kinetochores,

  • which are attached to 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 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 mom's and that's the dad's.They don't have to.

  • All of the ones from my dad might have ended up on one side

  • and 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.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 I,and everything aligns.

  • Now we're going to enter anaphase I.

  • Now,anaphase I 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 I here in meiosis.

  • So when we enter anaphase I,

  • you have just the homologous pairs separate,

  • so the chromatids stay with their sister chromatids.

  • So on this side,you'll have these to go there.

  • 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 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.

  • So let me just draw this.

  • So then you have your microtubules.

  • Some are connected to these kinetochores.You have your centromeres.

  • Of course,all of this is occurring within the cell

  • and these are getting pulled apart.

  • So it's analogous to anaphase in 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.

  • And then as you could imagine,telophase 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 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,in the early part of telophase,

  • they're still pushing the cell apart,

  • and at the same time,you have cytokinesis happening.

  • So by the end of telophase I,

  • you have the actual cytoplasm 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,right?

  • It had two pairs of homologous chromosome,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 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's 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.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.

  • Then this guy has one chromatid like that and one chromatid like that.

  • Now,when 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.

  • 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.

  • 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 spindles 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,and my brain keeps confusing them.

  • The centromeres,and maybe this'll 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 the--

  • that generate the spindle fibers.

  • 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.

  • 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 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 II.

  • It's just like anaphase in mitosis.

  • These things get pulled apart by the kinetochore microtubules,

  • while the other microtubules keep growing

  • and push and these two things further apart.

  • So let me show that.And they the key here:

  • this is the difference between anaphase II and anaphase I.

  • Anaphase I,the homologous pairs were broken up,

  • but the chromosomes themselves were not.

  • Now,in anaphase II,we don't have homologous pairs.

  • We just have chromatid pairs,sister chromatids.

  • Now,those are separated,which is very similar to anaphase in mitosis.

  • So now,this guy gets pulled in that direction

  • so it look 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 with 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 kinetochore structures

  • by a centrosome as kind of the coordinating body.

  • It's all being pulled apart.

  • Anaphase has always involved the pulling apart of

  • the chromosomes or pulling apart of something.

  • Let me 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.

  • The other cell's a little bit cleaner.

  • It didn't have that crossover occur.

  • So 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.The cell elongates.

  • You start having this cleavage occur right here.

  • So at the same time that in telophase II these get pulled part,

  • 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's 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 in the rest of the cell.

  • The other person that was his kind of partner in this meiosis II,

  • he's going to have a short purple and a 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.

  • You see this first guy,

  • this first one right here has two long purple ones.

  • They get separated.

  • So let me see,you have one long purple in that cell

  • and you have another long purple in this cell.

  • In that top one,you have a short green one,

  • and in this bottom,you have a short green one

  • that had got a little bit of one of my dad's--

  • a homologous part of one of my dad's chromosomes on it.

  • And,of course,these also have nuclear membranes, nuclear membranes,

  • and,of course,it has a cytoplasm in the rest of the cell,

  • which we'll learn more about all those 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,

  • and we went through two stages of division.

  • The first stage split up homologous pairs,

  • but it started over with that crossing over,

  • that genetic combination,which is a key feature of meiosis,

  • which adds a lot a 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.

We've gone over the general idea behind mitosis and meiosis.

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B1 中級 英國腔

減數分裂的階段 (Phases of Meiosis)

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    bilal chiba 發佈於 2021 年 01 月 14 日
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