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

    これまで

  • It's a good idea in this video to go a little bit more in detail.

    有糸分裂と、減数分裂の

  • I've already done a video on mitosis,

    一般概念を、勉強してきたね

  • and in this one,we'll go into the details of meiosis.

    この動画では

  • Just as a review,mitosis,you start with a diploid cell,

    もう少し細部を、みていこう

  • and you end up with two diploid cells.

    別の動画で、有糸分裂を勉強したから

  • Essentially, it just duplicates itself.And formally,

    今度は、減数分裂を詳しくみていこう

  • mitosis is really the process of the duplication of the nucleus,

    有糸分裂は 1個の、二倍体細胞から始まって

  • but it normally ends up with two entire cells.

    2個の、二倍体細胞でおわる

  • Cytokinesis takes place.So this is mitosis.

    つまり、自己複製

  • We have a video on it where we go into the phases of it:

    正確にいうと

  • prophase,metaphase,anaphase and telophase.

    有糸分裂は、核の複製だ

  • Mitosis occurs in pretty much all of our somatic cells

    最後に、2個の細胞が、つくられる

  • as our skin cells replicate,and our hair cells

    細胞質分裂も起きる

  • and all the tissue in our body as it duplicates itself,

    それが、有糸分裂

  • it goes through mitosis.

    有糸分裂の段階も、勉強したね

  • Meiosis occurs in the germ cells and it's used essentially to

    前期、中期

  • produce gametes to facilitate sexual reproduction.

    後期、終期

  • So if I start off with a diploid cell,

    有糸分裂は 体のいろんな細胞で起こる

  • and that's my diploid cell right there,this would be a germ cell.

    皮膚細胞や、毛髪細胞や

  • It's not just any cell in the body.It's a germ cell.

    あらゆる組織の細胞が

  • It could undergo mitosis to produce more germ cells,

    有糸分裂を経て、複製される

  • but we'll talk about how it produces the gametes.

    一方の減数分裂は、生殖細胞で起こる

  • It actually goes under two rounds.

    有性生殖に必要な、配偶子を作るために

  • They're combined,called meiosis,

    減数分裂が必要なんだ

  • but the first round you could call it meiosis 1,so I'll call that M1.

    二倍体細胞から、スタートしよう

  • I'm not talking about the money supply here.

    これは二倍体細胞の、生殖細胞

  • And in the first round of meiosis,

    体の他の部分の、細胞じゃない

  • this diploid cell essentially splits into two haploid cells.

    卵子や精子をつくる、生殖細胞

  • So if you started off with 43 chromosomes,

    生殖細胞を増やすだけなら 有糸分裂だが

  • you formally have 23 chromosomes in each one,

    ここでは、配偶子をつくる

  • or you can almost view it if you have 23 pairs here,

    そのために、2段階の分裂が行なわれる

  • each have two chromosomes,those pairs get split into this stage.

    2段階を含めて、減数分裂というが

  • And then in meiosis 2,

    最初のは、M1(Meiosis 1)

  • these things get split in a mechanism very similar to mitosis.

    経済用語のM1じゃないよ

  • We'll see that when we actually go through the phases.

    M1で、この二倍体細胞が

  • In fact,the prophase,metaphase,anaphase,telophase

    2個の一倍体細胞に、分裂する

  • also exist in each of these phases of meiosis.

    染色体を、46本もってたら

  • So let me just draw the end product.

    (正確には、それぞれを23本)

  • The end product is you have four cells and each of them are haploid.

    つまり、23対もってたら

  • And you could already see,this process right here,

    M1で、2つに分かれる

  • you essentially split up your chromosomes,

    つぎのM2で、染色体は

  • because you end up with half in each one,but here,

    有糸分裂と似た方法で、分裂する

  • you start with N and you end up with two chromosomes that each have N,

    この段階を

  • so it's very similar to this.You preserve the number of chromosomes.

    実際にみてみよう

  • So let's delve into the details of how it all happens.

    前期、中期、後期、終期は

  • So all cells spend most of their time in interphase.

    減数分裂の、各プロセスにもある

  • Interphase is just a time when the cell is

    最後に何が作られるか、描いてみるよ

  • living and transcribing and doing what it needs to do.

    最終的に、4個の細胞ができる

  • But just like in mitosis,

    4個とも

  • one key thing does happen during the interphase, and actually,

    一倍体細胞だ

  • it happens during the same thing, the S phase of the interphase.

    この段階で

  • So if that's my cell,that's my nucleus right here.

    染色体が、2つに分かれてる

  • And I'm going to draw it as chromosomes,but you have to remember that

    半分になった染色体、N本から分裂が始まり

  • when we're outside of mitosis or meiosis formally,

    N本ずつの染色体が、2本になった

  • the chromosomes are all unwound,

    有糸分裂と、似ているね

  • and they exist as chromatin,which we've talked about before.

    染色体数は、そのままだ

  • It's kind of the unwound state of the DNA.

    何が起きてるか、くわしくみてみよう

  • But I'm going to draw them wound up

    細胞は、間期の段階が長かったね

  • because I need to show you that they replicate.

    間期は、細胞が生きつづけ

  • Now,I'm going to be a little careful here.

    転写とか、必要なことをする時期だ

  • In the mitosis video,I just had two chromosomes.

    有糸分裂と、同じように

  • They replicated and then they split apart.

    間期には、重要なことが起こる

  • When we talk about meiosis,

    間期の、S期だ

  • we have to be careful to show the homologous pairs.

    これが細胞で、これが核

  • So let's say that I have two homologous pairs.

    染色体で、あらわすけど

  • So let's say I have--let me do it in appropriate colors.

    有糸分裂期や、減数分裂期以外では

  • So this is the one I got from my dad.

    染色体は、ほどかれていて

  • This is the one I got from my mom. They're homologous.

    実際は、染色質の状態だったね

  • And let's say that I have another one that I got from my dad.

    DNAが、ほどかれている

  • Let me do it in blue.

    複製が、わかりやすいように

  • Actually,maybe I should do all the ones from my dad in this color.

    ねじれたDNAを、描くよ

  • Maybe it's a little bit longer. You get the idea.

    ここで、注意すること!

  • And then a homologous one for my mom that's also a little bit longer.

    有糸分裂では、2本の染色体と言ったね

  • Now,during the S phase of the interphase--

    それが複製されて、2つに分かれる

  • and this is just like what happens in mitosis,

    減数分裂では

  • so you can almost view it as it always happens during interphase.

    相同染色体の対だ

  • It doesn't happen in either meiosis or mitosis.

    2対の、相同染色体がある

  • You have replication of your DNA.

    色を変えるよ

  • So each of these from the homologous pair--and remember,

    これが、父からの染色体

  • homologous pairs mean that they're not identical chromosomes,

    これが、母からの染色体

  • but they do code for the same genes.

    相同だ

  • They might have different versions or different alleles

    もう1対ある

  • for a gene or for a certain trait,

    父からの

  • but they code essentially for the same kind of stuff.

    染色体

  • Now,replication of these,

    青で描こう

  • so each of these chromosomes in this pair replicate.

    父からの、染色体を

  • So that one from my dad replicates like this,

    青で描くよ

  • it replicates and it's connected by a centromere,

    こっちは

  • and the one from my mom replicates like that,

    少し長い

  • and it's connected by a centromere like that,

    これは、母から

  • and then the other one does as well.

    少しだけ長い

  • That's the shorter one.

    間期のなかの、S期

  • Oh,that's the longer one,actually.

    有糸分裂と、同じで

  • That's the longer one.

    これは、間期に起きる

  • I should be a little bit more explicit in

    有糸分裂期でも、減数分裂期でもない

  • which one's shorter and longer.

    DNAが、複製されてる

  • The one from my mom does the same thing.

    相同染色体のそれぞれが、複製される

  • This is in the S phase of interphase.

    相同染色体は、同一のものではない

  • We haven't entered the actual cell division yet.

    同じ遺伝子を

  • And the same thing is true--

    コードしてるんだ

  • and this is kind of a little bit of a sideshow--of the centrosomes.

    異なる型、異なる対立遺伝子であれ

  • And we saw in the mitosis video that these are involved in

    1つの遺伝子、1つの形質にかかわる

  • kind of eventually creating the microtubule structure

    同じ種類を、コードする

  • in pulling everything apart,

    さあ、こいつらの複製だ

  • but you'll have one centrosome that's hanging out here,

    それぞれの相同染色体が 複製される

  • and then it facilitates its own replication,

    父からの染色体が、複製されて

  • so then you have two centrosomes.

    セントロメアで、くっつく

  • So this is all occurring in the interphase,

    母からのが、複製されて

  • and particularly in the S part of the interphase,not the growth part.

    セントロメアで、くっつく

  • But once that's happens,we're ready--

    こっちが、短いほう

  • in fact,we're ready for either mitosis or meiosis,

    こっちが、長いほう

  • but we're going to do meiosis now.This is a germ cell.

    これも、長いほう

  • So what happens is we enter into prophase I.

    長いのと、短いのを

  • So if you remember,in my--let me write this down

    もっとわかりやすく描こうか

  • because I think it's important.

    母からのも、同じようにする

  • In mitosis you have prophase,metaphase, anaphase and telophase.

    間期のなかの、S期だ

  • I won't keep writing phase down.PMAT.

    まだ、細胞は分裂していない

  • In meiosis,you experience these in each stage,

    有糸分裂と、同じことが起きるよ

  • so you have to prophase I,followed by metaphase I,

    中心体の、活躍だね

  • followed by anaphase I,followed by telophase I.

    有糸分裂では

  • Then after you've done meiosis 1,then it all happens again.

    中心体は、微小管を形成して

  • You have prophase II,followed by metaphase II,

    いろいろなものを、引っ張ったね

  • followed by anaphase II,followed by telophase.

    中心体は、ここにあって

  • So if you really just want to memorize the names,

    自己複製して、2個になる

  • which you unfortunately have to do in this,

    これはみんな、間期に起こること

  • especially if you're going to get tested on it,

    間期のなかの、S期

  • although it's not that important to kind of understand the concept of

    成長期じゃないよ

  • what's happening, you just have to remember

    有糸分裂でも、減数分裂でも

  • prophase,metaphase,anaphase,telophase,

    中心体の、複製は起きるけど

  • and it'll really cover everything.

    今は、減数分裂

  • You just after memorize in meiosis,it's happening twice.

    これは、生殖細胞だからね

  • And what's happening is a little bit different,

    前期Ⅰに、入るよ

  • and that's what I really want to focus on here.

    書いておこう

  • So let's enter prophase I of meiosis I.

    大事だから

  • So let me call this prophase I.So what's going to happen?

    有糸分裂には

  • So just like in prophase and mitosis,

    前期、中期、後期、終期がある

  • a couple of things start happening.

    全部書くのは、めんどうだな

  • Your nuclear envelope starts disappearing.

    PMAT

  • The centromeres--sorry,not centromeres.

    減数分裂では 各ステージにこの段階がある

  • I'm getting confused now.The centrosomes.

    前期Ⅰ、つづいて中期Ⅰ

  • The centromeres are these things connecting these sister chromatids.

    つづいて後期Ⅰ、終期Ⅰ

  • The centrosomes start facilitating the development of the spindles,

    減数分裂のM1のあとで、また繰り返す

  • and they start pushing apart a little bit from the spindles.

    前期Ⅱ、つづいて中期Ⅱ

  • They start pushing apart

    つづいて後期Ⅱ、終期Ⅱ

  • and going to opposite sides of the chromosomes.

    段階の名前は

  • And this is the really important thing in prophase I.

    試験のために

  • And actually,I'll make this point.

    覚えておかないとね

  • Remember,in interface,even though I drew it this way,

    実際に起こることが、重要だけど

  • they don't exist in this state,the actual chromosomes.

    名称も、覚えておこう

  • They exist more in a chromatin state.

    前期、中期、後期、終期

  • So if I were to really draw it,it would look like this.

    ひととおりの段階

  • The chromosomes,it would all be all over the place,and it actually

    いちど覚えれば 減数分裂では2度起きる

  • would be very difficult to actually see it in a microscope.

    つぎに、変わったことが起こるよ

  • It would just be a big mess of proteins and of histones,

    これが、肝心なんだ

  • which are proteins,and the actual DNA.

    M1の、前期Ⅰに入ろう

  • And that's what's actually referred to as the chromatin.

    前期Ⅰ

  • Now,in prophase,that starts to form into the chromosomes.

    何が、起こるかな

  • It starts to have a little bit of structure,and this is

    有糸分裂の前期と、同様に

  • completely analogous to what happens in prophase in mitosis.

    いくつかのことが、起きるよ

  • Now,the one interesting thing that happens

    核膜が

  • is that the homologous pairs line up.And actually,

    分解をはじめる

  • I drew it like that over here and maybe I should just cut and paste it

    セントロメア、ごめん

  • Let me just do that.

    じゃなくて

  • If I just cut and paste that,

    中心体

  • although I said that the nucleus is disappearing,

    セントロメアは

  • so let me get rid of the nucleus.I already said that.

    姉妹染色分体が、くっつく場所だ

  • The nucleus is slowly disassembling.

    中心体は

  • The proteins are coming apart during this prophase I.

    微小管を、形成して

  • I won't draw the whole cell,

    紡錘体同士を、引き離す

  • because what's interesting here is happening at the nuclear,

    引き離された、紡錘体は

  • or what once was the nucleus level.

    染色体の両側へ、移動する

  • So the interesting thing here that's different from mitosis

    前期Ⅰの

  • is that the homologous pairs line up next to each other.

    大事な部分だ

  • Not only do they line up,but they can actually share--

    こうやって、描いてるけど

  • they can actually have genetic recombination.

    実際の、染色体は

  • So you have these points where analogous--

    染色質の、状態だからね

  • or I guess you could say homologous-- points

    描くとしたら

  • on two of these chromosomes will cross over each other.

    いろんなところに、散らばってて

  • So let me draw that in detail.

    顕微鏡では

  • So let me just focus on maybe these two right here.

    観察しにくい

  • So I have one chromosome from my dad,

    ヒストンという、タンパク質と

  • and it's made up of two chromatids,

    DNAの、雑然としたかたまりだ

  • so it's already replicated,

    そういった染色質は

  • but we only consider it one chromosome,

    前期には、染色体に変わって

  • and then I have one from my mom in green.

    構造が、はっきりしてくる

  • I'm going to draw it like that.

    有糸分裂の、前期と

  • One from my mom in green,and it also has two chromatids.

    まったく同じ

  • Sometimes this is called a tetrad

    この、並んだ相同染色体対に

  • because it has four chromatids in it,

    面白いことが、起こるんだ

  • but it's in a pair of homologous chromosomes.

    こっちのほうに、描くよ

  • These are the centromeres,of course.

    ここにペーストしよう

  • What happens is you have crossing over,

    やってみるね

  • and it's a surprisingly organized process.

    ペーストする

  • When I say organized,it crosses over at a homologous point.

    核は

  • It crosses over at a point where,

    消失する

  • for the most part,you're exchanging similar genes.

    核は、ゆっくり消えていく

  • It's not like one is getting two versions of a gene

    前期Ⅰで、タンパク質は分解する

  • and the other is getting two versions of another gene.

    細胞全体は、描かないよ

  • You're changing in a way that

    核の中で、興味深いことが起きるから

  • both chromosomes are still coding for the different genes,

    核だった場所でね

  • but they're getting different versions of those genes

    有糸分裂にはなかった 興味深いものは

  • or different alleles, which are just versions of those genes.

    この一列に並んだ、相同染色体だ

  • So once this is done,

    ただ、並ぶだけじゃなく

  • the ones from my father are now not completely from my father,

    遺伝子の組み換えが、起きる

  • so it might look something like this.

    相同の部分で

  • Let me see,it'll look like this.

    2本の染色体に

  • The one from my father now has this little bit from my mother,

    乗換え(交叉)が、起きる

  • and the one from my--oh,no,my mother's chromosome is green--

    くわしく描くよ

  • a little bit from my mother,

    ここの2本を、見てみよう

  • and the one from my mother has a little bit from my father.

    父からの染色体が、1本

  • And this is really amazing because it shows you that

    染色分体は、すでに複製されてる

  • this is so favorable for creating variation in a population

    1本の、染色体だからね

  • that it has really become a formal part of the meiosis process.

    母からの染色体は、緑で描くよ

  • It happens so frequently.This isn't just some random fluke,

    こうやって、描こう

  • and it happens in a reasonably organized way.

    緑のは母からで、2本の染色分体をもつ

  • It actually happens at a point

    4本の染色分体を、もつので

  • where it doesn't kind of create junk genes.

    四分染色体.とも、よばれるけど

  • Because you can imagine,this cut-off point,which is called a chiasma,

    1対の、染色体だ

  • it could have happened in the middle of some gene,

    セントロメアもある

  • and it could have created some random noise,

    乗換えで、何が起きるか

  • and it could have broken down some protein development

    驚くほど、効率的なプロセスなんだ

  • in the future or who knows what.But it doesn't happen that way.

    効率的というのは

  • It happens in a reasonably organized way,

    相同部分で、乗換えが起こること

  • which kind of speaks to the idea that it's part of the process.

    乗換えは

  • So in prophase I,you also have this happening.

    類似する遺伝子の、交換だ

  • So once that happens

    1本の染色体が 2種類の遺伝子をもらうことはなく

  • you could have this guy's got a little bit of that chromatid

    もう一方の染色体も、同様

  • and then this guy's got a little bit of that chromatid.

    2本の、染色体は

  • So all of this stuff happens in prophase I.

    別々の遺伝子を、コードするんだ

  • You have this crossing over.

    別々の、対立遺伝子の

  • The nuclear envelope starts to disassemble,

    異なる種類を、もらう

  • and then all of these guys align and the chromatin starts

    いったん、乗換えが起きると

  • forming into these more tightly wound structures of chromosomes.

    父からの、染色体は

  • And really,that's all--when we talk about even mitosis,

    元のとは、違ってくる

  • that's where a lot of the action really took place.

    えーと

  • Once that happens,then we're ready to enter into the metaphase I,

    父からの、染色体には

  • so let's go down to metaphase I.

    母からの、緑の部分が入って

  • In metaphase I--let me just copy and paste what I've already done--

    母からの、染色体には

  • the nuclear envelope is now gone.

    父からの部分が、入る

  • The centrosomes have gone to opposite sides of the cell itself.

    驚くべきことだね