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