WEBVTT

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We've gone over the
general idea

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behind mitosis and meiosis.

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It's a good idea in
this video to go a

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little bit more in detail.

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I've already done a video on
mitosis, and in this one,

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we'll go into the details
of meiosis.

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

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you end up with two
diploid cells.

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Essentially, it just
duplicates itself.

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And formally, mitosis is really
the process of the

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duplication of the nucleus, but
it normally ends up with

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two entire cells.

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Cytokinesis takes place.

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So this is mitosis.

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

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prophase, metaphase, anaphase
and telophase.

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Mitosis occurs in pretty much
all of our somatic cells as

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our skin cells replicate, and
our hair cells and all the

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tissue in our body as it
duplicates itself, it goes

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through mitosis.

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Meiosis occurs in the germ
cells and it's used

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essentially to produce gametes
to facilitate sexual

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reproduction.

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So if I start off with a diploid
cell, and that's my

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diploid cell right there, this
would be a germ cell.

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It's not just any cell
in the body.

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It's a germ cell.

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It could undergo mitosis to
produce more germ cells, but

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we'll talk about how it
produces the gametes.

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It actually goes under
two rounds.

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They're combined, called
meiosis, but the first round

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you could call it meiosis
1, so I'll call that M1.

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I'm not talking about the
money supply here.

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And in the first round of
meiosis, this diploid cell

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essentially splits into
two haploid cells.

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So if you started off with 43
chromosomes, formally have 23

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chromosomes in each one, or you
can almost view it if you

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have 23 pairs here, each have
two chromosomes, those pairs

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get split into this stage.

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And then in meiosis 2, these
things get split in a

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mechanism very similar
to mitosis.

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We'll see that when we actually

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go through the phases.

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In fact, the prophase,
metaphase, anaphase, telophase

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also exist in each of these
phases of meiosis.

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So let me just draw
the end product.

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The end product is you have four
cells and each of them

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are haploid.

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

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essentially split up your
chromosomes, because you end

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up with half in each one, but
here, you start with N and you

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end up with two chromosomes that
each have N, so it's very

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similar to this.

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You preserve the number
of chromosomes.

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

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So all cells spend most of
their time in interphase.

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Interphase is just a time when
the cell is living and

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transcribing and doing
what it needs to do.

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But just like in mitosis, one
key thing does happen during

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the interphase, and actually,
it happens during the same

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thing, the S phase of
the interphase.

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

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

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remember that when we're outside
of mitosis or meiosis

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formally, the chromosomes are
all unwound, and they exist as

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chromatin, which we've
talked about before.

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

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But I'm going to draw them wound
up because I need to

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show you that they replicate.

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

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In the mitosis video, I just
had two chromosomes.

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They replicated and then
they split apart.

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When we talk about meiosis, we
have to be careful to show the

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homologous pairs.

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

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

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So this is the one I
got from my dad.

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This is the one I
got from my mom.

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They're homologous.

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And let's say that I have
another one that

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I got from my dad.

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Let me do it in blue.

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Actually, maybe I should do all
the ones from my dad in

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this color.

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Maybe it's a little
bit longer.

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You get the idea.

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And then a homologous one for
my mom that's also a little

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bit longer.

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Now, during the S phase of the
interphase-- and this is just

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like what happens in mitosis,
so you can almost view it as

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it always happens during
interphase.

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It doesn't happen in either
meiosis or mitosis.

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You have replication
of your DNA.

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So each of these from the
homologous pair-- and

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remember, homologous pairs
mean that they're not

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identical chromosomes,
but they do code

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for the same genes.

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They might have different
versions or different alleles

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for a gene or for a certain
trait, but they code

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essentially for the same
kind of stuff.

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Now, replication of these, so
each of these chromosomes in

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this pair replicate.

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So that one from my dad
replicates like this, it

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replicates and it's connected
by a centromere, and the one

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from my mom replicates like
that, and it's connected by a

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centromere like that, and then
the other one does as well.

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That's the shorter one.

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Oh, that's the longer
one, actually.

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That's the longer one.

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I should be a little bit more
explicit in which one's

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shorter and longer.

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The one from my mom does
the same thing.

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This is in the S phase
of interphase.

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We haven't entered the actual
cell division yet.

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And the same thing is true-- and
this is kind of a little

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bit of a sideshow-- of
the centrosomes.

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And we saw in the mitosis video
that these are involved

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in kind of eventually creating
the microtubule structure in

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pulling everything apart, but
you'll have one centrosome

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that's hanging out here, and
then it facilitates its own

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replication, so then you
have two centrosomes.

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So this is all occurring
in the interphase, and

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particularly in the S part
of the interphase,

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not the growth part.

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But once that's happens, we're
ready-- in fact, we're ready

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for either mitosis or meiosis,
but we're going

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to do meiosis now.

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This is a germ cell.

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So what happens is we enter
into prophase I.

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

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I think it's important.

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In mitosis you have prophase,
metaphase,

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anaphase and telophase.

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I won't keep writing
phase down.

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PMAT.

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In meiosis, you experience these
in each stage, so you

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have to prophase I, followed
by metaphase I, followed by

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anaphase I, followed
by telophase I.

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

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You have prophase II, followed
by metaphase II, followed by

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anaphase II, followed
by telophase.

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

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unfortunately have to do in
this, especially if you're

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going to get tested on it,
although it's not that

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important to kind of understand
the concept of

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what's happening, you just have
to remember prophase,

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metaphase, anaphase, telophase,
and it'll really

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cover everything.

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

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

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

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So let's enter prophase
I of meiosis I.

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So let me call this
prophase I.

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So what's going to happen?

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So just like in prophase and
mitosis, a couple of things

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start happening.

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Your nuclear envelope
starts disappearing.

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The centromeres-- sorry,
not centromeres.

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I'm getting confused now.

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The centrosomes.

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The centromeres are these things
connecting these sister

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chromatids.

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The centrosomes start
facilitating the development

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of the spindles, and they start
pushing apart a little

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bit from the spindles.

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They start pushing apart and
going to opposite sides of the

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chromosomes.

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And this is the really important
thing in prophase I.

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And actually, I'll
make this point.

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

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

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They exist more in a
chromatin state.

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

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

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actually would be very difficult
to actually see it

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in a microscope.

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

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which are proteins, and
the actual DNA.

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

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

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

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completely analogous
to what happens

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in prophase in mitosis.

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Now, the one interesting thing
that happens is that the

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homologous pairs line up.

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And actually, I drew it like
that over here and maybe I

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should just cut and paste it.

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Let me just do that.

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If I just cut and paste that,
although I said that the

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nucleus is disappearing, so let
me get rid of the nucleus.

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I already said that.

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The nucleus is slowly
disassembling.

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The proteins are coming apart
during this prophase I.

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I won't draw the whole cell,
because what's interesting

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here is happening at the
nuclear, or what once was the

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nucleus level.

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So the interesting thing here
that's different from mitosis

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

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

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can actually have genetic
recombination.

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So you have these points where
analogous-- or I guess you

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could say homologous-- points
on two of these chromosomes

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will cross over each other.

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So let me draw that in detail.

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

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So I have one chromosome from
my dad, and it's made up of

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two chromatids, so it's already
replicated, but we

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only consider it one chromosome,
and then I have

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one from my mom in green.

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I'm going to draw
it like that.

00:10:18.920 --> 00:10:23.370
One from my mom in green, and
it also has two chromatids.

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Sometimes this is called a
tetrad because it has four

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chromatids in it, but it's
in a pair of homologous

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chromosomes.

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These are the centromeres,
of course.

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What happens is you have
crossing over, and it's a

00:10:38.730 --> 00:10:42.770
surprisingly organized
process.

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When I say organized,
it crosses over at

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a homologous point.

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It crosses over at a point
where, for the most part,

00:10:49.450 --> 00:10:52.350
you're exchanging
similar genes.

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

00:10:54.690 --> 00:10:56.500
other is getting two versions
of another gene.

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You're changing in a way that
both chromosomes are still

00:10:59.850 --> 00:11:01.610
coding for the different genes,
but they're getting

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different versions of those
genes or different alleles,

00:11:05.670 --> 00:11:07.240
which are just versions
of those genes.

00:11:07.240 --> 00:11:12.020
So once this is done, the ones
from my father are now not

00:11:12.020 --> 00:11:13.880
completely from my father,
so it might look

00:11:13.880 --> 00:11:15.140
something like this.

00:11:15.140 --> 00:11:18.460
Let me see, it'll
look like this.

00:11:18.460 --> 00:11:20.860
The one from my father now has
this little bit from my

00:11:20.860 --> 00:11:25.480
mother, and the one from my--
oh, no, my mother's chromosome

00:11:25.480 --> 00:11:29.000
is green-- a little bit from my
mother, and the one from my

00:11:29.000 --> 00:11:33.360
mother has a little bit
from my father.

00:11:33.360 --> 00:11:36.560
And this is really amazing
because it shows you that this

00:11:36.560 --> 00:11:40.230
is so favorable for creating
variation in a population that

00:11:40.230 --> 00:11:44.230
it has really become a formal
part of the meiosis process.

00:11:44.230 --> 00:11:45.500
It happens so frequently.

00:11:45.500 --> 00:11:48.120
This isn't just some random
fluke, and it happens in a

00:11:48.120 --> 00:11:49.490
reasonably organized way.

00:11:49.490 --> 00:11:53.840
It actually happens at a point
where it doesn't kind of

00:11:53.840 --> 00:11:56.120
create junk genes.

00:11:56.120 --> 00:11:59.670
Because you can imagine, this
cut-off point, which is called

00:11:59.670 --> 00:12:02.690
a chiasma, it could have
happened in the middle of some

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gene, and it could have created
some random noise, and

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it could have broken down some
protein development in the

00:12:07.980 --> 00:12:09.010
future or who knows what.

00:12:09.010 --> 00:12:09.965
But it doesn't happen
that way.

00:12:09.965 --> 00:12:12.360
It happens in a reasonably
organized way, which kind of

00:12:12.360 --> 00:12:15.710
speaks to the idea that it's
part of the process.

00:12:15.710 --> 00:12:17.970
So prophase in I, you also
have this happening.

00:12:17.970 --> 00:12:21.210
So once that happens you could
have this guy's got a little

00:12:21.210 --> 00:12:26.810
bit of that chromatid and then
this guy's got a little bit of

00:12:26.810 --> 00:12:28.350
that chromatid.

00:12:28.350 --> 00:12:30.700
So all of this stuff happens
in prophase I.

00:12:30.700 --> 00:12:32.550
You have this crossing over.

00:12:32.550 --> 00:12:36.890
The nuclear envelope starts to
disassemble, and then all of

00:12:36.890 --> 00:12:40.460
these guys align and the
chromatin starts forming into

00:12:40.460 --> 00:12:42.990
these more tightly wound
structures of chromosomes.

00:12:42.990 --> 00:12:45.840
And really, that's all-- when
we talk about even mitosis,

00:12:45.840 --> 00:12:48.000
that's where a lot of the action
really took place.

00:12:48.000 --> 00:12:51.020
Once that happens, then we're
ready to enter into the

00:12:51.020 --> 00:12:54.920
metaphase I, so let's go
down to metaphase I.

00:12:54.920 --> 00:13:00.100
In metaphase I-- let me just
copy and paste what I've

00:13:00.100 --> 00:13:03.710
already done-- the nuclear
envelope is now gone.

00:13:03.710 --> 00:13:09.740


00:13:09.740 --> 00:13:12.820
The centrosomes have
gone to opposite

00:13:12.820 --> 00:13:17.130
sides of the cell itself.

00:13:17.130 --> 00:13:20.010
Maybe I should draw the
entire cell now

00:13:20.010 --> 00:13:22.030
that there's no nucleus.

00:13:22.030 --> 00:13:24.090
Let me erase the nucleus
a little bit

00:13:24.090 --> 00:13:26.990
better than I've done.

00:13:26.990 --> 00:13:30.070
Let me erase all of that.

00:13:30.070 --> 00:13:34.460
And, of course, we have the
spindles fibers that have been

00:13:34.460 --> 00:13:39.750
generated by now with the
help of the centrosomes.

00:13:39.750 --> 00:13:41.940
And some of them, as we learned,
this is exactly what

00:13:41.940 --> 00:13:44.310
happened in mitosis.

00:13:44.310 --> 00:13:46.290
They attach to the kinetochores,
which are

00:13:46.290 --> 00:13:52.550
attached to the centromeres
of these chromosomes.

00:13:52.550 --> 00:13:55.970
Now, what's interesting here is
that they each attach-- so

00:13:55.970 --> 00:13:59.510
this guy's going to attach to--
and actually, let me do

00:13:59.510 --> 00:14:00.950
something interesting here.

00:14:00.950 --> 00:14:02.770
Instead of doing it this way,
because I want to show that

00:14:02.770 --> 00:14:05.070
all my dad's chromosomes don't
go to one side and all my

00:14:05.070 --> 00:14:06.760
mom's chromosomes don't
go to the other side.

00:14:06.760 --> 00:14:10.000
So instead of drawing these two
guys like this, let me see

00:14:10.000 --> 00:14:12.310
if I can flip them.

00:14:12.310 --> 00:14:12.880
Let me see.

00:14:12.880 --> 00:14:15.150
Let me just flip them
the other way.

00:14:15.150 --> 00:14:17.560
Whether or not which direction
they're flipped is completely

00:14:17.560 --> 00:14:20.500
random, and that's what
adds to the variation.

00:14:20.500 --> 00:14:23.550
As we said before, sexual
reproduction is key to

00:14:23.550 --> 00:14:25.610
introducing variation
into a population.

00:14:25.610 --> 00:14:28.205
So that's the mom's and
that's the dad's.

00:14:28.205 --> 00:14:28.790
They don't have to.

00:14:28.790 --> 00:14:30.700
All of the ones from my dad
might have ended up on one

00:14:30.700 --> 00:14:33.300
side and all of them from my mom
might end up on one side,

00:14:33.300 --> 00:14:35.490
although when you're talking
about 23 pairs, the

00:14:35.490 --> 00:14:38.720
probability becomes
a lot, lot lower.

00:14:38.720 --> 00:14:41.560
So this is one from my dad.

00:14:41.560 --> 00:14:44.930
Of course, it has some
centromeres.

00:14:44.930 --> 00:14:46.790
Let me draw that there.

00:14:46.790 --> 00:14:50.160
And so these microspindles,
some of them attach to

00:14:50.160 --> 00:14:52.480
kinetochores, which are these
protein structures on the

00:14:52.480 --> 00:14:54.560
centromeres.

00:14:54.560 --> 00:14:57.100
And this is just
like metaphase.

00:14:57.100 --> 00:15:00.050
It's very similar to metaphase
in mitosis.

00:15:00.050 --> 00:15:05.160
This is called metaphase I,
and everything aligns.

00:15:05.160 --> 00:15:08.410
Now we're going to
enter anaphase I.

00:15:08.410 --> 00:15:13.050
Now, anaphase I is interesting,
because remember,

00:15:13.050 --> 00:15:16.710
in mitosis in anaphase, the
actual chromatids, the sister

00:15:16.710 --> 00:15:18.880
chromatids separated
from each other.

00:15:18.880 --> 00:15:22.990
That's not the case in anaphase
I here in meiosis.

00:15:22.990 --> 00:15:28.020
So when we enter anaphase I, you
have just the homologous

00:15:28.020 --> 00:15:30.840
pairs separate, so the
chromatids stay with their

00:15:30.840 --> 00:15:32.630
sister chromatids.

00:15:32.630 --> 00:15:37.130
So on this side, you'll have
these to go there.

00:15:37.130 --> 00:15:42.620
While I have the green out, let
me see if I can draw this

00:15:42.620 --> 00:15:45.070
respectably.

00:15:45.070 --> 00:15:46.780
I have the purple.

00:15:46.780 --> 00:15:49.780
It's a little bit shorter
version here.

00:15:49.780 --> 00:15:53.480
He's got a little bit of
a stub of green there.

00:15:53.480 --> 00:15:56.180
This guy's got little stub
of purple there.

00:15:56.180 --> 00:16:00.000
And then they have this longer
purple chromosome here.

00:16:00.000 --> 00:16:01.510
This is anaphase I.

00:16:01.510 --> 00:16:03.290
They're being pulled apart,
but they're being pulled

00:16:03.290 --> 00:16:07.250
apart-- the homologous pair is
being pulled apart, not the

00:16:07.250 --> 00:16:10.610
actual chromosomes, not
the chromatids.

00:16:10.610 --> 00:16:13.230
So let me just draw this.

00:16:13.230 --> 00:16:14.900
So then you have your
microtubules.

00:16:14.900 --> 00:16:16.060
Some are connected to
these kinetochores.

00:16:16.060 --> 00:16:17.310
You have your centromeres.

00:16:17.310 --> 00:16:19.780


00:16:19.780 --> 00:16:21.640
Of course, all of this is
occurring within the cell and

00:16:21.640 --> 00:16:23.570
these are getting
pulled apart.

00:16:23.570 --> 00:16:28.090
So it's analogous to anaphase
in mitosis, but the key

00:16:28.090 --> 00:16:31.830
difference is you're pulling
apart homologous pairs.

00:16:31.830 --> 00:16:34.070
You're not actually splitting
the chromosomes into their

00:16:34.070 --> 00:16:37.240
constituent chromatids,
and that's key.

00:16:37.240 --> 00:16:40.300
And if you forget that, you can
review the mitosis video.

00:16:40.300 --> 00:16:41.840
So this is anaphase I.

00:16:41.840 --> 00:16:48.400


00:16:48.400 --> 00:16:51.060
And then as you could imagine,
telophase I is essentially

00:16:51.060 --> 00:16:57.990
once these guys are at their
respective ends of the cell--

00:16:57.990 --> 00:17:01.190
it's getting tiring redrawing
all of these, but I guess it

00:17:01.190 --> 00:17:05.050
gives you time to let
it all sink in.

00:17:05.050 --> 00:17:09.240
So these guys are now at the
left end of the cell and these

00:17:09.240 --> 00:17:15.490
guys are now at the right
end of the cell.

00:17:15.490 --> 00:17:18.099
Now, the microtubules start
to disassemble.

00:17:18.099 --> 00:17:21.109
So maybe they're there a
little bit, but they're

00:17:21.109 --> 00:17:21.800
disassembling.

00:17:21.800 --> 00:17:24.170
You still have your centromeres
here and they're

00:17:24.170 --> 00:17:25.380
at opposite poles.

00:17:25.380 --> 00:17:27.339
And to some degree, in the
early part of telophase,

00:17:27.339 --> 00:17:29.870
they're still pushing the cell
apart, and at the same time,

00:17:29.870 --> 00:17:32.510
you have cytokinesis
happening.

00:17:32.510 --> 00:17:38.030
So by the end of telophase I,
you have the actual cytoplasm

00:17:38.030 --> 00:17:42.480
splitting during telophase right
there, and the nuclear

00:17:42.480 --> 00:17:44.810
envelope is forming.

00:17:44.810 --> 00:17:46.640
You can almost view it as the
opposite of prophase.

00:17:46.640 --> 00:17:50.020


00:17:50.020 --> 00:17:52.300
The nuclear envelope is forming,
and by the end of

00:17:52.300 --> 00:17:55.530
telophase I, it will have
completely divided.

00:17:55.530 --> 00:17:58.030
So this is telophase I.

00:17:58.030 --> 00:18:02.710
Now, notice: we started off with
a diploid cell, right?

00:18:02.710 --> 00:18:07.610
It had two pairs of homologous
chromosome, but it had four

00:18:07.610 --> 00:18:09.310
chromosomes.

00:18:09.310 --> 00:18:13.330
Now, each cell only has
two chromosomes.

00:18:13.330 --> 00:18:18.790
Essentially, each cell got one
of the pair of each of those

00:18:18.790 --> 00:18:20.960
homologous pairs, but it was
done randomly, and that's

00:18:20.960 --> 00:18:23.690
where a lot of the variation
is introduced.

00:18:23.690 --> 00:18:26.680
Now, once we're at this stage,
each of these cells now

00:18:26.680 --> 00:18:29.320
undergo meiosis II,
which is actually

00:18:29.320 --> 00:18:32.540
very similar to mitosis.

00:18:32.540 --> 00:18:34.550
And sometimes, there's actually
an in-between stage

00:18:34.550 --> 00:18:37.500
called interphase II, where
the cell kind of rests and

00:18:37.500 --> 00:18:40.790
whatever else, and actually the
centromeres now have to

00:18:40.790 --> 00:18:41.570
duplicate again.

00:18:41.570 --> 00:18:44.610
So these two cells-- I've drawn
them separately-- let's

00:18:44.610 --> 00:18:46.100
see what happens next.

00:18:46.100 --> 00:18:48.420
So let's say that the
centromere-- actually, I

00:18:48.420 --> 00:18:49.820
shouldn't have drawn the
centromere inside the

00:18:49.820 --> 00:18:50.740
nucleus like that.

00:18:50.740 --> 00:18:55.360
The centromere's going to be
outside the nucleus, outside

00:18:55.360 --> 00:19:00.880
of the newly formed nucleus
there and there.

00:19:00.880 --> 00:19:04.320
And then it'll actually
replicate itself at

00:19:04.320 --> 00:19:06.450
this point as well.

00:19:06.450 --> 00:19:07.700
So now we have two cells.

00:19:07.700 --> 00:19:10.470


00:19:10.470 --> 00:19:17.900
Let me just cut and paste what
I have. I have this one, this

00:19:17.900 --> 00:19:20.380
chromosome right here.

00:19:20.380 --> 00:19:22.650
It's got this little green stub
there and then I have

00:19:22.650 --> 00:19:25.630
this longer fully green
chromosome there.

00:19:25.630 --> 00:19:32.360
Now, this guy, he's got this
little purple stub here.

00:19:32.360 --> 00:19:35.770
Let me draw this whole purple
chromosome there.

00:19:35.770 --> 00:19:39.510
Then this guy has one chromatid
like that and one

00:19:39.510 --> 00:19:41.370
chromatid like that.

00:19:41.370 --> 00:19:47.060
Now, when we enter prophase
II, what do you

00:19:47.060 --> 00:19:48.610
think is going to happen?

00:19:48.610 --> 00:19:53.830
Well, just like before, you
have your nuclear envelope

00:19:53.830 --> 00:19:56.090
that formed in telophase I.

00:19:56.090 --> 00:19:57.830
It's kind of a temporary
thing.

00:19:57.830 --> 00:19:59.470
It starts to disintegrate
again.

00:19:59.470 --> 00:20:06.330


00:20:06.330 --> 00:20:10.520
And then you have your
centromeres.

00:20:10.520 --> 00:20:13.550
They start pushing apart so
now I had two centromeres.

00:20:13.550 --> 00:20:17.290
They replicated, and now they
start pushing apart while they

00:20:17.290 --> 00:20:19.260
generate their little
spindles.

00:20:19.260 --> 00:20:20.840
They push apart in opposite
directions.

00:20:20.840 --> 00:20:23.530
Now, this is happening in
two cells, of course.

00:20:23.530 --> 00:20:26.290
They go in opposite directions
while they generate their

00:20:26.290 --> 00:20:27.020
spindle fibers.

00:20:27.020 --> 00:20:29.310
And let me make it very clear
that this is two cells we're

00:20:29.310 --> 00:20:30.680
talking about.

00:20:30.680 --> 00:20:35.180
That's one of them and that's
the second of them.

00:20:35.180 --> 00:20:42.860
Now it's going to enter
metaphase II, which is

00:20:42.860 --> 00:20:45.920
analogous to metaphase I, or
metaphase in mitosis, where

00:20:45.920 --> 00:20:48.050
the chromosomes get lined up.

00:20:48.050 --> 00:20:49.050
Let me draw it this way.

00:20:49.050 --> 00:20:52.480
So now the centromeres, they've
migrated to the two

00:20:52.480 --> 00:20:54.810
poles of the cell.

00:20:54.810 --> 00:20:57.770
So those are my centromeres.

00:20:57.770 --> 00:21:00.085
I have all of my spindles
fibers.

00:21:00.085 --> 00:21:05.230


00:21:05.230 --> 00:21:06.630
Oh, sorry, did I call
those centromeres?

00:21:06.630 --> 00:21:07.880
The centrosomes.

00:21:07.880 --> 00:21:10.190


00:21:10.190 --> 00:21:11.655
I don't know how long I've been
calling them centromeres.

00:21:11.655 --> 00:21:14.830
These are centrosomes, and my
brain keeps confusing them.

00:21:14.830 --> 00:21:17.360
The centromeres, and maybe
this'll help you not do what I

00:21:17.360 --> 00:21:20.520
just did, the centromeres are
the things that are connecting

00:21:20.520 --> 00:21:22.090
the two sister chromatids.

00:21:22.090 --> 00:21:23.070
Those are centromeres.

00:21:23.070 --> 00:21:26.810
Centrosomes are the things that
are pushing back the--

00:21:26.810 --> 00:21:29.450
that generate the
spindle fibers.

00:21:29.450 --> 00:21:31.500
The chromosomes line up
during metaphase.

00:21:31.500 --> 00:21:34.200
Metaphase always involves the
lining up of chromosomes so

00:21:34.200 --> 00:21:35.790
that one-- let me
just draw it.

00:21:35.790 --> 00:21:38.790
So I have that and that.

00:21:38.790 --> 00:21:41.820
This one's got a purple
guy, too.

00:21:41.820 --> 00:21:44.920
This guy's got a purple guy,
a long purple guy, and then

00:21:44.920 --> 00:21:46.880
there's a little stub
for the other guy.

00:21:46.880 --> 00:21:51.410
This guy's got a long green
guy and this guy's got a

00:21:51.410 --> 00:21:56.540
little green stub, and then this
is the short green guy

00:21:56.540 --> 00:21:57.220
right there.

00:21:57.220 --> 00:21:58.650
And, of course, they're
being aligned.

00:21:58.650 --> 00:22:01.270
Some of these spindle fibers
have been attached to the

00:22:01.270 --> 00:22:03.360
centromeres or the kinetochores
that are on the

00:22:03.360 --> 00:22:08.460
centromeres that connect these
two chromatids, these sister

00:22:08.460 --> 00:22:09.650
chromatids.

00:22:09.650 --> 00:22:11.820
And, of course, we don't have
a nuclear membrane anymore,

00:22:11.820 --> 00:22:14.830
and these are, of course,
two separate cells.

00:22:14.830 --> 00:22:17.740
And then you can guess what
happens in anaphase II.

00:22:17.740 --> 00:22:19.970
It's just like anaphase
in mitosis.

00:22:19.970 --> 00:22:24.210


00:22:24.210 --> 00:22:27.340
These things get pulled apart
by the kinetochore

00:22:27.340 --> 00:22:30.190
microtubules, while the other
microtubules keep growing and

00:22:30.190 --> 00:22:33.140
push and these two things
further apart.

00:22:33.140 --> 00:22:34.480
So let me show that.

00:22:34.480 --> 00:22:36.530
And they the key here: this
is the difference between

00:22:36.530 --> 00:22:37.970
anaphase II and anaphase I.

00:22:37.970 --> 00:22:42.960
Anaphase I, the homologous pairs
were broken up, but the

00:22:42.960 --> 00:22:45.290
chromosomes themselves
were not.

00:22:45.290 --> 00:22:48.120
Now, in anaphase II, we don't
have homologous pairs.

00:22:48.120 --> 00:22:51.280
We just have chromatid pairs,
sister chromatids.

00:22:51.280 --> 00:22:54.910
Now, those are separated,
which is very similar to

00:22:54.910 --> 00:22:57.080
anaphase in mitosis.

00:22:57.080 --> 00:23:00.200
So now, this guy gets pulled in
that direction so it look

00:23:00.200 --> 00:23:01.940
something like this.

00:23:01.940 --> 00:23:04.830
The drawing here is the hardest
part of this video.

00:23:04.830 --> 00:23:06.200
So that guy gets pulled there.

00:23:06.200 --> 00:23:08.570
That guy's getting pulled
in that direction.

00:23:08.570 --> 00:23:10.840
He's got that little
green stub on him.

00:23:10.840 --> 00:23:13.470
And then you have one green
guy getting pulled in that

00:23:13.470 --> 00:23:16.210
direction with the longer
chromosome.

00:23:16.210 --> 00:23:18.220
And then one of the other longer
is getting pulled in

00:23:18.220 --> 00:23:21.190
that direction, and it's all
by these microtubules

00:23:21.190 --> 00:23:25.670
connected at the kinetochore
structures by a centrosome as

00:23:25.670 --> 00:23:28.850
kind of the coordinating body.

00:23:28.850 --> 00:23:30.350
It's all being pulled apart.

00:23:30.350 --> 00:23:32.850
Anaphase has always involved
the pulling apart of the

00:23:32.850 --> 00:23:36.440
chromosomes or pulling
apart of something.

00:23:36.440 --> 00:23:37.300
Let me put it that way.

00:23:37.300 --> 00:23:41.110
And it's happening on this
side of the cell as well.

00:23:41.110 --> 00:23:43.830
Of course, this is
all one cell.

00:23:43.830 --> 00:23:49.480
And just like in mitosis, as
soon as the sister chromatids

00:23:49.480 --> 00:23:52.450
are split apart, they are now
referred to as chromosomes, or

00:23:52.450 --> 00:23:53.910
sister chromosomes.

00:23:53.910 --> 00:23:55.530
And, of course, this
is happening twice.

00:23:55.530 --> 00:23:59.690
This is also happening
in the other cell.

00:23:59.690 --> 00:24:01.010
The other cell's a little
bit cleaner.

00:24:01.010 --> 00:24:03.730
It didn't have that
crossover occur.

00:24:03.730 --> 00:24:08.900
So you have the longer
purple one.

00:24:08.900 --> 00:24:12.600
He gets split up into two
chromatids, which we are now

00:24:12.600 --> 00:24:14.820
calling chromosomes, or
sister chromosomes.

00:24:14.820 --> 00:24:19.530
And then this guy up here, he
gets split up into this short

00:24:19.530 --> 00:24:23.270
green, and then there's a-- let
me do it this way-- this

00:24:23.270 --> 00:24:24.820
short green, and he's
got a little purple

00:24:24.820 --> 00:24:26.490
stub on it right there.

00:24:26.490 --> 00:24:29.090
And, of course, it's all being
pulled away by the same idea,

00:24:29.090 --> 00:24:30.885
by the centrosomes.

00:24:30.885 --> 00:24:33.700
I want to make sure I
get that word right.

00:24:33.700 --> 00:24:36.320
I'm afraid whether I used
centromeres for the whole

00:24:36.320 --> 00:24:39.110
first part of the video, but
hopefully, my confusion will

00:24:39.110 --> 00:24:42.540
help you from getting confused
because you'll realize that

00:24:42.540 --> 00:24:44.590
it's a pitfall to fall into.

00:24:44.590 --> 00:24:45.380
So that's anaphase.

00:24:45.380 --> 00:24:46.790
Everything is getting
pulled apart.

00:24:46.790 --> 00:24:49.810
And then you can imagine
what telophase II is.

00:24:49.810 --> 00:24:51.040
In fact, I won't
even redraw it.

00:24:51.040 --> 00:24:55.010
Telophase II, these things get
pulled apart even more, so

00:24:55.010 --> 00:24:59.170
this is telophase II.

00:24:59.170 --> 00:25:00.420
They get pulled apart
even more.

00:25:00.420 --> 00:25:02.180
The cell elongates.

00:25:02.180 --> 00:25:09.180
You start having this cleavage
occur right here.

00:25:09.180 --> 00:25:11.350
So at the same time that in
telophase II these get pulled

00:25:11.350 --> 00:25:13.560
part, you have the
cytokinesis.

00:25:13.560 --> 00:25:15.440
The tubules start disintegrating
and then you

00:25:15.440 --> 00:25:19.880
have a nucleus that forms
around these.

00:25:19.880 --> 00:25:22.720
So what is the end result
of all of these?

00:25:22.720 --> 00:25:29.280
Well, that guy's going to turn
into a nucleus that has this

00:25:29.280 --> 00:25:33.590
purple dude with a little green
stub, and then a long

00:25:33.590 --> 00:25:38.460
green guy, and then he's got
his nuclear membrane.

00:25:38.460 --> 00:25:41.620
And, of course, there's the
entire cytoplasm in the rest

00:25:41.620 --> 00:25:43.300
of the cell.

00:25:43.300 --> 00:25:46.280
The other person that was his
kind of partner in this

00:25:46.280 --> 00:25:50.180
meiosis II, he's going
to have a short

00:25:50.180 --> 00:25:55.980
purple and a long green.

00:25:55.980 --> 00:26:00.610
He has a nuclear membrane,
and, of course, it has

00:26:00.610 --> 00:26:03.340
cytoplasm around it.

00:26:03.340 --> 00:26:04.730
And then on this side, you have

00:26:04.730 --> 00:26:05.955
something similar happening.

00:26:05.955 --> 00:26:10.300
You see this first guy, this
first one right here has two

00:26:10.300 --> 00:26:11.840
long purple ones.

00:26:11.840 --> 00:26:13.080
They get separated.

00:26:13.080 --> 00:26:15.880
So let me see, you have one long
purple in that cell and

00:26:15.880 --> 00:26:19.670
you have another long
purple in this cell.

00:26:19.670 --> 00:26:22.780
In that top one, you have a
short green one, and in this

00:26:22.780 --> 00:26:25.260
bottom, you have a short green
one that had got a little bit

00:26:25.260 --> 00:26:28.960
of one of my dad's-- a
homologous part of one of my

00:26:28.960 --> 00:26:30.520
dad's chromosomes on it.

00:26:30.520 --> 00:26:33.740
And, of course, these also
have nuclear membranes,

00:26:33.740 --> 00:26:37.630
nuclear membranes, and, of
course, it has a cytoplasm in

00:26:37.630 --> 00:26:39.900
the rest of the cell, which
we'll learn more about all

00:26:39.900 --> 00:26:41.110
those other things.

00:26:41.110 --> 00:26:44.900
So what we see here is that we
went from a diploid starting

00:26:44.900 --> 00:26:46.530
way-- where did we start?

00:26:46.530 --> 00:26:50.890
We started up here with a
diploid germ cell, and we went

00:26:50.890 --> 00:26:52.820
through two stages
of division.

00:26:52.820 --> 00:26:56.220
The first stage split up
homologous pairs, but it

00:26:56.220 --> 00:26:58.590
started over with that crossing
over, that genetic

00:26:58.590 --> 00:27:01.380
combination, which is a key
feature of meiosis, which adds

00:27:01.380 --> 00:27:05.620
a lot a variation to a species
or to a gene pool.

00:27:05.620 --> 00:27:08.080
And then the second phase
separated the sister

00:27:08.080 --> 00:27:11.950
chromatids, just like what
happens in mitosis.

00:27:11.950 --> 00:27:15.980
And we end up with four haploid
cells because they

00:27:15.980 --> 00:27:18.750
have half the contingency of
chromosomes, and these are

00:27:18.750 --> 00:27:20.000
called gametes.

00:27:20.000 --> 00:27:22.499