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- [Voiceover] In the last video,

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we had just started to get into meiosis,

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and to be more precise, meiosis I,

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and to be even more precise
than that, prophase I,

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but we spent a good bit
of time on prophase I

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because some interesting things happened.

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Some things happened just
like prophase in mitosis where

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the nuclear envelope disappears
or starts to disappear,

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you have the chromosomes
going into their dense form

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that has kinda this classic shape that you

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could see from a microscope,

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but what was unique or
what was interesting

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about meiosis I and
prophase I in particular

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is that you have this
chromosomal crossover,

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that is a pretty typical
thing to happen in meiosis I,

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and it tends to happen
in a fairly clean way

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where homologous sections
of these homologous pairs

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crossover, so these
sections of the chromosome

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tend to code for the same genes.

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They're just different
variants of those same genes.

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They might have different alleles,

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and then once again, this
just adds more variation

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as we get into sexual reproduction,

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so it's a kind of neat
thing that happens here.

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But now let's continue with meiosis,

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and in particular meiosis
I, and you could guess

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what the next phase is going to be called.

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It is metaphase I, metaphase, metaphase I,

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and it has some similarities
with metaphase in mitosis.

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So in metaphase I, let me draw my cell,

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so this is the cellular
membrane right over there.

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I have my centrosomes,
which are now going to play

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more significant roles.

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The nuclear membrane is now gone,

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and just like in metaphase in mitosis,

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my chromosomes are going to line up

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along the, here I'll draw it,
kind of this up, down axis.

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So let's do that.

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So you have this one right over here.

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This is one chromosome,
two sister chromatids,

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and we had the chromosomal crossover,

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so it has a little bit of pink here.

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I'm gonna take a little bit of time

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to switch colors a little
bit more frequently.

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And then you have the one,
at least most of which

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you got from your mother,

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yeah but there's been a little bit

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of chromosomal crossover here as well.

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So let me draw that.

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Let me draw that.

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And then you have this one,

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and just for the sake of,

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so you have this one, this
chromosome from your father.

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It has replicated, so it's
now two sister chromatids.

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And this one from your mother,

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and I'm not gonna show the
chromosomal crossover here.

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Maybe it didn't happen over here.

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No homologous recombination over here.

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So these are, I guess, shorter.

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Now let me draw the centromeres.

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The centromeres I started
doing in this blue color.

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So the centromeres, the centromeres,

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and then the centrosomes,

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you have these microtubules that start,

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they can push the centrosomes
away from each other.

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But they also attach at the
kinetochores to the chromosomes,

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to the chromosomes, just like that.

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And these are, the microtubules,
you'll see people talk

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about oh these connect, and they're able

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to move things around,

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but I find this incredible
that you just have

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a bunch of proteins through
just kind of chemical

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and thermodynamic processes,

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are able to do really interesting things

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like move chromosomes to
different parts of the cell,

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so that we eventually can
get these gametes that can

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participate in sexual reproduction.

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This is an amazing thing,

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and it's developed over
billions of years of evolution,

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but it's just mind boggling
to think about the complexity,

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and not all of this is
completely understood

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exactly how all of this works.

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I mean you have these
kind of motor proteins

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that help move the chromosomes
along, these microtubules

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can elongate and shorten
in interesting ways.

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So it's a really fascinating process.

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But anyway, this is what's
happening in metaphase I.

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Now you can probably guess
what happens after that.

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We then move to anaphase I.

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So let me, we now go to anaphase I.

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I'll write that over here.

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Anaphase, anaphase I,

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and just like anaphase in mitosis,

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over here, the chromosomes
start getting pulled apart,

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except for one significant difference,

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and this is actually a very
significant difference.

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In mitosis, the sister
chromatids get pulled apart.

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The sister chromatids get pulled apart

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to become two daughter chromosomes.

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That does not happen in anaphase I.

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In anaphase I, the sister
chromatids stay together.

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It's the homologous pairs
that get pulled apart.

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So let me draw that.

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So this homologous pair
up here gets pulled apart.

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The two sister chromatids do
not get pulled apart here.

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So you have this one is
getting pulled onto this side.

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So this one's getting
pulled onto this side.

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It has a little bit from the original,

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so a little bit of that right over there.

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And then you have this one
getting pulled on this side.

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So draw it the best I can, the colors,

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alright, so it looks like that,

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although it's nice to have,
it's kinda easy to keep track of

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cause these switch colors like that.

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And then you have this one
getting pulled on this side.

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This one getting pulled on this side.

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And finally finally this one
getting pulled onto that side.

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And let me draw the centrosomes.

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So that's my, oops, centrosome,

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and once again, it's pulling,

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or I guess you could say the
chromosomes are being moved

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and these things are
pushing each other apart.

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The two centrosomes might be pushing apart

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to get to the opposite
ends of the actual cell,

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but they're bringing,

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there's all sorts of
interesting mechanisms

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that are bringing along
these microtubules,

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bringing the chromosomes,

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once again splitting the homologous pairs.

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And how they split is random.

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You know, this pink one could
have been on the right side,

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this orange one could have
been on the left side,

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or vice versa, and once again,

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this adds more variation
amongst the gametes,

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so even all of the resulting
gametes that get produced,

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they all will have different
genetic information.

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So this is anaphase I.

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You're pulling these apart, and

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then you could imagine what
happens in telophase I.

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So telophase I, telophase, telophase I.

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Telophase I, and this is fairly analogous

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to what happens in mitosis in telophase.

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So now you have your
cytokinesis is beginning,

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and actually, it might even begin earlier,

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in mitosis it happens
as early as anaphase,

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at least the cytokinesis is starting,

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but you're starting to see that.

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The homologous pairs
are fully split apart,

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and they're at opposite ends,

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and actually they can begin to unravel

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into their chromatin state,

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so this one began to unravel
into its chromatin state.

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It has a little bit of the magenta.

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Oops, it has a little bit of
the magenta right over here.

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This is unravelling as well.

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This is unravelling like that,

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once it gets into its chromatin state.

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The cellular, and let me
do the other ones as well.

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So this is this one right over here.

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It's beginning to unravel.

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This one over here, beginning to unravel.

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It's got a bit of orange on it.

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It's got a little bit of orange on it.

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The nuclear membrane begins to form again.

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The nuclear membrane begins to form again.

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In some ways, it's reversing
what happened in prophase I

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where the nuclear membrane disappeared,

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and the chromosomes condensed.

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And let me draw, let me
draw the centrosomes,

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which are outside the nuclear
membrane, just like that.

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And the microtubules are also dissolving.

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The microtubules are also dissolving.

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And you have your cytokinesis.

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So your cytokinesis, so these separate.

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These separate into two cells.

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So once again, when we did the overview

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of meiosis, we said look,
the first phase of meisosis,

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you go from a diploid germ
cell to two haploid cells.

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And these aren't quite
our end product yet.

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This right over here, what
we have just gone through,

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what we have just gone through,

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all of this combined that
we have just gone through,

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this is meiosis I.

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And in the next video, we're
gonna go through meiosis II.

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Whoops, I didn't mean to do that.

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This is, so let's see,
all of this is meiosis I.

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Let me write that in a
different color, in bold.

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So this is all meiosis, meiosis I here,

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and you can see each of these cells now

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have a haploid number.

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They now have a haploid, haploid number

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of two chromosomes each.

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Now each of those two chromosomes

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do have two sister chromatids,

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and as we'll see in meiosis II,

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which is very similar to mitosis,

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is going to split up the sister chromatids

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from each of these chromosomes,

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which gives us two daughter chromosomes.

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So we're gonna see that over here.

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So your haploid number here is two.

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You have two chromosomes here and

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you have two chromosomes there.

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And we'll explore meiosis
II in the next video.