- [Voiceover] In the last video,
we had just started to get into meiosis,
and to be more precise, meiosis I,
and to be even more precise than that, prophase I,
but we spent a good bit of time on prophase I
because some interesting things happened.
Some things happened just like prophase in mitosis where
the nuclear envelope disappears or starts to disappear,
you have the chromosomes going into their dense form
that has kinda this classic shape that you
could see from a microscope,
but what was unique or what was interesting
about meiosis I and prophase I in particular
is that you have this chromosomal crossover,
that is a pretty typical thing to happen in meiosis I,
and it tends to happen in a fairly clean way
where homologous sections of these homologous pairs
crossover, so these sections of the chromosome
tend to code for the same genes.
They're just different variants of those same genes.
They might have different alleles,
and then once again, this just adds more variation
as we get into sexual reproduction,
so it's a kind of neat thing that happens here.
But now let's continue with meiosis,
and in particular meiosis I, and you could guess
what the next phase is going to be called.
It is metaphase I, metaphase, metaphase I,
and it has some similarities with metaphase in mitosis.
So in metaphase I, let me draw my cell,
so this is the cellular membrane right over there.
I have my centrosomes, which are now going to play
more significant roles.
The nuclear membrane is now gone,
and just like in metaphase in mitosis,
my chromosomes are going to line up
along the, here I'll draw it, kind of this up, down axis.
So let's do that.
So you have this one right over here.
This is one chromosome, two sister chromatids,
and we had the chromosomal crossover,
so it has a little bit of pink here.
I'm gonna take a little bit of time
to switch colors a little bit more frequently.
And then you have the one, at least most of which
you got from your mother,
yeah but there's been a little bit
of chromosomal crossover here as well.
So let me draw that.
Let me draw that.
And then you have this one,
and just for the sake of,
so you have this one, this chromosome from your father.
It has replicated, so it's now two sister chromatids.
And this one from your mother,
and I'm not gonna show the chromosomal crossover here.
Maybe it didn't happen over here.
No homologous recombination over here.
So these are, I guess, shorter.
Now let me draw the centromeres.
The centromeres I started doing in this blue color.
So the centromeres, the centromeres,
and then the centrosomes,
you have these microtubules that start,
they can push the centrosomes away from each other.
But they also attach at the kinetochores to the chromosomes,
to the chromosomes, just like that.
And these are, the microtubules, you'll see people talk
about oh these connect, and they're able
to move things around,
but I find this incredible that you just have
a bunch of proteins through just kind of chemical
and thermodynamic processes,
are able to do really interesting things
like move chromosomes to different parts of the cell,
so that we eventually can get these gametes that can
participate in sexual reproduction.
This is an amazing thing,
and it's developed over billions of years of evolution,
but it's just mind boggling to think about the complexity,
and not all of this is completely understood
exactly how all of this works.
I mean you have these kind of motor proteins
that help move the chromosomes along, these microtubules
can elongate and shorten in interesting ways.
So it's a really fascinating process.
But anyway, this is what's happening in metaphase I.
Now you can probably guess what happens after that.
We then move to anaphase I.
So let me, we now go to anaphase I.
I'll write that over here.
Anaphase, anaphase I,
and just like anaphase in mitosis,
over here, the chromosomes start getting pulled apart,
except for one significant difference,
and this is actually a very significant difference.
In mitosis, the sister chromatids get pulled apart.
The sister chromatids get pulled apart
to become two daughter chromosomes.
That does not happen in anaphase I.
In anaphase I, the sister chromatids stay together.
It's the homologous pairs that get pulled apart.
So let me draw that.
So this homologous pair up here gets pulled apart.
The two sister chromatids do not get pulled apart here.
So you have this one is getting pulled onto this side.
So this one's getting pulled onto this side.
It has a little bit from the original,
so a little bit of that right over there.
And then you have this one getting pulled on this side.
So draw it the best I can, the colors,
alright, so it looks like that,
although it's nice to have, it's kinda easy to keep track of
cause these switch colors like that.
And then you have this one getting pulled on this side.
This one getting pulled on this side.
And finally finally this one getting pulled onto that side.
And let me draw the centrosomes.
So that's my, oops, centrosome,
and once again, it's pulling,
or I guess you could say the chromosomes are being moved
and these things are pushing each other apart.
The two centrosomes might be pushing apart
to get to the opposite ends of the actual cell,
but they're bringing,
there's all sorts of interesting mechanisms
that are bringing along these microtubules,
bringing the chromosomes,
once again splitting the homologous pairs.
And how they split is random.
You know, this pink one could have been on the right side,
this orange one could have been on the left side,
or vice versa, and once again,
this adds more variation amongst the gametes,
so even all of the resulting gametes that get produced,
they all will have different genetic information.
So this is anaphase I.
You're pulling these apart, and
then you could imagine what happens in telophase I.
So telophase I, telophase, telophase I.
Telophase I, and this is fairly analogous
to what happens in mitosis in telophase.
So now you have your cytokinesis is beginning,
and actually, it might even begin earlier,
in mitosis it happens as early as anaphase,
at least the cytokinesis is starting,
but you're starting to see that.
The homologous pairs are fully split apart,
and they're at opposite ends,
and actually they can begin to unravel
into their chromatin state,
so this one began to unravel into its chromatin state.
It has a little bit of the magenta.
Oops, it has a little bit of the magenta right over here.
This is unravelling as well.
This is unravelling like that,
once it gets into its chromatin state.
The cellular, and let me do the other ones as well.
So this is this one right over here.
It's beginning to unravel.
This one over here, beginning to unravel.
It's got a bit of orange on it.
It's got a little bit of orange on it.
The nuclear membrane begins to form again.
In some ways, it's reversing what happened in prophase I
where the nuclear membrane disappeared,
and the chromosomes condensed.
And let me draw, let me draw the centrosomes,
which are outside the nuclear membrane, just like that.
And the microtubules are also dissolving.
The microtubules are also dissolving.
And you have your cytokinesis.
So your cytokinesis, so these separate.
These separate into two cells.
So once again, when we did the overview
of meiosis, we said look, the first phase of meisosis,
you go from a diploid germ cell to two haploid cells.
And these aren't quite our end product yet.
This right over here, what we have just gone through,
what we have just gone through,
all of this combined that we have just gone through,
this is meiosis I.
And in the next video, we're gonna go through meiosis II.
Whoops, I didn't mean to do that.
This is, so let's see, all of this is meiosis I.
Let me write that in a different color, in bold.
So this is all meiosis, meiosis I here,
and you can see each of these cells now
have a haploid number.
They now have a haploid, haploid number
of two chromosomes each.
Now each of those two chromosomes
do have two sister chromatids,
and as we'll see in meiosis II,
which is very similar to mitosis,
is going to split up the sister chromatids
from each of these chromosomes,
which gives us two daughter chromosomes.
So we're gonna see that over here.
So your haploid number here is two.
You have two chromosomes here and
you have two chromosomes there.
And we'll explore meiosis II in the next video.