We've gone over the general idea
behind mitosis and meiosis.
It's a good idea in this video to go a
little bit more in detail.
I've already done a video on mitosis, and in this one,
we'll go into the details of meiosis.
Just as a review, mitosis, you start with a diploid cell, and
you end up with two diploid cells.
Essentially, it just duplicates itself.
And formally, mitosis is really the process of the
duplication of the nucleus, but it normally ends up with
two entire cells.
Cytokinesis takes place.
So this is mitosis.
We have a video on it where we go into the phases of it:
prophase, metaphase, anaphase and telophase.
Mitosis occurs in pretty much all of our somatic cells as
our skin cells replicate, and our hair cells and all the
tissue in our body as it duplicates itself, it goes
through mitosis.
Meiosis occurs in the germ cells and it's used
essentially to produce gametes to facilitate sexual
reproduction.
So if I start off with a diploid cell, and that's my
diploid cell right there, this would be a germ cell.
It's not just any cell in the body.
It's a germ cell.
It could undergo mitosis to produce more germ cells, but
we'll talk about how it produces the gametes.
It actually goes under two rounds.
They're combined, called meiosis, but the first round
you could call it meiosis 1, so I'll call that M1.
I'm not talking about the money supply here.
And in the first round of meiosis, this diploid cell
essentially splits into two haploid cells.
So if you started off with 43 chromosomes, formally have 23
chromosomes in each one, or you can almost view it if you
have 23 pairs here, each have two chromosomes, those pairs
get split into this stage.
And then in meiosis 2, these things get split in a
mechanism very similar to mitosis.
We'll see that when we actually
go through the phases.
In fact, the prophase, metaphase, anaphase, telophase
also exist in each of these phases of meiosis.
So let me just draw the end product.
The end product is you have four cells and each of them
are haploid.
And you could already see, this process right here, you
essentially split up your chromosomes, because you end
up with half in each one, but here, you start with N and you
end up with two chromosomes that each have N, so it's very
similar to this.
You preserve the number of chromosomes.
So let's delve into the details of how it all happens.
So all cells spend most of their time in interphase.
Interphase is just a time when the cell is living and
transcribing and doing what it needs to do.
But just like in mitosis, one key thing does happen during
the interphase, and actually, it happens during the same
thing, the S phase of the interphase.
So if that's my cell, that's my nucleus right here.
And I'm going to draw it as chromosomes, but you have to
remember that when we're outside of mitosis or meiosis
formally, the chromosomes are all unwound, and they exist as
chromatin, which we've talked about before.
It's kind of the unwound state of the DNA.
But I'm going to draw them wound up because I need to
show you that they replicate.
Now, I'm going to be a little careful here.
In the mitosis video, I just had two chromosomes.
They replicated and then they split apart.
When we talk about meiosis, we have to be careful to show the
homologous pairs.
So let's say that I have two homologous pairs.
So let's say I have-- let me do it in appropriate colors.
So this is the one I got from my dad.
This is the one I got from my mom.
They're homologous.
And let's say that I have another one that
I got from my dad.
Let me do it in blue.
Actually, maybe I should do all the ones from my dad in
this color.
Maybe it's a little bit longer.
You get the idea.
And then a homologous one for my mom that's also a little
bit longer.
Now, during the S phase of the interphase-- and this is just
like what happens in mitosis, so you can almost view it as
it always happens during interphase.
It doesn't happen in either meiosis or mitosis.
You have replication of your DNA.
So each of these from the homologous pair-- and
remember, homologous pairs mean that they're not
identical chromosomes, but they do code
for the same genes.
They might have different versions or different alleles
for a gene or for a certain trait, but they code
essentially for the same kind of stuff.
Now, replication of these, so each of these chromosomes in
this pair replicate.
So that one from my dad replicates like this, it
replicates and it's connected by a centromere, and the one
from my mom replicates like that, and it's connected by a
centromere like that, and then the other one does as well.
That's the shorter one.
Oh, that's the longer one, actually.
That's the longer one.
I should be a little bit more explicit in which one's
shorter and longer.
The one from my mom does the same thing.
This is in the S phase of interphase.
We haven't entered the actual cell division yet.
And the same thing is true-- and this is kind of a little
bit of a sideshow-- of the centrosomes.
And we saw in the mitosis video that these are involved
in kind of eventually creating the microtubule structure in
pulling everything apart, but you'll have one centrosome
that's hanging out here, and then it facilitates its own
replication, so then you have two centrosomes.
So this is all occurring in the interphase, and
particularly in the S part of the interphase,
not the growth part.
But once that's happens, we're ready-- in fact, we're ready
for either mitosis or meiosis, but we're going
to do meiosis now.
This is a germ cell.
So what happens is we enter into prophase I.
So if you remember, in my-- let me write this down because
I think it's important.
In mitosis you have prophase, metaphase,
anaphase and telophase.
I won't keep writing phase down.
PMAT.
In meiosis, you experience these in each stage, so you
have to prophase I, followed by metaphase I, followed by
anaphase I, followed by telophase I.
Then after you've done meiosis 1, then it all happens again.
You have prophase II, followed by metaphase II, followed by
anaphase II, followed by telophase.
So if you really just want to memorize the names, which you
unfortunately have to do in this, especially if you're
going to get tested on it, although it's not that
important to kind of understand the concept of
what's happening, you just have to remember prophase,
metaphase, anaphase, telophase, and it'll really
cover everything.
You just after memorize in meiosis, it's happening twice.
And what's happening is a little bit different, and
that's what I really want to focus on here.
So let's enter prophase I of meiosis I.
So let me call this prophase I.
So what's going to happen?
So just like in prophase and mitosis, a couple of things
start happening.
Your nuclear envelope starts disappearing.
The centromeres-- sorry, not centromeres.
I'm getting confused now.
The centrosomes.
The centromeres are these things connecting these sister
chromatids.
The centrosomes start facilitating the development
of the spindles, and they start pushing apart a little
bit from the spindles.
They start pushing apart and going to opposite sides of the
chromosomes.
And this is the really important thing in prophase I.
And actually, I'll make this point.
Remember, in interphase, even though I drew it this way,
they don't exist in this state, the actual chromosomes.
They exist more in a chromatin state.
So if I were to really draw it, it would look like this.
The chromosomes, it would all be all over the place, and it
actually would be very difficult to actually see it
in a microscope.
It would just be a big mess of proteins and of histones,
which are proteins, and the actual DNA.
And that's what's actually referred to as the chromatin.
Now, in prophase, that starts to form into the chromosomes.
It starts to have a little bit of structure, and this is
completely analogous to what happens
in prophase in mitosis.
Now, the one interesting thing that happens is that the
homologous pairs line up.
And actually, I drew it like that over here and maybe I
should just cut and paste it.
Let me just do that.
If I just cut and paste that, although I said that the
nucleus is disappearing, so let me get rid of the nucleus.
I already said that.
The nucleus is slowly disassembling.
The proteins are coming apart during this prophase I.
I won't draw the whole cell, because what's interesting
here is happening at the nuclear, or what once was the
nucleus level.
So the interesting thing here that's different from mitosis
is that the homologous pairs line up next to each other.
Not only do they line up, but they can actually share-- they
can actually have genetic recombination.
So you have these points where analogous-- or I guess you
could say homologous-- points on two of these chromosomes
will cross over each other.
So let me draw that in detail.
So let me just focus on maybe these two right here.
So I have one chromosome from my dad, and it's made up of
two chromatids, so it's already replicated, but we
only consider it one chromosome, and then I have
one from my mom in green.
I'm going to draw it like that.
One from my mom in green, and it also has two chromatids.
Sometimes this is called a tetrad because it has four
chromatids in it, but it's in a pair of homologous
chromosomes.
These are the centromeres, of course.
What happens is you have crossing over, and it's a
surprisingly organized process.
When I say organized, it crosses over at
a homologous point.
It crosses over at a point where, for the most part,
you're exchanging similar genes.
It's not like one is getting two versions of a gene and the
other is getting two versions of another gene.
You're changing in a way that both chromosomes are still
coding for the different genes, but they're getting
different versions of those genes or different alleles,
which are just versions of those genes.
So once this is done, the ones from my father are now not
completely from my father, so it might look
something like this.
Let me see, it'll look like this.
The one from my father now has this little bit from my
mother, and the one from my-- oh, no, my mother's chromosome
is green-- a little bit from my mother, and the one from my
mother has a little bit from my father.
And this is really amazing because it shows you that this
is so favorable for creating variation in a population that
it has really become a formal part of the meiosis process.
It happens so frequently.
This isn't just some random fluke, and it happens in a
reasonably organized way.
It actually happens at a point where it doesn't kind of
create junk genes.
Because you can imagine, this cut-off point, which is called
a chiasma, it could have happened in the middle of some
gene, and it could have created some random noise, and
it could have broken down some protein development in the
future or who knows what.
But it doesn't happen that way.
It happens in a reasonably organized way, which kind of
speaks to the idea that it's part of the process.
So prophase in I, you also have this happening.
So once that happens you could have this guy's got a little
bit of that chromatid and then this guy's got a little bit of
that chromatid.
So all of this stuff happens in prophase I.
You have this crossing over.
The nuclear envelope starts to disassemble, and then all of
these guys align and the chromatin starts forming into
these more tightly wound structures of chromosomes.
And really, that's all-- when we talk about even mitosis,
that's where a lot of the action really took place.
Once that happens, then we're ready to enter into the
metaphase I, so let's go down to metaphase I.
In metaphase I-- let me just copy and paste what I've
already done-- the nuclear envelope is now gone.
The centrosomes have gone to opposite
sides of the cell itself.
Maybe I should draw the entire cell now
that there's no nucleus.
Let me erase the nucleus a little bit
better than I've done.
Let me erase all of that.
And, of course, we have the spindles fibers that have been
generated by now with the help of the centrosomes.
And some of them, as we learned, this is exactly what
happened in mitosis.
They attach to the kinetochores, which are
attached to the centromeres of these chromosomes.
Now, what's interesting here is that they each attach-- so
this guy's going to attach to-- and actually, let me do
something interesting here.
Instead of doing it this way, because I want to show that
all my dad's chromosomes don't go to one side and all my
mom's chromosomes don't go to the other side.
So instead of drawing these two guys like this, let me see
if I can flip them.
Let me see.
Let me just flip them the other way.
Whether or not which direction they're flipped is completely
random, and that's what adds to the variation.
As we said before, sexual reproduction is key to
introducing variation into a population.
So that's the mom's and that's the dad's.
They don't have to.
All of the ones from my dad might have ended up on one
side and all of them from my mom might end up on one side,
although when you're talking about 23 pairs, the
probability becomes a lot, lot lower.
So this is one from my dad.
Of course, it has some centromeres.
Let me draw that there.
And so these microspindles, some of them attach to
kinetochores, which are these protein structures on the
centromeres.
And this is just like metaphase.
It's very similar to metaphase in mitosis.
This is called metaphase I, and everything aligns.
Now we're going to enter anaphase I.
Now, anaphase I is interesting, because remember,
in mitosis in anaphase, the actual chromatids, the sister
chromatids separated from each other.
That's not the case in anaphase I here in meiosis.
So when we enter anaphase I, you have just the homologous
pairs separate, so the chromatids stay with their
sister chromatids.
So on this side, you'll have these to go there.
While I have the green out, let me see if I can draw this
respectably.
I have the purple.
It's a little bit shorter version here.
He's got a little bit of a stub of green there.
This guy's got little stub of purple there.
And then they have this longer purple chromosome here.
This is anaphase I.
They're being pulled apart, but they're being pulled
apart-- the homologous pair is being pulled apart, not the
actual chromosomes, not the chromatids.
So let me just draw this.
So then you have your microtubules.
Some are connected to these kinetochores.
You have your centromeres.
Of course, all of this is occurring within the cell and
these are getting pulled apart.
So it's analogous to anaphase in mitosis, but the key
difference is you're pulling apart homologous pairs.
You're not actually splitting the chromosomes into their
constituent chromatids, and that's key.
And if you forget that, you can review the mitosis video.
So this is anaphase I.
And then as you could imagine, telophase I is essentially
once these guys are at their respective ends of the cell--
it's getting tiring redrawing all of these, but I guess it
gives you time to let it all sink in.
So these guys are now at the left end of the cell and these
guys are now at the right end of the cell.
Now, the microtubules start to disassemble.
So maybe they're there a little bit, but they're
disassembling.
You still have your centromeres here and they're
at opposite poles.
And to some degree, in the early part of telophase,
they're still pushing the cell apart, and at the same time,
you have cytokinesis happening.
So by the end of telophase I, you have the actual cytoplasm
splitting during telophase right there, and the nuclear
envelope is forming.
You can almost view it as the opposite of prophase.
The nuclear envelope is forming, and by the end of
telophase I, it will have completely divided.
So this is telophase I.
Now, notice: we started off with a diploid cell, right?
It had two pairs of homologous chromosome, but it had four
chromosomes.
Now, each cell only has two chromosomes.
Essentially, each cell got one of the pair of each of those
homologous pairs, but it was done randomly, and that's
where a lot of the variation is introduced.
Now, once we're at this stage, each of these cells now
undergo meiosis II, which is actually
very similar to mitosis.
And sometimes, there's actually an in-between stage
called interphase II, where the cell kind of rests and
whatever else, and actually the centromeres now have to
duplicate again.
So these two cells-- I've drawn them separately-- let's
see what happens next.
So let's say that the centromere-- actually, I
shouldn't have drawn the centromere inside the
nucleus like that.
The centromere's going to be outside the nucleus, outside
of the newly formed nucleus there and there.
And then it'll actually replicate itself at
this point as well.
So now we have two cells.
Let me just cut and paste what I have. I have this one, this
chromosome right here.
It's got this little green stub there and then I have
this longer fully green chromosome there.
Now, this guy, he's got this little purple stub here.
Let me draw this whole purple chromosome there.
Then this guy has one chromatid like that and one
chromatid like that.
Now, when we enter prophase II, what do you
think is going to happen?
Well, just like before, you have your nuclear envelope
that formed in telophase I.
It's kind of a temporary thing.
It starts to disintegrate again.
And then you have your centromeres.
They start pushing apart so now I had two centromeres.
They replicated, and now they start pushing apart while they
generate their little spindles.
They push apart in opposite directions.
Now, this is happening in two cells, of course.
They go in opposite directions while they generate their
spindle fibers.
And let me make it very clear that this is two cells we're
talking about.
That's one of them and that's the second of them.
Now it's going to enter metaphase II, which is
analogous to metaphase I, or metaphase in mitosis, where
the chromosomes get lined up.
Let me draw it this way.
So now the centromeres, they've migrated to the two
poles of the cell.
So those are my centromeres.
I have all of my spindles fibers.
Oh, sorry, did I call those centromeres?
The centrosomes.
I don't know how long I've been calling them centromeres.
These are centrosomes, and my brain keeps confusing them.
The centromeres, and maybe this'll help you not do what I
just did, the centromeres are the things that are connecting
the two sister chromatids.
Those are centromeres.
Centrosomes are the things that are pushing back the--
that generate the spindle fibers.
The chromosomes line up during metaphase.
Metaphase always involves the lining up of chromosomes so
that one-- let me just draw it.
So I have that and that.
This one's got a purple guy, too.
This guy's got a purple guy, a long purple guy, and then
there's a little stub for the other guy.
This guy's got a long green guy and this guy's got a
little green stub, and then this is the short green guy
right there.
And, of course, they're being aligned.
Some of these spindle fibers have been attached to the
centromeres or the kinetochores that are on the
centromeres that connect these two chromatids, these sister
chromatids.
And, of course, we don't have a nuclear membrane anymore,
and these are, of course, two separate cells.
And then you can guess what happens in anaphase II.
It's just like anaphase in mitosis.
These things get pulled apart by the kinetochore
microtubules, while the other microtubules keep growing and
push and these two things further apart.
So let me show that.
And they the key here: this is the difference between
anaphase II and anaphase I.
Anaphase I, the homologous pairs were broken up, but the
chromosomes themselves were not.
Now, in anaphase II, we don't have homologous pairs.
We just have chromatid pairs, sister chromatids.
Now, those are separated, which is very similar to
anaphase in mitosis.
So now, this guy gets pulled in that direction so it look
something like this.
The drawing here is the hardest part of this video.
So that guy gets pulled there.
That guy's getting pulled in that direction.
He's got that little green stub on him.
And then you have one green guy getting pulled in that
direction with the longer chromosome.
And then one of the other longer is getting pulled in
that direction, and it's all by these microtubules
connected at the kinetochore structures by a centrosome as
kind of the coordinating body.
It's all being pulled apart.
Anaphase has always involved the pulling apart of the
chromosomes or pulling apart of something.
Let me put it that way.
And it's happening on this side of the cell as well.
Of course, this is all one cell.
And just like in mitosis, as soon as the sister chromatids
are split apart, they are now referred to as chromosomes, or
sister chromosomes.
And, of course, this is happening twice.
This is also happening in the other cell.
The other cell's a little bit cleaner.
It didn't have that crossover occur.
So you have the longer purple one.
He gets split up into two chromatids, which we are now
calling chromosomes, or sister chromosomes.
And then this guy up here, he gets split up into this short
green, and then there's a-- let me do it this way-- this
short green, and he's got a little purple
stub on it right there.
And, of course, it's all being pulled away by the same idea,
by the centrosomes.
I want to make sure I get that word right.
I'm afraid whether I used centromeres for the whole
first part of the video, but hopefully, my confusion will
help you from getting confused because you'll realize that
it's a pitfall to fall into.
So that's anaphase.
Everything is getting pulled apart.
And then you can imagine what telophase II is.
In fact, I won't even redraw it.
Telophase II, these things get pulled apart even more, so
this is telophase II.
They get pulled apart even more.
The cell elongates.
You start having this cleavage occur right here.
So at the same time that in telophase II these get pulled
part, you have the cytokinesis.
The tubules start disintegrating and then you
have a nucleus that forms around these.
So what is the end result of all of these?
Well, that guy's going to turn into a nucleus that has this
purple dude with a little green stub, and then a long
green guy, and then he's got his nuclear membrane.
And, of course, there's the entire cytoplasm in the rest
of the cell.
The other person that was his kind of partner in this
meiosis II, he's going to have a short
purple and a long green.
He has a nuclear membrane, and, of course, it has
cytoplasm around it.
And then on this side, you have
something similar happening.
You see this first guy, this first one right here has two
long purple ones.
They get separated.
So let me see, you have one long purple in that cell and
you have another long purple in this cell.
In that top one, you have a short green one, and in this
bottom, you have a short green one that had got a little bit
of one of my dad's-- a homologous part of one of my
dad's chromosomes on it.
And, of course, these also have nuclear membranes,
nuclear membranes, and, of course, it has a cytoplasm in
the rest of the cell, which we'll learn more about all
those other things.
So what we see here is that we went from a diploid starting
way-- where did we start?
We started up here with a diploid germ cell, and we went
through two stages of division.
The first stage split up homologous pairs, but it
started over with that crossing over, that genetic
combination, which is a key feature of meiosis, which adds
a lot a variation to a species or to a gene pool.
And then the second phase separated the sister
chromatids, just like what happens in mitosis.
And we end up with four haploid cells because they
have half the contingency of chromosomes, and these are
called gametes.