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.
