In its most popular sense, when people talk about
mitosis, they're referring to a cell, a diploid cell.
So diploid just means it has its full complement of
chromosomes, so it has 2N chromosomes.
So that's the nucleus.
This is the whole cell.
And so most people are saying, look, the cell itself
replicates into two diploid cells, so it turns into two
cells, each that have a full complement of chromosomes, 2N
chromosomes.
And so when people say a cell has experienced mitosis, they
normally mean this.
But I want to make one slight clarification, that formally,
mitosis only refers to the process of the replication of
the genetic material and the nucleus.
So, for example, if I were to draw this-- let me draw the
cell-- and it has now two nucleuses, each with the
diploid number of chromosomes, this cell
has experienced mitosis.
It has not experienced cytokinesis, which we will
talk about in a few moments, but that's the process of the
actual cytoplasm of the cell being split into
two different cells.
And just as a clarity, the cytoplasm is all the stuff
outside of the nucleus.
So I'll talk about that in a second, but just know in
everyday usage, this is normally the case when people
talk about mitosis.
But if you've got a teacher that likes to get you on a
technicality, this is technically what mitosis is.
It's the splitting of the nucleus or the replication of
the nucleus into two separate nucleuses.
That's normally accompanied by cytokinesis where the
cytoplasms of the cells actually separate.
Now, with that said, let's go into the mechanics of mitosis.
So the first steps that are really necessary for mitosis
actually occur outside of mitosis when the cell is just
doing its day-to-day life, and that's during the interphase.
And the interphase, literally it's not a phase of mitosis.
It's literally when the cell is just living.
Let's say we have some new cell.
Let me do it in green.
That's a new cell here.
Maybe this is its nucleus.
It's got 2N chromosomes, and then it grows.
It brings in nutrients from the outside and builds
proteins and does whatever, and so it grows a bit.
It's obviously got its full chromosomal complement still.
And then at some point during this life cycle, and I'll
label these actually, so this phase in interphase, and this
might not even be covered in some biology classes, but they
give it a label.
They call it G1, which is really just
when the cell is growing.
It's just growing, accumulating materials and
building itself out, and then it actually replicates its
chromosomes.
So you still have a diploid number of chromosomes.
So let me zoom in.
So let me draw this.
This is called the S phase of interphase, so this is S.
And S is where you have replication of the actual
chromosomes.
Once again, we're not even in mitosis yet.
So S, you have replication of your chromosomes.
So if I were to zoom in on the nucleus during the S phase, if
I were to start off-- let me just start with some organism
that has two chromosomes.
So let's say that at the beginning of S phase, and I'll
draw things as chromosomes just to make it clear that
things are being replicated.
So let me say it has this chromosome right here and then
let's say it has this chromosome right here.
As it goes through S phase, these chromosomes replicate.
And I'm just drawing the nucleus here.
I've zoomed in on just this part right here, where N is 1,
where our full diploid complement is two chomosomes.
During S phase, our chromosomes will replicate and
will have-- so that green one will completely replicate and
generate a copy of itself, and we've learned this a little
bit, they're connected at the centromere.
Now, each of those copies are called chromatids, and that
magenta one will do the same thing.
Even though we have two chromatids, one for each
chromosome, now we have four chromatids, two for each
chromosome, we still say we only have two chromosomes.
That's its centromere right there.
This occurs in the S phase, and then the cell will just
continue to grow more.
So the cell was already big-- I'll focus on the cell again.
The cell was already big and it gets bigger.
It gets bigger, and that's during the G2 phase, so it's
just growing more.
Now, there's another little part of the cell we haven't
even talked about yet, but I'll talk
about it a little bit.
It's not super-duper important, but it's the idea
of these centrosomes.
These are going to be very important later on when the
cell is actually dividing, and those also duplicate.
So let's say I have a little centrosome here.
It has centrioles inside it.
You don't have to worry too much about that, but they're
these little cylindrical-looking things.
But I just want to-- so you don't get confused if you see
the word centriole and centrosomes, not to be
confused with centromeres, which are these little points
where the two chromatids attach.
Unfortunately, they named many things in this process very
similarly, or a lot of the parts
of a cell very similarly.
But you have these things called centrosomes that are
going to enter the picture very soon, that are sitting
outside of the nucleus, and they also replicate.
They also replicate during the interphase.
So you had one before, now you have two of them.
And, of course, they each have their two little centrioles
inside, but we're not going to focus too much
on those just yet.
So that's what happened in the interphase.
This is most of the cell's life, and it's kind of growing
and doing what it wants.
Actually, I'll make a slight point here.
When I drew the DNA here, I drew them as chromosomes.
But the reality is when we're sitting in the interphase,
this is not what the DNA would actually look like.
The DNA, if I were to actually draw this, it's in its
chromatin form.
It's not all tightly wound like I drew it here.
I drew it tightly wound so that you can see that it got
replicated, but the reality is that that green chromosome
would actually be all unwound, and if you were looking in a
microscope, you would even have trouble seeing it.
This is its chromatin form.
We'll talk a little bit about where it actually organizes
itself back into a chromosome, but in its chromatin form,
it's just a bunch of DNA and proteins that the DNA is
wrapped around a little bit, so you might have some
proteins here that the DNA is wrapped around a little bit.
But if you're looking at it in a microscope, it just looks
like a big blur of DNA and proteins.
Same thing for the magenta molecule.
Really, for DNA to do anything, it
has to be like this.
It has to be open to its environment in order for the
mRNA and the different types of helper proteins to really
be able to function with it.
And even for it to be able to replicate, it has to be
unwound like this in order for it to function.
It only gets tightly wound like this later on.
I just drew it like this, so really it had one green one,
and it's going to replicate to form another green one, and
they're going to be attached at some point.
That magenta one is going to replicate to form another
magenta one, and they'll be attached at some point, but
it's not going to be clear.
I just drew it this way to show that it really happened.
This is the reality.
It's in its chromatin form.
Now, we're ready for mitosis.
So the first stage of mitosis is
essentially-- let me draw this.
So I'll draw the cell in green.
I'm going to draw the nucleus a lot bigger than it normally
is relative to the cell just because, at least right now, a
lot of the action is going in the nucleus.
So the first stage of mitosis is the prophase.
These are somewhat arbitrary names that were assigned.
People looked in a microscope.
Oh, that's a certain type of step that we always see when a
nucleus is dividing so we'll call this the prophase.
What happens in the prophase is that the actual chromatin
starts actually turning into this type of form.
So as I just said, when we're in the interphase, the DNA's
in this form where it's all separated and unwound.
It actually starts to wind together, so this is where
you'll actually have-- and remember, it's already
replicated.
The replication happened before mitosis begins.
So I had that one chromosome there, and then I
have another one here.
It has two sister chromatids that we'll see
soon get pulled apart.
Now, during prophase, you also start to have these
centromeres appear that I was touching on before.
These guys over here, they start to facilitate the
generation of what you call microtubules, and you can kind
of view these as these sticks or these ropes that are going
to be key in moving things around as we divide the cell.
All of this is pretty amazing.
I mean, you think of a cell, you think of something that's
inherently pretty simple.
It's the most basic living structure in us or in life.
But even here, you have these complex mechanics going on,
and a lot of it still isn't understood.
I mean, we can observe it, but we really don't know what's
happening at the atomic level or at the protein level that
allows these things to move around in such a nicely
choreographed way.
It's still an area of research.
Some of this is understood, some of it isn't.
But you have these two centrosomes, and they
facilitate the development of these microtubules, which are
literally like these little microstructures.
You can view them as tubes or as some type of rope.
Now as prophase progresses, it eventually gets to the point
where-- let me do it.
I don't want this word replication written here.
It makes it confusing.
Let me delete that.
Let me get rid of this replication.
So as prophase progresses, the nuclear envelope actually
disappears.
So let me redraw this.
Let me copy and paste what I've done before.
Put it there.
So as prophase progresses-- the nuclear envelope actually
starts to disassemble.
So this starts to actually dissolve and disassemble, and
then these things start to grow and attach themselves to
the centromere.
So actually, let me do that.
So this is all during prophase.
Since all of this happens during prophase, this latter
part of prophase, sometimes they'll call it late prophase,
sometimes it'll be called prometaphase.
Sometimes it's considered-- I don't think there's a hyphen
really there.
So sometimes it's actually considered a separate phase of
mitosis, although when I learned it in school, they
didn't bother with prometaphase.
They just called it all prophase.
But by the end of prophase, or actually by the end of
prometaphase, depending on how you want to view it, the whole
situation is going to look something like this.
You have your overall cell.
The nuclear envelope has disassembled, so to some
degree, it doesn't exist anymore.
Although the proteins that formed it are still there and
they're going to be used later on.
And you have your two chromosomes in this case.
In a human's case, you would have 46 of them.
You have your two chomosomes, each made with sister
chromatids, each made with two sister chromatids.
Two chromosomes.
They, of course, have their centromeres right there, and
then these centrosomes will have migrated roughly on
opposite sides of what was once the nucleus.
And these things have kind of spread apart, these
microtubules, so they're doing two functions, really.
At this point, they're kind of pushing these
two centrosomes apart.
So you have all of these things, and they're connecting
the-- you know, some of them are coming from this
centrosome, some are coming from this centrosome, some are
connecting the two.
And then some of these microtubules, these tubes or
these ropes, however you want to view them, attach
themselves to the centromeres of the actual chromosomes, and
the protein structure that they attach them to is called
the kinetochore.
So there's the kinetochore there, and that may or may not
be-- kinetochore.
It's a protein structure.
It's actually fascinating.
It's still an open area of research on how exactly the
microtubule attaches to the kinetochore, and as we'll see
in a second, it's at the kinetochore that the
microtubules essentially start to pull at the two separate
sister chromatids and actually pull them apart.
And it's actually not understood
exactly how that works.
It's just been observed that this actually happens.
Once prophase is done, essentially the cells then
just make sure that the chromosomes are well aligned.
I kind of drew them well aligned here, but that just
kind of formally occurs during metaphase,
which is the next phase.
The first one was prophase.
Now we're in metaphase, and metaphase really is just an
aligning of the chromosomes, so all of the chromosomes are
going to be aligned at the center of the cell.
So I have my magenta one here, I have my magenta one here,
and I have my other one here, my green one there, and, of
course, you have your centrosomes, the microspindles
that are coming off of them.
Some of them are kinetochore microspindles that are
actually attaching to the centromeres of the actual
chromosomes.
It's very confusing, right?
The centrosomes are these structures that help direct
what happens to these microtubules.
Centrioles are these little structures, these little
can-shaped structures inside the centrosomes, and the
centromere are the center points where the two
chromatids attached to each other within a chromosome.
So this is one sister chromatid, that's another
sister chromatid, and they attach at the centromere.
But this is metaphase.
It's fairly easy.
Metaphase, you just have this aligning of the cells, and
there's actually some theories, how does the cell
know to progress past this point?
How does it know that everything
is aligned and attached?
And then there are some theories that there's actually
some signaling mechanism that if one of these kinetochore
proteins isn't properly attached to one of these
ropes, that somehow a signal is sent that mitosis should
not continue.
So this is a very intricate process.
You can imagine if you have 46 chromosomes and you have all
of this stuff going on in the cell, and it's not like
there's some individual pushing stuff, or some
computer here.
It's really directed by chemistry and by
thermodynamic processes.
But just by the intricacy or the elegance of how these
things are, it happens spontaneously with all of the
proper checks and balances, so that most of the time, nothing
bad happens, which is all quite amazing.
So after metaphase, now we're ready to pull the stuff apart,
and that's anaphase.
So in anaphase-- let me write that down.
I've changed the color of my cell.
These guys get pulled apart.
And as soon as they get pulled apart-- so let's see, this
guy's getting pulled.
Let me do it in green.
So one of the sister-- nope, that's not green.
One of the sister chromatids is pulling in that direction.
One is getting pulled in that direction.
And then the same is true for the magenta ones.
Pulled in that direction, and one is getting
pulled in that direction.
And, of course, you have your centrosomes here and then
they're connected to the kinetochores that are right
there and that's where they're pulling.
There's also a whole microtubule structure that
isn't connected to the actual chromosomes, but they're
helping to actually push apart these two centrosomes so that
everything is going to opposite sides of the cell.
And so as soon as these two chromatids are separated, and
I touched on this a little bit before when we talked about
the vocabulary of DNA, then as soon as that happens, these
are each referred to as chromosomes.
So now you can say that the cell has what it
used to have here.
It has two chromosomes.
It now has four chromosomes.
Because as soon as a chromatid is no longer connected to its
sister chromatid, they're then considered sister chromosomes,
which is just a naming convention.
I mean, they were there before, they were there after.
They were just attached before.
Now they're not attached, so you kind of consider them
their own individual entity.
And then we're almost done.
The last stage is telophase.
I'm going to draw the cell a little bit different here
because something is happening simultaneously with telophase
most of the time.
So telophase, and actually I'll
rotate the cell 90 degrees.
Let's say that this was one centromere.
This is the other centromere.
So at this point, it's essentially
pulled the DNA to itself.
So this guy has pulled one copy of that chromosome and
one copy of this chromosome.
That guy's done the same up here.
He's pulled over one copy of each-- oh, I used a different
color-- one copy of each chromosome to himself.
Let me draw that right there like that.
And now the nuclear membranes start forming around each of
these two ends.
So now you start having a nuclear membrane form around
each of these two ends.
And so by the end of the telophase-- that's what we're
in, the telophase-- we will have completed mitosis.
We will have completely replicated our two original
nucleuses and all of the genetic content inside of it.
Now, at the same time telophase is happening, you
also normally have this cytokinesis, where this
cleavage furrow forms, where essentially-- during
telophase, these things are getting pushed further and
further apart by those microtubules so that they're
already at the ends of the cell, of the cytoplasm of the
cell, and you can almost view them as pushing on the sides
to elongate the cell.
As that is happening, you have this furrow forming, this
little indentation.
By the end of telophase in mitosis, you also have this
process of cytokinesis, where this cleavage furrow forms and
deepens, deepens, deepens until the cytoplasm is
actually split into two separate cells.
So this is cytokinesis, which is formally not a part of
mitosis, but it normally occurs with the telophase, so
right at the end of mitosis, you do normally have two
complete identical cells.
Once you have each of these two cells, then they, each
individually, enter their own interphase.
Or they each individually, if we look at just this one, he
will then be in his G1 phase.
At some point, these two things are going to replicate,
and that's the S phase, and you go to the G2 phase, and
then this guy will experience mitosis all over again.