Phases of Mitosis

Explanation of the phases of mitosis.

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.