- In the previous video, we talked about interphase
which is the bulk of a cell's life cycle as it grows
and its DNA replicates, and it grows some more.
And now, we're gonna talk about the actual cell division.
We're gonna talk about mitosis.
And if you wanna be precise, mitosis is the process
by which this one nucleus will turn into two nuclei
that each have the original genetic information.
Now, as we exit mitosis, we get into cytokineses
which will then split each of the nuclei
into a separate cell when we split the cytoplasm
right over here.
We split or the cell as it turns into to cells.
Well, let's see how all of this happens.
So the first phase, and I'll leave the end of interphase
right over here.
We have this big cell, our DNA has been replicated.
We have two centrosomes right over here.
The first phase of mitosis involves the cell
and I might draw a little bit smaller just so I have
enough space here.
So, involve.
So, this is the cell right over here.
So we're gonna go to this phase right over here.
And a few things start happening.
One, the DNA, the chromosomes go from being in their
chromatin form where they're all spread out
to kind of a more condensed form that you can actually see
from a light microscope.
So for example, that magenta chromosome which is now made up
of two sister chromatids after replication,
we talked about that in the interphase video.
It might look something like this in a ...
It might look something like this
if you were to look in a microscope.
It's unlikely to be magenta but it's gonna have kind of that
classic chromosome shape that you're used to seeing
in textbooks.
And it has the centromere that connects
these two sister chromatids.
Right now, both of these two sister chromatids combined
are considered to be one chromosome.
Even though before replication it was still considered,
the magenta stuff was still considered to be one chromosome.
And we can draw the blue chromosome.
Once again, it's now in the condensed form.
That's one sister chromatid right over there.
That's another sister chromatid.
They are connected at the centromere.
So they're condensing now as we enter into mitosis.
And the nuclear membrane
starts to go away.
So the nuclear membrane is starting to go away.
And these two centrosomes are starting to migrate
to opposite sides of the cell.
So one of them is going over here and one of them
is maybe going to go over here.
So they're migrating to opposite sides of the cell.
And it's pretty incredible.
It's easy to say, "Oh, this happens and then that happens."
Remember, this cell doesn't have a brain.
This is all happening through chemical and thermodynamic
reactions and the way, just based on certain triggers
of where the cell is and its life cycle.
It's amazing that this is happening.
It's amazing that the structures,
and what sometimes we consider to be a simple thing
but it's actually incredibly complex thing.
It kind of knows what to do even though
it has no intelligence here.
And a lot of what I'm talking about,
the general overview of the process is well understood
but scientists are still understanding exactly
how do the different things happen when they should happen
and by what mechanism and what's actually happening.
Sometimes, molecular or atomic basis.
But anyway, this first phase of mitosis,
the nuclear envelope, the nuclear membrane
starts to disappear.
The centrosomes migrate to the opposite ends of the cell.
And our DNA condenses into kind of the condensed form
of the chromosomes.
And we call this prophase.
Prophase.
Prophase of mitosis.
Now, in the next phase.
In the next phase, let me draw my cell again.
Drawing that same green color.
In the next phase,
your nuclear membrane is now gone.
And the chromosomes start lining up
in the middle of the cells.
So you have the blue one right over here,
the blue one that's one sister chromatid.
Here's another sister chromatid
and they're connected at the centromere,
not to be confused with a centrosome.
And then, here's the magenta, one of the
magenta sister chromatids and another one.
And once again, it's not magenta in real life,
I'm just making it a magenta because it looks nice.
That's the centromere right over there.
Our centrosomes are at opposite ends
of the cell at this point.
At opposite ends of the cell.
And you might have heard of the word,
let me label this again.
I labeled it in a previous video.
That's centrosomes.
Centrosomes where the two sister chromatids are connected.
That's a centromere.
Centromere.
Now, you might heard of the word centrioles.
Centrioles are actually, they help,
they are exist inside the centrosomes.
They're these two kind of cylindrical looking structures.
Each of the centrosomes have two centrioles.
But for the sake of this video, you can say,
"Well, the centrioles are just part of the centrosomes."
But I'm just listing you all the words
that involves centri something.
Centrioles right over there and you have two of them
per centrosome.
So hopefully, that helps clarify some confusion.
But these things line up.
And a lot of what you're about to see, this orchestration,
these things moving around in the cell, things lining up
and soon things pulling apart, these are all coordinated
with actually a fairly sophisticated mechanism of kind of a
scaffold of this kind of ropes, these microtubules.
And the centrosomes' role, until now
I've just been kind of drawing.
I mean, what do they do?
Well, the centrosomes' role is these microtubules
extend from them to each other and to the centromeres
of these chromosomes and to a large degree,
they're not the only actors here.
They help pull and push things in the right way.
So these help keep make sure that the two centrosomes
push away from each other.
And then as we'll see in the next phase,
that they're able to pull one of each of the
sister chromatids from each of the pairs towards
each of them.
So this right over here where you see
where the nuclear envelope is now gone, the chromosomes
have been lined up just like this and your centrosomes
are on opposite side of your cell.
We call this the metaphase.
We call this the metaphase of mitosis.
And then you can imagine what's going to happen next.
What's going to happen next?
What's going to happen next is ...
Let me, I don't wanna draw it too big 'cause I wanna be able
to fit it all in one page.
What's going to happen next is that
those microtubules are going to start pulling
on each of the sister chromatids.
Let me draw that.
So you have this centrosome right over here.
You have all these microtubules in your cell.
You have the centrosome right over here,
all those microtubules.
And this one is going to be pulling,
is going to be getting one of the blue chromatids
to pull towards it or to move towards it.
So let me draw that.
So this is one blue chromatid right over here.
aAd this one is going to be pulling the other blue
chromatid towards it.
And same thing for the magenta.
So that one's being pulled to that way.
And this one is being pulled that way.
And just in case you're concerned about the,
some of more of the words of vocabulary involved,
the point it was this microtubules connect to
what used to be sister chromatids,
but now that they're apart, we now call them chromosomes.
When they emerge, this is one chromosome and they have
two sister chromatids.
But now they're apart, we would actually now consider this
each an independent chromosome.
So now, you actually have four chromosomes over here.
This point right over here, we call it kinetochore.
And even exactly how that interphase works
and exactly how things move is not fully understood.
Some of this stuff is understood but some of this
is still an area of research.
So something even as basic as cell division,
not so basic after all.
So this right over here where you can start to see the DNA
kind of migrating to their respective sides of the cell.
We call this the anaphase.
And that takes us to the last formal phase of mitosis
and that is called telophase.
Telophase.
And in telophase, I'm gonna do my best shot to draw it.
And you could already see I have started to draw that
the cellular membrane is starting to pinch in kind of in
preparation for cytokineses, in preparation for the cell
splitting into two cells.
So I'll do it a little bit more.
Cytokineses is usually described as kinda being a separate
process in mitosis although it's obviously they are,
they kind of together help the cell fully turn
into two cells.
So now in telophase, so you have this,
what used to be a sister chromatid,
now we can call it a chromosome on its own.
And we have this chromosome.
And we have this chromosome
right over there.
We have to do it on both sides.
So you're there and you have
this right over here.
And actually, let me draw it a little bit different.
Because at this phase, you're really starting to unwind
what happened at prophase.
So prophase, you have the disappearance
of the nuclear membrane and you have the condensation
of the chromosomes into this form here.
Telophase, that's unwinding a little bit.
So actually, let me draw the two centrosomes.
So you have one centrosome with that same color that
I had before.
So you have one centrosome right here.
You have another centrosome right over there.
And now, the DNA, this blue DNA,
this chromosome is now here but it's starting to get
a little under wind a little bit,
same thing on this side right over here.
Magenta chromosome is on here but once again,
it's starting to get a little bit unwound.
Same thing over here.
And you start to have nuclear membranes forming
around the DNA.
So once again, it's kind of redoing what
was undone in prophase.
Undone in prophase.
And so, when you're done, essentially,
you're gonna have these nuclear membranes.
The DNA is gonna go back to its chromatin form.
And then you have cytokineses.
And cytokineses is the process by which this gets fully
pinched together and you have two separate cells.
And some folks will say, "Oh, it kind of begins in anaphase
"and finishes after telophase."
But it's kind of happening near the end of mitosis
in parallel with it.
So this is cytokineses.
Cytokineses.
Cytokineses right over here.
This essentially is how this larger cell
that had two nuclei, how this divides, fully divides
into two cells.
And at that point, you're back to this phase
of the cell cycle.
Now, each of these two cells
are going to go through interphase, G1,
S-phase replicate their DNA, and G2 phase goes some more
and then it go through mitosis again.
And then these two will turn into four cells.
Anyway, hopefully, you enjoyed that.