Hi. It's Mr. Andersen and in this video we're
going to go through the phases of mitosis. If you ask anybody what they remember from
biology invariably it's the word mitosis. But they probably couldn't identify the different
phases of mitosis. And you should be able to by the end of this video, look right here
and say oh that's going to be telophase. Or that's going to be metaphase right there.
And also you should understand what's the point of mitosis. Mitosis is technically the
division of the nuclei. But what we're doing is really making sure that we can make two
exact daughter cells that are identical to that parent cell. And that's how you went
from that fertilized egg that was a zygote that was you to the billions of cells in your
body. And it's how you replace cells as they die over time. And so you should know this,
that mitosis is technically part of the cell cycle. And so a cell cycle is how a cell duplicates
itself. And most of the time it's in this orange phase which is called interphase.
Now what's a cell doing during that phase? During the G1 part of interphase the cell
is growing, so it is getting larger. It then enters into the S phase or the synthesis phase.
What's going on there? It's duplicating all of the DNA. And then finally it goes into
the G2 phase where the cell continues to grow. Now some cells don't go through this. They
don't copy themselves, like nerve and muscle cells. And so they enter into what's called
a G0 phase. They never divide. But if you are going to divide, after the G2 phase then
you go into what's called the M phase. And that's the mitosis, where we're dividing the
nuclei. What do you get when if you go through this whole cell cycle? You get two cells and
each of those independently go into their own cell cycle. And so you can think of it
almost like a clock. And so cells are spending various amounts of time in each of these different
phases. Where are they spending most of their time? In the interphase. The cell is growing
and copying the DNA. And then growing again. And so a classic lab that you do in biology
is to look at cells that are undergoing mitosis. And so what we're looking at here are onion
cells. And so we've dyed the chromosomes. And so what you see is that most of these
cells, not much is going on. You just have the nuclei on the inside. And so they're in
what's called interphase. But some of them are going through this division of the nuclei.
And so this one would be in prophase. This would be anaphase. And that's metaphase. And
so since they're spending less of their time there in this one picture, they're going to
be represented a smaller amount. And so the big thing that you should understand is that
mitosis is not individual phases. It's not just quickly changing between them. But it's
a movie. And this is a pretty amazing movie. This right here is a sea urchin undergoing
cell division. Becoming a larva of a sea urchin and eventually an adult sea urchin. And so
in this picture or in this time lapse the yellow is going to be dyed to represent the
chromosomes. Then the blue is actually the microtubules that make up the spindle that
allows us to divide that nuclei in half. And so if you think of it like a movie it makes
more sense to go through the whole movie before we get into each of the individual phases.
And so what we're going to go through are all the steps of mitosis. And we're just going
to play it like a movie. And we're going to concentrate on what goes on on four different
structures. So let's start with the cell membrane. So if we look through the different phases
during the cell membrane, just keep an eye on that. Don't look at anything else. So watch
what happens to the cell membrane overtime. Nothing really. And then boom the whole thing
divides in half. And so the cell is getting larger and then it will divide in half. So
we're forming what's called a cleavage furrow in the middle. And now we have two cells.
And so now let's look at a different structure. So we understand what's going on with the
cell membrane, what happens to the nuclear envelope or that membrane around the nuclei?
Let's watch that. So not much is happening. It got a little bit smaller there. And then
boom that whole nuclear envelope fragments apart. And it's gone. So we don't see it anymore.
And then it reforms. Once those new cells form then we have the new nuclei forming on
each side. Alright. Let's go through that movie again. This time let's look at the centrosome.
Now what is a centrosome? A centrosome is going to be made of two things. These microtubules.
And those are going to form a spindle. And it is going to allow us to equally divide
those chromosomes to either side. And then in the middle you have what's called a centrioles.
And we think that the centrioles organize those microtubules and organize that spindle.
Now if this were a plant cell there would be some differences like a cell wall. But
they don't have centrioles. We think that the nuclear envelope is important in basically
maintaining those microtubules as it goes through mitosis. And so let's watch what happens
to those centrosomes. And it's going to happen right away. So let's watch this. So the first
thing that happens is that it actually replicates. And so we're going to have one centrosome.
And then quickly we're going to have two. Now let's watch what happens to those centrosomes.
They form microtubules between them. That kind of pushes them apart. And then each cell
is going to have a centrosome in each one. So that would be replicated again. And then
this thing goes over and over and over again. Now let's look at the most important thing
as we switch through these slides. And that's the chromosomes themselves. So chromosomes
are the genetic information. And so it's the DNA inside the nuclei. Remember that DNA is
wrapped around these histone proteins. And so when we want to condense the chromosome
we just pack it more tightly and more tightly over time. And eventually it looks like this.
So this is that characteristic shape of a chromosome. Generally when this cell is in
interphase it's just going to be loose DNA doing its job. But when it replicates then
we have to make sure that it's condensed and we can equally share it. And so what's important
to know is that in this chromosome each of these sides is called a sister chromatid and
they are attached together at the centromere in the middle. So they're exact copies, since
during that S phase we copied all the DNA. Each of the sister chromatids are exactly
the same. Now one other thing that's going to show up as we go through mitosis is the
kinetochore. And the kinetochore is a protein that is going to be found on the center of
each of those sister chromatids. And it's going to attach to the microtubules. And I'll
point that out as we switch through it. And so let's look at these chromosomes now and
watch them over time. So they're real loose at this point. Now we've duplicated it so
we've copied the DNA. So you can see way more chromosomes. Now they're condensing together,
those chromosomes in the middle. At this point you can see that the chromosomes, so the kinetochore
right here is attaching to the microtubules of that spindle. Now some of those microtubules
go all the way across the cell. But you can see a lot of them are attaching to the kinetochores.
What happened now is that they've all lined up right across the middle. There's this straight
line along the middle of the cell. Watch what happens now to those sister chromatids. They're
pulling apart. And now they're going to each of their individual nuclei. You can see that
they were condensed by now they're starting to loosen up again. And now we've just got
loose DNA. And so those are the big things that are going on. And so now let's go through
the whole phases. We're going to add names to it and then just kind of talk about the
most important things that occur in each of those different phases. And so we'll start
with interphase which you know it's technically not part of mitosis. And so let's watch what
happens during interphase. And so this would be early in interphase and now this is later.
So what's one big thing that happened? We copied the DNA. We duplicated the DNA. And
then another thing that occurred during interphase ,that those centrosomes were replicated. So
now we have two centrosomes and we had one just before. So now keep an eye on that cell
and see what happens as we move into mitosis or prophase. So in prophase you can see two
big things happening. You can see the DNA starting to condense here. And then we're
attaching those microtubules all the way across from the centrosomes. So the chromosomes condense.
What else happens at this point? The mitotic spindle is starting to form. So when we we're
looking back at that sea urchin video, that was that dyed blue in color. It's really organizing
the division of the nuclei itself. Now let's go into what's called prometaphase. And so
during prometaphase what are the big things that happen? You can see right away that that
nuclear envelope is fragmenting, so it's breaking it apart. And then the other big thing that's
happening at this point is that those microtubules are starting to attach to the kinetochores
in the middle. Not all of them. Some of them are reaching all the across the cell. But
you know it's prometaphase if you see kind of that explosion of that nuclear envelope.
What happens during metaphase? Well you can see now that they've all lined up in the middle.
And that's how I remember metaphase means meet in the middle. And so what we get is
all of those lining up along what's called the metaphase plate. It's one of the straightest
lines in all of nature. Let's keep an eye on those chromatids as we move into anaphase.
So what's happening is that they're moving apart. I always think of the "A" standing
for apart. The chromatids are separating. And so we're getting equal amounts of DNA
in each of those daughter cells. And then finally we go into telophase and cytokinesis.
During telophase what we're going to form is a new nuclei on each side. The cells are
elongating and that cleavage furrow is forming. So this is that cleavage furrow right here.
It's basically taking that one cell and pinching it in half. Now if this was in a plant cell
they form what's called a cell plate which is a new cell wall right in the middle. What
are the other big things? Remember those microtubules I pointed to earlier that were not attached
to the chromosomes, they're actually elongating the cell and allowing it to divide. And then
the other important thing that happens is that we're forming a new nuclei around each
of those daughter chromosomes. Now where did that nuclear envelope come from? Remember
when we fragmented that nuclear envelope before, those fragments are coming back to make the
two new daughter nuclei. Eventually we will go back into interphase. Now where are each
of these cells headed? They're going back into the cell cycle again. And so the way
I ask my students to remember this is IPPMATC. And so you can just write that on the top
of a test. There's mnemonics to remember it, but this tends to work for me. Remember technically
interphase is not part of mitosis. Either is cytokinesis on the end. And so you should
be able to point at each of these. So if I point at this one right here, what phase is
that going to be? That is anaphase. And so what's going on? You can see those chromatids
moving apart. If we were to look back here, what's this going to be? That would be prometaphase.
That's where we get the explosion of those nuclear envelopes. And so that's mitosis.
It makes sure that we equally divide the DNA in those daughter cells. It allows cells to
replicate. It made all of the cells in your body. And I hope that was helpful.