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.