Biology: Mitosis: The Phases

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Cell Reproduction
An Introduction to the Cell Cycle and Mitosis
Mitosis: The Phases

As we start to look outside of interphase and we go into mitotic division, let’s quickly review what a chromosome looks
like. Some organisms have two of these; some organisms have more than two. When the chromosome is doubled
and we’re at this stage, and we have our centromere with its kinetochore, we officially call this a “doubled
chromosome,” if you will. But even more importantly, I want to refer to each side of this chromosome as a
“chromatid.” Don’t get the vocabulary confused. Remember, before it was a chromosome it was chromatin.
Chromatin doubled to give you chromosomes. But we have to refer to each side of these as something, so we’re
going to call each of these a chromatid, and in fact they are going to be called “sister chromatids.” That just makes
the explanation and the understanding that much easier because now we can refer to them as something. And as
you know, these are eventually going to split and gradually become chromosomes, but let’s not get ahead of
ourselves.
Okay, let’s talk mitosis. Mitosis is going to be divided into literally five phases. Let’s list the five phases. First of all,
we’ve already talked about interphase. That’s not officially a phase of cell division. But we’re going to go into
something called prophase, prometaphase—and you know, it really becomes a question of what decade are you
learning this in, because we used to take prophase and divide it up into early, middle and late prophase, and then go
right into this next phase called metaphase. And there’s a reason I’m stressing this to you. There is no such thing as
phases in the sense that, okay, time to stop one and move on to the other. This is a dynamic, continuous, very
gradual process. But once again, for the sake of discussion and publication and communication, we have to call
these things something. So when I say “a metaphase chromosome” to you, you’ve got something pictured in your
head, as opposed to a prophase chromosome. So we need to have these words. After metaphase, we’re going to
leave metaphase and go into anaphase, and then our last phase will be telophase at the end. So what we have,
therefore, is this division of time. It’s a division over time that we’re going to look at this. Okay, so let’s go through it a
phase at a time. Let’s take a look at the vocabulary, but most importantly, what do we want? We want to see the
events.
First of all, what happened in interphase G, S1, G2? The cell got ready for this. And there’s the thing. Early in
interphase, the only thing—everything is important, but the one thing that’s going to affect us is that area called the
centrosome. In interphase, you guys recall that there is a structure in cells called the centrosome. And in animal
cells, within the centrosome are these structures called centrioles. Early in interphase the centrosome region doubles,
so you get two of them. And indeed, in animals the centrioles also double. The centrioles are kind of a puzzle for us.
If you remove them—like if you laser beam them—mitosis just works fine. And plants don’t even have centrioles. So
they’re certainly associated with the centrosome but the centrosome is the business end of what’s going to happen in
mitosis.
Okay, so what do we have here? Well, right here we have what typically I’m going to refer to as an interphase cell.
Let’s review some cell plumbing here, if you will. All right, so we’ve got the centrosome and centrioles. Let’s move
this up a wee bit and take a look at some other parts. Remember the nucleus, and you know that you have, within the
nucleus, chromatin, which as we enter mitosis has doubled but has not necessarily started its supercoiling. We have
the cell membrane, which I’ll call CM. We have the nuclear envelope, which I’ll call NE, for lack of space and time.
And we have this structure right up here called the nucleolus, and I’m going to write that down for you in case you
don’t remember that name, nucleolus.
Let’s take a look at what happens as we move from interphase to prophase. As we move from interphase to
prophase, you notice that what begins to occur here is the centrosomes are starting to separate. They’re starting to
move, and it looks like they’re starting to grow some fibers out of them. And in fact we call those asters, “aster” as in
star. So we have the beginning of asters. And we will see in just a bit that the asters are literally microtubules that are
going to make up—well, I won’t give you names, but they’re microtubules, suffice it to say. Look what’s happening to
our chromatin. It’s already starting to look like that chromosome I showed you a second ago with its centromere in the
center. So what we have here, therefore, is right here, these two lines are denoting sister chromatids. And of course,
we have a doubled chromosome.
All right, that’s nice. But what does this really look like in a cell? Well, let’s take a look at a cell. Here is a typical
animal cell in interphase. You can see the centrosome. You can see the nucleus. You can see the dark staining
chromatin. And you know, as I look at this, if I’m looking at it through a microscope I’m saying, “Wow, it looks like it
maybe has started to enter prophase,” because I’m starting to see not the chromatin stained as an amorphous blob
but it’s starting to look like the chromatin is starting to supercoil and perhaps form chromosomes. And so we start toget into prophase where the chromosomes begin to form. And look at this, you guys. Look at this. We have the
centrosomes that have started to come across and they’ve started to migrate. And in fact, this is well advanced. And
the asters are radiating out. So this is prophase as we begin to enter the next phase, which is going to be
prometaphase, and eventually into metaphase.
Let’s take a look at that. In prometaphase, something starts to happen to the nucleus. The nuclear membrane starts
to break down—the nuclear envelope. So the nuclear envelope breaks down, and we’ve denoted this with these
chunks of nuclear envelope, which allows these spindle fibers that are growing to literally start to penetrate into what
used to be the nucleus. So in prometaphase now we’re getting the breakdown of the nucleus, so fragments of the
nuclear envelope. Look at our centrosomes. They’re at either pole, if you will. And you have the chromosomes as
clearly doubled sets of chromatids. Interestingly enough, at this point, look what’s happened. Some of its fibers have
attached to the kinetochore, and so we have here—I’m going to say that this is a kinetochore fiber or tubule—or fiber,
whatever. And look, here are some tubules or fibers that have not attached to kinetochores. They may end up kind of
important in a little bit, too. So we have what I’m going to refer to as nonkinetichore tubules or fibers. And there’s my
centrosome at either end.
And then as we enter metaphase, a very cool thing begins: that tug-of-war I showed you before. And by the time
we’re at metaphase, we’re ready to start dividing because look what’s happened. Very simply, we have lined all of our
chromosomes—the chromosomes are as tightly packed as they’re ever going to get, and we have lined all of our
chromosomes on what now seems to be a spindle. See, it looks kind of like a spindle, if you know what a spindle is.
And they’re called spindle fibers. Some of these fibers go across the cell and don’t attach to a chromosome, and
some are directly attached to the kinetochores of the chromosomes. And in fact we refer to this so-called equator, or
this imaginary line across there where the centromeres, indeed, are lined up—we call that the “metaphase plate.” So
that’s going to be called the metaphase plate, that imaginary line, if you will, where they’re all lined up.
Well, what does this really look like? Glad you asked. Let’s take a look at a cell in metaphase. How nice. There’s
my cell. There are my two centrosomes. There are my spindles. And there are my chromosomes lined up on the
metaphase plate. Beautiful. When you see this stuff through a microscope, it’s downright exciting.
Well, we’re almost done because now we get to go to anaphase. In anaphase, the cell begins to pinch inward and we
start to get the cleaving of the cell. Let’s see what’s going to happen here. The cell begins to elongate—this is
anaphase—and finally we’re going to reach something—well, eventually we’ll get to a phase called telephase where
we’re going to pinch that thing, but not completely, apart. But let’s look at anaphase. Very simply, what has begun to
happen is that reeling-in process I told you about. The chromosomes, like little Pac-Men, the motor molecules along
the kinetochore—remember, there’s a kinetochore in each chromatid—the little Pac-Men along the kinetochore just
are going to eat their way along that fiber and look like they’re getting reeled in. So if we look at it with this
chromosome here, this is going to separate and start to pull like that, which explains the way I have these
chromosomes arranged. Let me just show you this real quick. This is so cool.
To put it like I had it right there, they start to work their way in, and look at the directions the chromatids point because
of the forces. And indeed, that’s exactly how we’ve drawn it for you. So in anaphase, they start to pull in and then
what’s going to happen is you’re literally going to start elongating the cell. The cell elongates, and that’s because of
the nonkinetochore microtubules that are literally kind of pushing that cell. By pushing on the middle, the cell is going
to be pushed inward. So it’s elongating. So the cell begins to elongate and we finally get to telephase. But before we
get to telephase, let me just show you one more thing. Let me show you what anaphase looks like in a cell. See, they
start to pull inward like so. And here’s a later anaphase right here, but I have a close-up of that cell. Here’s another
early anaphase right there. But let’s look at this bottom cell. Look at that, how they’re pulling all the way across.
And now finally we get to telephase. And in telephase we start to form something called the “cleavage furrow.” And in
the cleavage furrow, the cell begins to cleave. It begins to break—well, “break” is not a good work. It begins to divide.
And we’re getting ready for the next step, the final step, which we’ll talk about later, called “cytokinesis.” But look what
starts to happen in telephase. In telephase, we’re going to start to get the reforming of the nuclear envelope, and
that’s happening right here. The nuclear envelope begins to reform. Look, the centrosomes are divided. Look, we’re
starting to get the division of the cell right here. Look, oh, what’s that? My gosh, the nucleolus is reforming. What are
we getting, you guys? We’re getting a new cell. But what have you accomplished?

In summary, what have you accomplished? You have taken these chromosomes and literally split them so that now
no longer are these chromatids of a pair, but they are now in separate cells—one, one. And a new nucleus has
formed around them—one, two. A new nucleolus appears. And what can these cells do? Well, we’re going to see
next how we got to these two separate cells from that one, and that process is called “cytokinesis.” But look at what
you’ve done! You’ve doubled; you’ve split. The concept is there. We need the names so we can describe it.