Biology: Mitosis: An Overview

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Cell Reproduction
An Introductio to the Cell Cycle and Mitosis
Mitosis: An Overview

As we start to delve into this whole idea of the way cells divide, let’s just do a quick review of the cell cycle and make
sure you know where we’re going and where we’ve been. Remember that generally speaking, a cell doesn’t spend
much time dividing at all. It spends about 10 percent of its life. Some cells, in fact, never divide, and we’ll talk about
that later. But the bottom line is here, G1, is where most of your cell’s activities are going to occur. Then the cell
starts to get ready for replication. It grows a little bit more in G2—the G standing for “gap”—but we want to spend our
time right now in this section right here, this section, which his going to be called “mitosis” and “cytokinesis.”
Now, immediately the vocabulary starts coming out like asteroids out of the sky. And yes, biology does have a lot of
vocabulary, and yes, the word “mitosis” is important and the word “cytokinesis” is important. But I haven’t even written
them down yet. So am I derelict in my teaching here? No, absolutely not. We’re going to use those words plenty.
But what I want you to understand at this point is the cellular events that are going to happen right here in this stage
called mitosis, and then we’re going to talk about the events that happen in the cytokinesis stage. All of this stuff—the
interphase, the phase when the cell is not actively dividing—we spent a lot of time and will spend a lot more time on
that as we study cell physiology. But now we want to talk about right there, that M phase. What’s going to happen
when a cell divides? Later there will be plenty of time for the names of the phases, but right now let’s see what
happens.
Well, let’s quickly review what’s up with chromosomes. What’s a chromosome all about? Remember the
chromosomes are about packing of DNA. You only pack DNA into a chromosome when you’re going to do mitosis.
Mitosis. Better write that down. Sorry, guys, can’t speak without vocabulary. So in this mitotic division or in this
mitosis, we’re going to form chromosomes. And chromosomes end up looking something like this. And let’s talk
about what makes something a chromosome. Well, remember, it has its DNA, and its DNA is packed. And there is a
structure on that chromosome. All of this would be one molecule of DNA all the way down, all the way down, all the
way down, all the way down. Yet in the center of the chromosome there’s this kind of pinched-in section, and that’s
called the centromere. And within the centromere is going to be a very important structure, and we’ll see why it’s so
important in a second. And that is going to be called the kinetochore, within the centromere. The kinetochore—
kinetic energy. Starting to think it might have something to do with motion, right? Well, if you’re thinking that, you’re
absolutely correct.
So what mitosis is about, number one, is forming chromosomes. Just so I can show you, really you can form
chromosomes in three different ways. The centromere doesn’t always have to be in the center. This one in particular
has its chromosome in the center but some can be off-center. And indeed, some can be at the very top of the
chromosome. So anyway, these three chromosomes—this metacentric chromosome, this acrocentric chromosome
and this telocentric chromosome—all have their centromeres in different places, but that’s not as important as what’s
going to happen here. What’s going to happen is going to involve the kinetochore.
Now, once again I want you to think about the concept of mitosis rather than memorizing phases. What’s your job?
What’s your goal? If these are the chromosomes in their doubled form, what is it you want to do with these
chromosomes? If you’re a cell, remember the purpose of mitosis: to form an identical copy. And therefore your role
is to get these chromosomes so that you can actually halve them, because they’re doubled, and then split them.
That’s all it is. We’ve doubled, we split. We double, we split. We double, we split. That’s all you do your whole life
with your cells. Well, that doesn’t sound so hard. And really the whole idea of mitosis, phases aside, is more a
concept of what happens in your cytoskeleton and in your cytosol than what happens to your chromosomes, because
that’s very important.
I want to talk to you a little bit about how chromosomes move. There’s the magic of mitosis: chromosomal
movement. One of the things you’re going to notice, as I just said, all right, we’ve got our chromosomes doubled and
now we’ve got to split them. Well, in order to split them, you’ve got to move them. You’ve got to take a chromosome
like this and somehow get it to go like that. And you’ve got to somehow organize these before you can make a move,
and that is where spindle fibers come in. Spindle fibers are all about movement. Now, you know about spindle fibers
a little bit when we talked about—you may want to link back and read about tubulin, because spindle fibers are made
out of tubulin. Tubulin is a protein that makes up microtubules. So spindle fibers, therefore, are microtubules.
You remember that the cytoskeleton consists of microfilaments and microtubules. I know you know that. Well, here’s
the thing. As we enter mitosis, what’s going to happen is the microtubules of the cytoskeleton are going to break
down. Why? Well, here’s the thing. From these cytoskeleton microtubules, you are going to make these things
called spindle fibers. Why? Because they’re going to be made out of the same material. I’m using the British spelling
of “fibres” there; I guess it’s okay. And so the point here is that you have a cytoskeleton in your cell. That
cytoskeleton is there during G and S and G2, but as G2 progresses it starts to break down. Why? You’re going to
use the raw materials of this microtubule, the tubulin, to make fibers across the cell called spindle fibers. You’ll see
those and what they do—well, I’ll tell you right now.
What’s the purpose of the spindle fibers? Well, one of the things that I said has to happen is that your chromosomes
have to get organized. They somehow have to get set up—and let’s just take this randomly occurring cell, this
chromosome laying in the cell right now. And you know what? Let’s just say I want to somehow get these
chromosomes to organize and line up before I split the cell. Well, right here, denoted by this paper clip, is going to be
the kinetochore, okay? So what you’re going to see is that as the microtubules grow, they’re going to hook to a
kinetochore. We’ll put it all in sequence when we learn the phases, but they’re going to hook. And then what starts to
happen is—remember what tubulin is? Tubulin is a protein that sits in a circle that makes this long tubular
arrangement. In fact, take a look at the box right now. You can see that whole tubulin arrangement. What happens
is, apparently these have an ability to shorten. But you know what? If this is a microtubule and this is where it’s
hooked to—and you know what, we might be able to even make this just a little more dramatic by putting it onto an
orange background. If this is a microtubule and it’s hooked over here, it doesn’t shorten at its anchor site. It turns out
it shortens at the kinetochore. So we have a microtubule hooked to the kinetochore and there is an involvement of a
motor molecule there. So like a Pac-Man, what’s going to happen is this. This is going to shorten here by digestion,
but the paper clip, if you will, is going to eat its way along there and it’s going to cause chromosomal movement.
What does that look like to someone watching this? Well, it looks something like this. Here’s a microtubule hooked to
a kinetochore, and literally it looks like it’s being reeled in like a fishing line. But then another microtubule hooks to the
kinetochore on the other side of the chromosome and it starts to reel back. Now, these are hooked in the center of
the cell. And then this one reels; and then this one reels; and then this one reels; and then this one reels. And what
you eventually get is that double chromosome in the center of the cell. And you do that with all of your chromosomes.
Do you know how many chromosomes humans have? We have 46 chromosomes. All 46 of your chromosomes will
be lined up. How, and what do we call this? Names can come later.
But now that you have them lined up, guys, it’s easy. Since they’re all attached by their kinetochores to microtubules,
the next step is, once again, the motor molecules—and one, two, three. You split yourself into two groups of
chromosomes. See, that’s the principle. We’re going to put some names on it soon, but that’s all mitosis is about:
doubling chromosomes and then splitting them with the help of microtubules.