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Have you ever looked down at your leg or your arm to find some cut and you have no idea
where it came from or how you got it? So you put a bandage on it and a few days later,
it’s gone. It’s all healed, and you don’t even think anything of it of this amazing process
that causes this to happen. Or let’s say you're looking at your nails and you notice
they're a lot longer than the last time you cut them.
Or let’s say you're looking in the mirror and you notice that you are a lot bigger than
you were when you were five years old. What do all of these things have in common?
One major thing they both have in common is mitosis.
Mitosis is a type of cell division done by most of your body cells and it’s really
important for your cells to divide. If they didn’t divide, you wouldn’t grow. I mean, how do
you grow if you can’t make more cells, right? So one reason why you’re bigger than you were
when you were 5 is mitosis. Mitosis also is great for repair of damage. If you have some
kind of accident like when we were talking about that cut on your arm or leg, well you want
to make sure it can get repaired well so you have to make more cells to do that.
Mitosis is great for that. Now it’s really important to understand what it is not---mitosis
is not a process that makes sperm or eggs cells, because that's something different called meiosis,
which sounds like mitosis…unfortunately, but it is a different process. Mitosis is
done to produce body cells.
Mitosis makes identical cells, that is the goal, identical cells. So if you’re
trying to make more skin cells, to replace worn out or damaged skin cells, you don’t want
to start suddenly making stomach cells there. That would be ridiculous! You want to make
sure you have identical cells replacing what was lost, so mitosis makes identical cells.
It’s a really important thing. Now, it’s also important to understand that your cells
are not dividing all the time. If all they did was divide, it would just be rapid crazy
growth. In fact, this is kind of what cancer is. Cancer is uncontrolled cell growth. We
have a clip on the cell cycle and what the cell is usually doing most of its daily life,
which is actually a phase called interphase where it’s growing and replicating its dna
and carrying out its daily cell functions. That’s where cells spends most of their time in respect to the whole cell cycle.
Mitosis is a very short amount of time in respect to the whole cell cycle. But mitosis
is a critical process because this is where it is going to divide and make more cells.
Before we get into the steps of division, it’s really important to understand that
your cells have something inside them – an organelle called the nucleus. And the nucleus
holds your DNA. DNA is really important because it’s your genetic information. And if you’re
going to make more cells, you need to have the same DNA in those new cells as you did
in your original cells. You want it to be identical, no mistakes. Very important.
The problem is you’ve got a LOT of DNA. And we’ve got to get that DNA into the new
cells using mitosis. So there has to be a better way to organize that DNA. Well, what
actually happens is that DNA can be organized into these condensed units called chromosomes.
Chromosomes are made of DNA and protein. You’ve probably heard before that humans have 46
chromosomes. That means 46 chromosomes are found in most human body cell nuclei. What
are nuclei? Well it’s the plural of nucleus. You don’t say nucleuses; you say nuclei.
Well in the nuclei, there are 46 chromosomes. Organizing DNA into condensed chromosomes makes
it a lot easier to move over when you’re making new cells.
So if you have 46 chromosomes in a human body cell, you have to duplicate those chromosomes in interphase
before mitosis starts. That basically means you’re duplicating your DNA, since chromosomes
are made of DNA and protein. You have to do this before mitosis starts, because if you’re going
to make an identical cell that has 46 chromosomes just like the original, well it makes sense you
have to duplicate the genetic material before splitting. So if you look at our cell cycle video clip
we talk about interphase. That's a stage where most of the time, cells are spending their time.
They're actually duplicating their DNA during that time. So ready for the tricky part?
Because we tend to count chromosomes by the number of centromeres present, when the 46
chromosomes duplicate, we still say there are 46 chromosomes as the sister chromatids
are still attached and we’re counting by centromeres. So 46 chromosomes here, they
replicate in interphase, and you still have 46 chromosomes in this picture. But you went
from 46 to 92 chromatids. We have a video explaining that in more depth and how that
factors in for mitosis.
Ok so now we can get right into mitosis. I like to tell students to remember PMAT. It’s
a little acronym that helps you remember. The P is for prophase. The M is for metaphase.
The A is for anaphase. The T is for telophase. So remember: PMAT. The stages in order.
The very first step is prophase. Prophase because it’s the beginning step, the nucleus
is still there and it’s going to go away later on but this is a stage
where it's actually still there. The chromosomes are visible; in fact, we say they’re condensing
which means they are thickening and visible.
The next stage is metaphase. M for metaphase, but I also like to remember M for middle
because in this stage the chromosomes line up in the middle of the cell. The nucleus
has been disassembled, it’s no longer there so we’ve got the chromosomes in the middle
waiting there.
Next the A is for anaphase. In anaphase, I like to think as the A for “away.”
The chromosomes move away, they are moving to opposite sides of the cell, so they
are moving towards the poles of the cells.
Now one thing to point out, these chromosomes...they're not moving
by themselves, they actually have something called spindles. These spindles are fibers
that help move the chromosomes to the ends. Kind of helps them move along.
The last stage of mitosis---think T is for telophase. In telophase, the chromosomes are
actually at the complete opposite ends and new nuclei are forming on each side to make
these two new cells. The nuclei are starting to surround the chromosomes on both sides.
I like to think the T is for “two” because you can really see in this step that
the end goal is going to be two cells and in the human body, they're each going to have 46 chromosomes. And, again, remember,
they are identical.
Cytokinesis is responsible for the final separation into two cells by splitting the cytoplasm,
which completes after the PMAT mitosis stages.
So why did all of matter? Without understanding cell division, we wouldn’t understand how
growth and repair happens---because they both require more cells to be made. Understanding
mitosis is also very important for cancer research too. Cancer itself is uncontrolled cell growth
- so in other words, uncontrolled mitosis. Well, that’s it for the amoeba sisters and
we remind you to stay curious.