Captions are on! Click the CC button at bottom right to turn off. Follow us on Twitter (@AmoebaSisters) and Facebook! 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.