Hi. It's Mr. Andersen. In this podcast I'm going to talk about mitosis. Mitosis
in general is used to mean exact copying of cells. Technically it means division of the
nucleus. But in biology we tend to refer to mitosis as making exact copies and then meiosis
as making sex cells. Why is mitosis important? Well when you were born you were a tiny little
zygote. And you moved from that to an embryo to a fetus and to you through a process of
mitosis. And when this lizard loses its tail it's going to regenerate a new tail using
mitosis. And when this anemone divides in half through asexual reproduction, you guessed
it, that's mitosis. Or when you even form a scab, that's mitosis. So mitosis is important.
And it's also important that we ensure that the DNA in that first cell is in all the cells
that are copied from that. Because that's how life works. You start life as an individual
cell with DNA in it. And you eventually end up with this complex organism that has billions
and billions of cells. And each of them have the exact same DNA inside it. Before we can
talk too much about mitosis we need to get this out of the way. And it's the idea of
diploid versus haploid. Basically if we're looking at an organism or a cell that's haploid,
it essentially means that it has one copy of all of it's DNA. And so in eukaryotic cells,
our DNA is organized into what are called chromosomes or linear stretches of DNA. And
so if I were to name these I would name this as one, two three. We generally do that from
the longest to the smallest. This would be chromosome 1, chromosome 2, chromosome 3.
But this cell would just have one copy of each of those. And we refer to that as a haploid
cell. Or its n. Or it has n copies of chromosomes. In this case n would equal 3. An diploid cell
is going to have two copies of it. In other words it's going to have two of chromosome
1, two of chromosome 2, and 2 of chromosome 3. And so we call that diploid. And I remember
di just means two. So it has two chromosomes. In this case 2n equals 6, since we have 6
total chromosomes. Let's apply that into humans. And so in humans a sperm like this or an egg
like this are haploid. And so for each of those, n=23. Same thing here. But a baby comes
from a cell that copies itself over and over and over again. And since we're fertilizing
a sperm with an egg, this is what the chromosomes in a human look like. In other words 2n=46.
You have 46 total chromosomes. You have two chromosome 1, two chromosome 2, two chromosome
3 and it just keeps going like that. Now this is just staining on the chromosomes. Each
of the chromosomes will have individual genes that are on it. It will be thousands of genes
that are found on each of that. But you have inside every cell in your body two chromosome
number 1s. And you got those from your dad. Excuse me, you get one from your dad and you
get one from your mom. And likewise with all the chromosomes in your body you have two
copies of each. And so hopefully that helps. That's what diploid and haploid means. Now
the cell cycle is basically how we go from one cell, and if I were to draw that in it
would be like one cell here with a nucleus. And how we eventually make 2 cells. And that,
let's start from the outside of the cell cycle. And the cell cycles really kind of just like
a photocopy machine. Once it starts we make copy after copy after copy after copy. And
so basically what we do is a cell starts here. And the first thing it's going to enter into
is G1 phase. Now that whole G1 phase, right here, and it just stands for gap. But I remember
G1 stands for grow. Because that's what the cell is going to do. So this G1 phase is a
cell just making, getting larger and larger and larger. And so it's getting bigger and
bigger. Now this is actually the first step in what's called interphase. And so this would
be all interphase. So from here all the way back around again, is interphase. And then
this would be mitosis right here. So basically what's next? Well the next phase is going
to be the S phase. So in this S phase, which is again part of interphase, that's going
to be where we actually copy the DNA. So this is going to be where we're doing DNA replication
and we're copying all the DNA. And then we're going to have G2 or gap 2 or we call this
grow 2. And so these are all the phases of interphase. So basically what does a cell
do in G1? It's going to grow. What's it do during S? It's going to stop growing. It's
going to copy all of its DNA. And the in G2 it's going to continue to grow. And so at
the end of this, so after we've gone through all of interphase, a cell is ready to divide.
And so at this point it's actually going to go through mitosis. What we would say as prophase,
metaphase, anaphase, telephase and then cytokinesis. And we'll talk about all of the specifics
of how that works. And so if you look at it, where is most of the time spent? It's actually
spent in interphase. Now there are some cells that never really copy themselves. So the
nerves for example in your central nervous system, so the nerves in your brain, aren't
going to make copies of themselves. And muscles are the same way. So they are stuck in what's
called a G0 phase. But all of the other cells in your body are making copies of themselves
because they wear out and they have to make extra copies. So before we can even look at
the parts of mitosis, we should understand what a chromosome really is. And so this the
characteristic shape of a chromosome. It's that X shape. If we were to go to the specific
parts of it, basically one side of it is going to be a chromatid. And so if you're to look
just on onside of it, so this one right here, is a chromatid. So this would be one chromatid
and this would be the sister chromatid over on this side. So where do we get chromosomes
looking like this? This is going to be after the S phase. So after the S phase remember
we've copied all of our DNA. And so basically if you've got DNA over here, you have the
exact same DNA over on the other side. So basically what are some other parts? You're
going to have this centromere which is going to be in the middle. And then there's not
going to be any genes right there. And then we're going to have a short arm and then a
long arm on this side. But again when you look at a chromosome like that, that's a chromosome
way down here. And sometimes we get confused on where's the DNA? Well the DNA is that double
helix. It's wrapped around these histone proteins. And then you've got have these beads and beads
upon beads. And lines upon lines. And so really that DNA is folded and folded and folded upon
itself before it eventually gets to the level of a chromosome. And so basically it's usually
going to look something like this. When it's in a cell, it's what we call chromatin. So
it's all loose in there and it's doing it's job. It's actually making proteins. It's controlling
the cell. But before we can copy it has to coalesce even more. So it gets this characteristic
chromosome shape. Okay. So now let's go through the phases of mitosis. I remember this as
IPMAT. It's interphase, prophase, metaphase, anaphase, telephase. The one thing that you
should remember is that mitosis is not specific phases where it's moving jerky. It's a movie
that just kind of moves from interphase to prophase to metaphase to anaphase. And so
there's some great movies out there. You could see cells going through mitosis. And you could
watch this process occur. But what I'm going to do is step you through the important phases
and then what's happening there. And so let's look at this. This is a cell that's now in
prophase. And so what happens so far? Well you can look here. We've already gone through
gap 1. We've copied all the DNA. And then we've going through gap 2. So now were in
prophase. So what's going to happen in prophase? Well all the chromatin, in other words that
loose DNA and proteins you can see has coalesced together. And it's in chromosomes. There's
no more nucleolus. Nucleolus remember makes ribosomes and that's kind of dissolved. We
have separation of, it should say centrosomes. So there are centrosomes which a moving to
the side. We're going to find this is animal cells. And then we start to get the formation
of the spindle. So the spindle are these blue tubes. And those are going to eventually attach
to the chromosomes. So let's look at the next phase and see what's happened. Now we're in
what's called pro-metaphase. So we're moving to metaphase. So what are some things that
have happened. Well you can see that that nuclear envelope has started to break apart.
And then the chromosomes are forming what are called kinetochores. And those kinetochores
are right here on the side. You can see on there. You can see one there. I like to think
of kinetochores almost like a Pac-Man. So they basically will sit there like this. They're
going to sit on the inside of the chromosome. So like that. And then the tubules that make
up the spindle, let me find a different color, so if we find blue, basically the spindles
are going to attach to the kinetochore and it looks kind of like that. And so basically
I live to think of this as like Pac-Man eating it's way out to the other side. It doesn't
really occur that way, but that's how I remember it. Let's look at some of the other important
things. We've got microtubules starting to attach. You can see we're starting to get
some attachment to the chromosomes. So this is prophase, pro-metaphase, excuse me. So
we're still not to metaphase. If we go to metaphase, metaphase, and I have some pneumonic
devices to remember this. So during metaphase I always remember that they're going to meet,
so the m in metaphase, excuse me, stands for meet. So they're coming together to meet.
And they're going to meet in the middle. And so basically all the chromosomes are going
to line up right here along what's called the metaphase plate. And so there's going
to be this imaginary line that goes right down through the middle of the cell. And you
can see that they're all lined up right along there. It's kind of like a tug of war. It's
pulling on it in either side. And they're eventually lining up right in the middle.
Now we have the anaphase. And so what happens in anaphase? Well you can see that they've
pulled apart. And so they're moving in either direction. So I remember the A in anaphase
stands for moving apart. So they're separating to either pole. Because remember what we have
to do is make sure that each of these cells have the exact same genetic material. If we
go to the next phase then that's telephase. Telephase I always think the T stands for
the end. And so we're getting close to the end. So what we're getting is now a new nuclear
envelope starting to form around each of those. They're starting to unfold from that characteristic
chromosome shape. And they're starting to form chromatin. We start to get the nucleoli
show up. And then the cell's continue to get bigger. And you could see here we've kind
of undergone mitosis. In other words the nucleus has divided and so what the last step? Well
that last step is cytokinesis. In other words that cytoplasm has to divide in half. And
so cytokinesis is the divvying up of the rest of the cell or the rest of the cytoplasm.
Being as there's important stuff in there. You can see here we've got some endoplasmic
reticulum. Some mitochondria, lysosomes. That's all going to be divvied up on either side.
Now if you think about it, what's the next step? Well the next step. So if you think
about it, what's the next step? Well the next step is this cell is going to go into G1 phase.
So it's going to start growing. It's going to copy it's DNA. And then it's going to grow
again. And then it's going to divide in half again. Now what we're looking at here is animal
cells. But plant cells will do something similar. The only difference is that instead of zipping
in half. Because the can't really do that. Because they have a cell wall, what they'll
do is they'll build something called a cell plate. That's going to form right here in-between
the cells. And that will eventually form a new cell wall. And eventually we'll have two
cells as they split in half. But aside from that it's pretty much the same way. In fact
here's some cells undergoing mitosis. This is just a diagram. But it's basically onion
root cells. So onions, when the roots grow, they do that by making more copies of the
cell mitotically. And that makes the cell get bigger and bigger and bigger. We could
go through and look at these cells and try to figure out what phase they're in. So I
would call that interphase and interphase and interphase and interphase and this one
I would call prophase. And then interphase. And you get the sense that almost all these
cells are in interphase. Which should make sense because when you look at the cell cycle
they were spending most of their time in interphase. But right here is one that's actually pulled
apart. And so I would call this telephase. Or this one, let's see, they're meeting right
in the middle. That would be metaphase. Or here they are pulling apart. So this would
be like in anaphase. And so basically we could count the cells and what phases they're in.
We could figure out kind of a pie chart of how much time they spend in each of those
phases. And this works great over and over and over again. But sometimes it works too
well. And so all cancer is is cells reproducing out of control. In other words we have all
these mechanisms inside our cells that stop cells from doing this. But occasionally there
will be a mutation and that will suppress some of these cancer suppressing genes and
eventually you have cancer. Which is a bunch of cells that make copies of themselves. The
troubling thing about that is these cells can move different places in your body and
they can activate other cells to become cancers. And so what do we call a group of cells that
are actively growing out of control? We call that a tumor. And you can see some tumors
forming in this person who has lung cancer and this person who has a tumor in their brain.
So you can see here this is a graph of cigarette consumption in men from 1900 to 1940. You
can see that it increased. And then this is lung cancer rates. And you can see that there
is roughly like a 20 to 40 year lag between how many cigarettes they were smoking and
the amount of lung cancer. And so there's genetic predisposition. But we can also have
carcinogens. In this case it's chemicals found within the smoke that can cause mutations
and cause cells to jump in and become cancers. But what is it? It's just uncontrolled cell
growth. Uncontrolled mitosis. And I hope that's helpful.