Okay, today I'm going to model mitosis and we're going to do it with a simple
organism. In this organism it's going to have, we'll make it four chromosomes, and I'll add
those other two in just a second. And so our diploid cell is going to have a diploid number
of four. What does diploid mean? Haploid mean? Well, in this case diploid means that you
have a chromosome from your father, that you get from your father, and a chromosome that
you get from your mother. Now we would call this chromosome 1 and that's because it's
the longest one. If we had two other chromosomes, this would be chromosome 1 and this would
be chromosome 2. Now in us, if we have 22 what are called autosomes and then we have
sex chromosomes. But the way chromosomes are named are just their number. 1 is the longest,
2 is the next longest, 3 is the next longest. So a few other things you should know about
a chromosome, this would be the centromere, the center where they're connected. And then
each of these beads represents a gene. Now here we have just a few genes, but in a human
cell we are going to have thousands of genes on each of the chromosomes. And so this is
a very simplified model. You can imagine if this was a human chromosome, my whole board
would be filled with these chromosomes. And so this is a very simple model. So this one
right here from dad and this one from mom are what are called homologous chromosomes.
And what that means is that they have the same length, but they don't necessarily do
the same thing. What does that mean? Chromosome 1 from dad and 1 from mom are going to be
exactly the same length, centromeres located in the same thing and the genes will be in
the same spots. So for example this is the gene for blue eyes. This is would be where
the other gene for blue eyes is. And so you may be asking yourself, well how are they
different then if everything seems to be the same? Well, this right here could be a recessive
gene for blue eyes and this could be a dominant gene. It doesn't give you blue eyes. And so
those alleles are going to be found on either side. Or this right here could be the gene
for hitchhiker's thumb, makes your thumb bend backwards like this, and over here, hitchhiker's
thumb here, this could be one for a straight thumb. So you don't have a hitchhikers thumb.
And so the chromosome you get from mom, dad and the chromosome you get from mom are homologous
and they never ever meet, except in meiosis, which will get to in just a second. Okay,
so what's our goal in mitosis? The goal in mitosis is to make an exact copy of the nucleus.
And so this circle right here, bounded by this, is going to be the nuclei and these
are going to be the chromosomes inside it. Now what does a cell do before it divides?
A cell will get bigger and then it will copy its DNA. So let me model how that works. When
it copies the DNA this chromosome will have an exact copy of it made. This occurs during
the S
phase. This chromosome will have an exact copy of itself. This one will have an exact
copy of itself and then this one will make an exact copy of itself. And so maybe this
is what you'll remember chromosomes looking like. And so after the
S phase, you'll eventually have a cell,
or a chromosome that's made an exact duplicate of itself. And so this is a gene here, there's
an exact copy on this side. In other words this side and this side are exactly the same.
In fact those are called sister chromatids at this point. And so this would be at the
end of the S phase
What happens after the S phase?
It goes into another growth phase called
G2 phase. So
G2 phase, all of that is part
of interphase. Now would it look like this during interphase, you wouldn't see the chromosomes
during interphase. All of that DNA would be loose within the cell doing its job, it's
a job that it normally does. And so you're really not going to see chromosomes look like
this until we get to prophase. Okay. What happens during prophase, all that DNA is going
to coalesce and we'll actually be able to see it. Okay, what happens next? So that would
be prophase. The next thing is going to be metaphase. What happens in metaphase is that
these will, little spindle fibers will attach to these, and they will line up and meet in
the middle. And so they're going to line up like this. This chromosome will line up like
that, this one will line up like that, this one will line up like that. And so the reason
they line up like that is that the spindle fibers attach to them, and they're going to
go from here to here and here to here. And then we're going to have attachments over
on the other side as well. Now these spindles are going to go outside of the nuclei, which
I would draw, but I can't quite fit that on my screen right here. And so there's going
to be a tug on either side by the spindles and that lines these all up in this perfect,
what is called the metaphase plate that goes right down the middle. So this would be metaphase.
What happens next, and next is going to be anaphase. And so in anaphase what happens
is these are going to be pulled to the side as those spindles shorten. They'll be pulled
like this. And these will be pulled, this all happens at the same time, and these will
be pulled like this as well. So they're going to move to the sides and then the spindle
fibers are going to start to disappear. And then we're going to have two brand new nuclei,
so this would be one nuclei and this would be another nuclei. Now if you look at the
chromosomes in each of these nuclei, we've got one like that, we've got one like that,
we've got one like that and we've got one like that. They're exactly the same. In other
words we have two brand new nuclei and each of them have the same duplicate DNA in each
one. And that's the goal of mitosis. The goal of mitosis is to make two exact copies of
the cell. Okay, so I am going to take a minute and clean this up for just a second and then
I am going to show you how that differs from meiosis. So in mitosis we are making two nuclei
and those nuclei are going to be exactly the same, but now let me show you what we do in
meiosis. In meiosis our goal is not to make duplicate cells but to make different cells.
We make genetically different cells. And so how does it start? Well it starts the same
way. The cells going to make a copy of itself, and so it's going to go through a growth phase,
so the cell will get larger, it's then going to duplicate its DNA, so its going to duplicate
its DNA like that and like that and like that and like that. So this is going to be at the
end of interphase. So the cell has grown, it's copied its DNA and then it's started
to grow again. But now we're going to have something different happen. And so in meiosis
we have two divisions. And so during the first division what will happen is the homologs
will come together and this forms something called a tetrad. And so we'll have this homolog
fit together and this homolog together. In other words the chromosome you get from your
dad and the chromosome you get from your mom will actually come together and they'll wrap
around each other like this. They line up along the middle so this would be metaphase
I, and they are going to, this actually happens a little bit before this during prophase,
but what'll happen is that they're going to wrap around each other so closely that portions
of one will switch with portions of another. And portions of this one will switch with
portions of another one. And so during prophase I they're going to switch bits of them. Or
this one right here, it might pop off here, this little gene and it might switch with
another gene over here. So switch like that. And what that gives us is variability. And
so this would be during prophase I. We cross over, and so we switch some of the chromosomes.
Another important thing happens at the next step, which is metaphase I. This might line
up like this. But it also might line up like this. And this might line up like this, but
it also might line up like this. And this is called independent assortment as I start
to loose one of my chromosomes here. In other words, depending on which side they line up
on, they're going to be pulled in a different direction. Now why is this important? Well,
let's say this right here is the gene for Huntington's Disease. Huntington's Disease
means your going to die when you get to be 40 years old. And if it lines up like this
and goes to a sperm or an egg over here, then you're going to get Huntington's. But if it
goes over here, you're not. And so this, when you draw a Punnett Square is where that actual
genetic 50/50 split takes place. That's called independent assortment. Okay, so now what
happens next. We're going to attach a spindle on each each of these. So there's going to
be a spindle here and there's going to be a spindle here and that spindle is going to
during Anaphase I, it's going to pull them apart. So these are going to pull apart. And
these are going to pull apart to the side. Now what happens next is we form two new nuclei.
So I'll do that like this. So we form two new nuclei. And now the next thing we go through
is, we don't go through another interphase. So there's no interphase, but what happens
next is it will actually line up again. So it'll meet in the middle and these will meet
in the middle like this, and then it will simply spilt them in half. So this one will
go this way, this one will go this way. This one will go this way. This one will go this
way. The same thing will happen here. These will get split to the side and these will
get split to the side. So what is that? Well, we now create four cells. So here's one cell,
here's another cell, here's another cell and here's another cell. So in meiosis what we
create are four nuclei. And how is this different from mitosis, well we've reduced the numbers
of chromosomes, there's two in each one and also we've made them all different. And so
if you look at the color combinations in each of these nuclei, it's totally different. And
so what do these become? Well, in males each of these four become a sperm. And so each
of these will swim off to be a sperm. But in eggs it's a little bit different. In eggs,
let me put this one back for just a second. In eggs, one of these will be chosen, let's
say this one. One of these will become the chosen one. And all the other ones will form
something called a polar body. In other words they won't be used at all. And the reason
that is in an egg is that there's a lot of other part to an egg. There's going to be
all the mitochondria out here and there's going to be all of the endoplasmic reticulum
and there's going to be the golgi apparatus. And so there's all this other parts in the
cell. And so in an egg you're going to choose just one for the nuclei and that's going to
be that one chosen one, we'll call it. Okay. What happens next in the circle of life? In
the circle of life the next thing that happens, let's imagine this doesn't come from the same
egg, this sperm is going to fertilize this egg. So this sperm is going to fertilize this
egg. And so this is going to make a brand 2n=4
fertilized egg called a zygote. And so what happens next , well now we've got
chromosome 1 that we got from dad. Chromosome 1 that we got from mom. Chromosome 2 that
we got from dad and chromosome 2 that we got from mom. And so we have a brand new egg.
And so how do we go from an egg to a brand new organism? Well this one will make an exact
copy of itself. This'll make an exact copy of itself and then it will undergo mitotic
division. And so mitotic division is used to make exact copies of cells. And meiotic
division is used to make four cells that are genetically different. And so I hope that's
helpful.