Mitosis and Meiosis Simulation


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.

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