Mitosis and Meiosis Simulation


Mr. Andersen uses chromosome beads to simulate both mitosis and meiosis. A brief discussion of gamete formation is also included.


Transcript Provided by YouTube:

00:07
Okay, today I’m going to model mitosis and we’re going to do it with a simple
00:14
organism. In this organism it’s going to have, we’ll make it four chromosomes, and I’ll add
00:19
those other two in just a second. And so our diploid cell is going to have a diploid number
00:25
of four. What does diploid mean? Haploid mean? Well, in this case diploid means that you
00:30
have a chromosome from your father, that you get from your father, and a chromosome that
00:34
you get from your mother. Now we would call this chromosome 1 and that’s because it’s
00:39
the longest one. If we had two other chromosomes, this would be chromosome 1 and this would
00:45
be chromosome 2. Now in us, if we have 22 what are called autosomes and then we have
00:50
sex chromosomes. But the way chromosomes are named are just their number. 1 is the longest,
00:54
2 is the next longest, 3 is the next longest. So a few other things you should know about
00:59
a chromosome, this would be the centromere, the center where they’re connected. And then
01:04
each of these beads represents a gene. Now here we have just a few genes, but in a human
01:09
cell we are going to have thousands of genes on each of the chromosomes. And so this is
01:12
a very simplified model. You can imagine if this was a human chromosome, my whole board
01:17
would be filled with these chromosomes. And so this is a very simple model. So this one
01:22
right here from dad and this one from mom are what are called homologous chromosomes.
01:26
And what that means is that they have the same length, but they don’t necessarily do
01:33
the same thing. What does that mean? Chromosome 1 from dad and 1 from mom are going to be
01:37
exactly the same length, centromeres located in the same thing and the genes will be in
01:42
the same spots. So for example this is the gene for blue eyes. This is would be where
01:46
the other gene for blue eyes is. And so you may be asking yourself, well how are they
01:49
different then if everything seems to be the same? Well, this right here could be a recessive
01:56
gene for blue eyes and this could be a dominant gene. It doesn’t give you blue eyes. And so
02:02
those alleles are going to be found on either side. Or this right here could be the gene
02:06
for hitchhiker’s thumb, makes your thumb bend backwards like this, and over here, hitchhiker’s
02:11
thumb here, this could be one for a straight thumb. So you don’t have a hitchhikers thumb.
02:15
And so the chromosome you get from mom, dad and the chromosome you get from mom are homologous
02:21
and they never ever meet, except in meiosis, which will get to in just a second. Okay,
02:25
so what’s our goal in mitosis? The goal in mitosis is to make an exact copy of the nucleus.
02:30
And so this circle right here, bounded by this, is going to be the nuclei and these
02:34
are going to be the chromosomes inside it. Now what does a cell do before it divides?
02:39
A cell will get bigger and then it will copy its DNA. So let me model how that works. When
02:45
it copies the DNA this chromosome will have an exact copy of it made. This occurs during
02:50
the S
02:50
phase. This chromosome will have an exact copy of itself. This one will have an exact
02:55
copy of itself and then this one will make an exact copy of itself. And so maybe this
02:59
is what you’ll remember chromosomes looking like. And so after the
03:03
S phase, you’ll eventually have a cell,
03:06
or a chromosome that’s made an exact duplicate of itself. And so this is a gene here, there’s
03:11
an exact copy on this side. In other words this side and this side are exactly the same.
03:17
In fact those are called sister chromatids at this point. And so this would be at the
03:21
end of the S phase
03:21
What happens after the S phase?
03:23
It goes into another growth phase called
03:25
G2 phase. So
03:27
G2 phase, all of that is part
03:29
of interphase. Now would it look like this during interphase, you wouldn’t see the chromosomes
03:35
during interphase. All of that DNA would be loose within the cell doing its job, it’s
03:40
a job that it normally does. And so you’re really not going to see chromosomes look like
03:43
this until we get to prophase. Okay. What happens during prophase, all that DNA is going
03:48
to coalesce and we’ll actually be able to see it. Okay, what happens next? So that would
03:53
be prophase. The next thing is going to be metaphase. What happens in metaphase is that
03:58
these will, little spindle fibers will attach to these, and they will line up and meet in
04:05
the middle. And so they’re going to line up like this. This chromosome will line up like
04:10
that, this one will line up like that, this one will line up like that. And so the reason
04:16
they line up like that is that the spindle fibers attach to them, and they’re going to
04:23
go from here to here and here to here. And then we’re going to have attachments over
04:29
on the other side as well. Now these spindles are going to go outside of the nuclei, which
04:33
I would draw, but I can’t quite fit that on my screen right here. And so there’s going
04:37
to be a tug on either side by the spindles and that lines these all up in this perfect,
04:43
what is called the metaphase plate that goes right down the middle. So this would be metaphase.
04:47
What happens next, and next is going to be anaphase. And so in anaphase what happens
04:52
is these are going to be pulled to the side as those spindles shorten. They’ll be pulled
04:58
like this. And these will be pulled, this all happens at the same time, and these will
05:04
be pulled like this as well. So they’re going to move to the sides and then the spindle
05:10
fibers are going to start to disappear. And then we’re going to have two brand new nuclei,
05:17
so this would be one nuclei and this would be another nuclei. Now if you look at the
05:22
chromosomes in each of these nuclei, we’ve got one like that, we’ve got one like that,
05:25
we’ve got one like that and we’ve got one like that. They’re exactly the same. In other
05:29
words we have two brand new nuclei and each of them have the same duplicate DNA in each
05:35
one. And that’s the goal of mitosis. The goal of mitosis is to make two exact copies of
05:41
the cell. Okay, so I am going to take a minute and clean this up for just a second and then
05:45
I am going to show you how that differs from meiosis. So in mitosis we are making two nuclei
05:52
and those nuclei are going to be exactly the same, but now let me show you what we do in
05:57
meiosis. In meiosis our goal is not to make duplicate cells but to make different cells.
06:05
We make genetically different cells. And so how does it start? Well it starts the same
06:09
way. The cells going to make a copy of itself, and so it’s going to go through a growth phase,
06:14
so the cell will get larger, it’s then going to duplicate its DNA, so its going to duplicate
06:19
its DNA like that and like that and like that and like that. So this is going to be at the
06:27
end of interphase. So the cell has grown, it’s copied its DNA and then it’s started
06:32
to grow again. But now we’re going to have something different happen. And so in meiosis
06:36
we have two divisions. And so during the first division what will happen is the homologs
06:42
will come together and this forms something called a tetrad. And so we’ll have this homolog
06:48
fit together and this homolog together. In other words the chromosome you get from your
06:53
dad and the chromosome you get from your mom will actually come together and they’ll wrap
06:58
around each other like this. They line up along the middle so this would be metaphase
07:04
I, and they are going to, this actually happens a little bit before this during prophase,
07:09
but what’ll happen is that they’re going to wrap around each other so closely that portions
07:14
of one will switch with portions of another. And portions of this one will switch with
07:22
portions of another one. And so during prophase I they’re going to switch bits of them. Or
07:29
this one right here, it might pop off here, this little gene and it might switch with
07:34
another gene over here. So switch like that. And what that gives us is variability. And
07:41
so this would be during prophase I. We cross over, and so we switch some of the chromosomes.
07:48
Another important thing happens at the next step, which is metaphase I. This might line
07:52
up like this. But it also might line up like this. And this might line up like this, but
08:00
it also might line up like this. And this is called independent assortment as I start
08:05
to loose one of my chromosomes here. In other words, depending on which side they line up
08:09
on, they’re going to be pulled in a different direction. Now why is this important? Well,
08:13
let’s say this right here is the gene for Huntington’s Disease. Huntington’s Disease
08:17
means your going to die when you get to be 40 years old. And if it lines up like this
08:22
and goes to a sperm or an egg over here, then you’re going to get Huntington’s. But if it
08:26
goes over here, you’re not. And so this, when you draw a Punnett Square is where that actual
08:31
genetic 50/50 split takes place. That’s called independent assortment. Okay, so now what
08:36
happens next. We’re going to attach a spindle on each each of these. So there’s going to
08:39
be a spindle here and there’s going to be a spindle here and that spindle is going to
08:44
during Anaphase I, it’s going to pull them apart. So these are going to pull apart. And
08:49
these are going to pull apart to the side. Now what happens next is we form two new nuclei.
08:56
So I’ll do that like this. So we form two new nuclei. And now the next thing we go through
09:05
is, we don’t go through another interphase. So there’s no interphase, but what happens
09:10
next is it will actually line up again. So it’ll meet in the middle and these will meet
09:17
in the middle like this, and then it will simply spilt them in half. So this one will
09:22
go this way, this one will go this way. This one will go this way. This one will go this
09:26
way. The same thing will happen here. These will get split to the side and these will
09:30
get split to the side. So what is that? Well, we now create four cells. So here’s one cell,
09:39
here’s another cell, here’s another cell and here’s another cell. So in meiosis what we
09:43
create are four nuclei. And how is this different from mitosis, well we’ve reduced the numbers
09:49
of chromosomes, there’s two in each one and also we’ve made them all different. And so
09:52
if you look at the color combinations in each of these nuclei, it’s totally different. And
09:57
so what do these become? Well, in males each of these four become a sperm. And so each
10:04
of these will swim off to be a sperm. But in eggs it’s a little bit different. In eggs,
10:14
let me put this one back for just a second. In eggs, one of these will be chosen, let’s
10:18
say this one. One of these will become the chosen one. And all the other ones will form
10:23
something called a polar body. In other words they won’t be used at all. And the reason
10:28
that is in an egg is that there’s a lot of other part to an egg. There’s going to be
10:33
all the mitochondria out here and there’s going to be all of the endoplasmic reticulum
10:39
and there’s going to be the golgi apparatus. And so there’s all this other parts in the
10:43
cell. And so in an egg you’re going to choose just one for the nuclei and that’s going to
10:48
be that one chosen one, we’ll call it. Okay. What happens next in the circle of life? In
10:54
the circle of life the next thing that happens, let’s imagine this doesn’t come from the same
10:58
egg, this sperm is going to fertilize this egg. So this sperm is going to fertilize this
11:04
egg. And so this is going to make a brand 2n=4
11:10
fertilized egg called a zygote. And so what happens next , well now we’ve got
11:18
chromosome 1 that we got from dad. Chromosome 1 that we got from mom. Chromosome 2 that
11:23
we got from dad and chromosome 2 that we got from mom. And so we have a brand new egg.
11:31
And so how do we go from an egg to a brand new organism? Well this one will make an exact
11:35
copy of itself. This’ll make an exact copy of itself and then it will undergo mitotic
11:40
division. And so mitotic division is used to make exact copies of cells. And meiotic
11:46
division is used to make four cells that are genetically different. And so I hope that’s
11:52
helpful.


This post was previously published on YouTube.

Photo credit: Screenshot from video