Mitosis & Meisos Simulation - Cell : Cycle & Cell Division Video Lecture - Class 11

[Music]
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 nut from your mother
now we would call this chromosome one
and that's because it's the longest one
if we have two other chromosomes this
would be chromosome one and this would
be chromosome two now in us we have 22
what are called autosomes and then we
have sex chromosomes but the way
chromosomes are named are just their
number one is the longest two is the
next longest three 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
where they're connected and then each of
these beads represents a gene now here
we just have a few genes but in a human
cell we're going to have thousands of
genes on each of the chromosomes and so
this is a very simplified model you can
imagine if this is a human chromosome my
whole board it'd be filled with these
chromosomes and so this is a very simple
model so this one right here from dad
and this one for 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 one
from dad and one from mom are going to
be exactly the same length centromere is
located in the same thing and the genes
will be in the same spot so for example
this is the gene for blue eyes this
would be where the other gene for blue
eyes is and so you may be asking
yourself well how are they different
than 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 so
Hitchhiker's thumb here this could be
one for a straight thumb so you don't
have a Hitchhiker's thumb and so the
chromosome you get from mom dad and the
chromosome you get from mom
or homologous and they never ever meet
except in meiosis which we'll 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 as 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 remember chromosomes looking
like and so after the S phase you'll
eventually have a cell or 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 what 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 this is 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 alignment like that
and this one will line up
that just want to line up like that and
so the reason they line up like that is
that the spindle fiber is attached to
them and they're going to go from here
to here and here to here and then we're
going to have a tap image 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 to next it
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 short and 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 I've got one like that 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 goal of
mitosis is to make two exact copies of
the cell okay so I'm going to take a
minute and clean this up for just a
second and then I'm going to show you
how that differs from meiosis so in
mitosis we're 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 actually
make different cells we make genetically
different cells and so how does it start
well it starts the same way 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 it's going to
duplicate its DNA like that and like
that like that and like that so this is
going to be at the end of interphase so
the cell is grown it's copied its DNA
and then it 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
homologous will come together and this
forms something called a tetrad and so
we'll have this homolog get together in
this homolog together in other words the
chromosome that you get from your dad
and the chromosome that you get here
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 in metaphase one
and they are going to this actually
happens a little before this in during
prophase but what will 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 one 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 it will switch like that and
what that gives us is variability and so
this would be during prophase one we
cross over and so we switch some of the
chromosomes
another important thing happens at the
next step which is metaphase one 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
lose 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 you're going
to die when you get to be a 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 the independent
Stormin okay so now what happens next
we're going to attach a spindle on 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 one'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 we
don't go through another interphase and
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'll simply split 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
so here's another cell here's another
cell and here's another cell so in
meiosis what we create our 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 others part two 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
on 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 and so this sperm is going to
fertilize this egg and so this is going
to make a brand new 2n equals four
fertilized egg called a zygote and so
what happens next well now we've got
chromosome one that we got from dad
chromosome one that we got from mom
chromosome two that we got from dad
chromosome two that we got from mom and
so we have a front 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 will
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 mitotic division is
used to make four cells that are
genetically different and so I hope
that's helpful