Hi. It's Mr. Andersen and welcome to Biology Essentials video 28. This is on
the cell cycle, mitosis and meiosis. In other words how we go from one cell to all the cells,
the trillions of cells inside our body. Meiosis is important because that's how we make sex
cells. Now let me digress a little bit. This is a whipped tail lizard from the desert southwest
and what's interesting about this is that it's a female lizard and when it wants to
make more lizards it will simply use mitosis to make an exact copy of a cell inside its
body. It's called parthenogenesis or virgin birth and it will make a brand new, a number
of baby lizards and they're all females. So they don't have males. It's rare to not have
males. It's rare to not have meiosis and the reason why is that all the whipped tail lizards
are genetically the same. But it works. And if you live in a fairly stable environment
it tends to work. Now we're not going to go into the specifics of mitosis and meiosis.
You can look, I've got videos on each of those that talks about the different phases. What
I want to talk about this is in general what do mitosis and meiosis do and how does the
cell cycle work and how is it controlled. And so a diploid cell is going to be a typical
human cell, or a typical cell inside an organism. It's going to be 2n and what that means is
it has 2 complete sets of chromosomes. So for example in humans 2n=46. That means we
have 23 pair of chromosomes. And so the goal of the cell cycle in mitosis is to make a
copy of that cell in other words to make a diploid cell. That diploid cell can enter
into the cell cycle again, make more cells and more cells and more cells. And so the
way that we make new cells in our body or replace cells that are damaged is mitosis.
In meiosis we're going to take a normal diploid cell and we're actually going to make a haploid
cell, we're going to make sperm and egg. So in humans n now equals 23. Now if we were
to just stop there, we wouldn't have diploid cells anymore, but fertilization where egg
meets sperm is going to combine those two cells to make a diploid cell and now that
diploid cell can enter into the cell cycle again. And so we've got kind of two loops.
We've got the mitosis loop and the meiosis loop. Mitosis is used to make all the cells
in our body, meiosis just makes gametes, or sex cells. I also in this video want to talk
about how we control the creation of diploid cells. So it may be better to come from here
and how we use cyclin, an example of that would be mitosis promoting factor (MPF) to
control the cell cycle in where it is and where it's headed next. Now if you're talking
about cell cycle the best place to watch, or start is with videos of actually cells
dividing. And so this is a cell undergoing division. So we start with one cell and it
makes an exact copy of itself. Now when you're watching it, let me go back a second, on these
first two videos what you don't see is everything that actually happens before the cell makes
a copy of itself. In other words before the cell is actually able to divide, it has to
duplicate all of the DNA and all of the machinery of the cell. This last one we're looking actually
inside the cell itself. And so the cell division actually has two parts to it. Part one is
going to be the division of the nucleus, and we call that in general mitosis. And then
after we've divided the nucleus you'll see the chromosomes actually separate here. Then
we actually have the division of the rest of it and that's called cytokinesis. Cytokinesis
is the break apart of all of the other parts of the cell, so the machinery of the cell,
the mitochondria, cytoplasm, things like that. And so let me see if I can start that up.
So first of all, there we go, first we have the division of the nucleus. You can see the
chromosomes pulling apart and then you have cytokinesis or the division of the cell itself.
And so all mitosis is is one cell forming two cells. And those two cells are identical
to that first cell. And that's how we go from that first fertilized egg inside us to the
trillion of cells we have inside an adult body. So when you're replacing cells in your
body you're doing it through mitosis. So let's look at the cell cycle. And so what happens
is you'll have a cell enter into this. So a cell's going to look like this. It's going
to enter into the cell cycle as one cell. Let me make a little better arrow. And then
it's eventually going to exit out as two cells, each of those cells remember could go back
into the cell cycle and so this is how we make all the cells in our body. Now I've heard
that a lot of the dust in a room actually are dead skin cells. And this is what skin
looks like. So skin is going to be creating new cells. They are going to migrate up to
the surface and then we're eventually going to lose skin cells at the top. But we keep
replacing those cells and to do that we use the cell cycle. So let's look at the parts
of the cell cycle. If we start first with that cell entering in, the first thing it'll
do is it'll actually enter into the G1phase. G1 phase the cell is actually going to grow.
It's going to get bigger and bigger and bigger and bigger. We then enter into the S phase.
During the S phase, we're going to actually, using DNA replication, we're going to copy
all of the DNA inside a cell. It then goes into the G2 or the growth 2 phase where it
continues to grow and gets ready for division of the actual cell. And so G1, S and G2 are
all part of what's called Interphase. And if you look at a cell, it's generally in interphase.
It's growing, it's copying its DNA, it's growing or maybe it's just working, it's doing what
a normal cell does. And so most of the life of a cell is in interphase. It's in these
three, G1, S and G2 and the actual copying of the nucleus and copying of the cell, this
mitotic phase is actually going to be really, really small. If it never divides again, it'll
actually stall out in something called the G0 phase. And so we've got cells inside our
body, you're heard of this maybe, cells of the central nervous system, muscle cells for
example, that never copy themselves during your whole life time. They're in what's called
the G0 phase or they're just waiting. And they're not going to make divisions. And so
how does a cell know when it's time to divide? And when it should go on and when it shouldn't?
The best analogy I can think of is it works kind of like an hour glass clock. And so there
are little proteins inside a cell and as those proteins accumulate throughout the life of
the cell eventually you get a critical number of these proteins at the bottom, and once
we have enough of those then it actually tells the cell to advance to the next stage. And
those proteins are called cyclins. And so let's look at what cyclins look like. A cyclin,
I'll represent it here, it's simply a protein. But if we look here at the G1 phase, the S
phase, the G2 phase and mitosis there's a set of cyclins or a set of these different
types of proteins and what they're going to do is they're going to build up. And so cyclin
A and B are ones that I'm really familiar with. Cyclin A will actually build up during
the S phase and then it'll drop off right as the cell divides, as it goes into this
mitotic phase. And so cyclins will actually accumulate and so those are like the sands
through an hourglass. They're going to get more and more and more and more cyclins as
a cell goes through the cell cycle. Now the other chemical that I want to talk about is
something called CDK. CDK is simply a cyclin dependent kinase and a kinase is simply going
to be a chemical that can speed up actions within a cell. And so CDKs are found in all
living organisms. And you can actually take CDKs from a yeast and put them in our cells
and they work just as fine. So they show homology through evolution. And so cyclin dependent
kinase, if you look at their name, are simply dependent upon cyclin. And so I made a little
animation of how that works. And so a typical cell in your body is going to have a bunch
of CDKs in it or cyclin dependent kinases and so we could say this is like right here.
But throughout the life of the cell, the cell is going to start building up and accumulating
larger amounts of cyclin. So the amount of cyclin is going to get larger and larger and
larger. So eventually what happens is the cyclin is going to fit into the cyclin dependent
kinases. Now we have an activated CDK cyclin complex. What does that mean? We have something,
a protein, that's able to do things. So now you can think of we've like mustered this
army and now the army is ready to do something. And so what does it do? Well, we're right
here, so we're just about to enter into the M phase, so we're just about to do mitosis
and so what these cyclin dependent kinases do is they act on the cell itself. A specific
type of CDK is called the mitosis promoting factor or MPF and what that does is when it
builds up enough of these cyclin dependent kinases, they're actually going to work on
the cell. So one thing they'll do is they'll actually break apart the nucleus. So we're
able to start dividing that cell. Another thing that CDKs will do is they'll actually
work on the microtubules that build this spindle. And so all of these together will work on
pushing that cell into the mitotic phase or into this next step of the cell cycle. The
neat thing about each of these is that after they've actually done that they'll actually
gobble themselves up. They'll disappear and then the whole cycle begins over again. Okay.
So if we kind of talk big picture about what happens in the cell cycle, a typical cell
right here is going to be, let's say this is a typical cell, a typical cell right here
is actually going to be 2n, it's diploid. You have one chromosome from mom, one chromosome
from dad. So in this case it's going to be 2n=2. It's then going to duplicate itself.
So during the S phase it's going to make copies of itself and so at this point right here
we'd actually have a 4n cell. It's made copies of that. And at this point we can either take
the path of mitosis or the path of meiosis. And so in the path of mitosis, that'll simply
split in half and now we'll have two 2n cells. And if you look at these two 2n cells they're
exactly the same as that first cell. So this is what's happening to the chromosomes. If
we look at that 4n cell as it goes into meiosis it'll actually line up. It will split in half
and then it will split in half again. And so what you have is actually n cells. Those
are called haploid cells and we have four of those in meiosis. And so the cell cycle
will take a diploid cell make two diploid cells in mitosis or make four haploid cells
in meiosis. That got a little messy so let's look at it in a little more detail. So mitosis
is how we replace cells in the body. Cells that are broken, cells that are broken down,
cells that we need to replace, we do that through mitosis. Or when we want to grow how
do we go from a very small organism to a very large organism? It's just making more cells.
And so this would be a real, to make it simple, we're going to start with a simple cell. This
is going to be a 2n, it's diploid, but 2n=2. So this would be that first parent cell. It
will then duplicate all of the DNA. So now we have this characteristic shape. It has
two sides to it and this side and that side are copies. That's what happened during the
S phase. They'll then meet in the middle. They'll divide in half, so this would be our
mitosis phase and now we have two diploid cells. And if you look at those cells, they're
exactly the same as the original cell. And each of those are 2n=2. So that's mitosis.
What happens to these 2n cells. Well, they can enter into the cell cycle again and it
goes over and over and over again. In meiosis what happens is a little more detailed. Remember
meiosis only deals with making sex cells in reproduction. And so in this case to make
it, to show you meiosis I had to increase the number of cells, or chromosomes. In this
case 2n=4. In other words we have two chromosomes from mom, two chromosomes from dad. So they'll
copy themselves, so now we actually have a 4n cell at this point, but since we're doing
meiosis, this is where actually crossing over occurs. And so parts of this chromosome will
swap places with this chromosome and vice versa. What that gives us is variability in
all of the sperm and the egg. They'll divide in half. Now we have 2n cells and then we'll
eventually have n cells equals 2. And so we started with 2n=4 and now we have n=2. Each
of these four things in a male become a sperm and in a female one of them will become an
egg and the other ones will actually form what are called polar bodies.
And so what do we get after meiosis? Well we get sperm and we get egg. And we can then
go from these original n=2 to a 2n=4 zygote. And so now we're back to a diploid cell and
that diploid cell can enter into the cell cycle. This is a zygote to make more cells
and eventually makes sex cells to make the next generation. So that's cell cycle, mitosis,
meiosis and I hope that's helpful.