Welcome once again as we discuss some
topics on anatomy and physiology and
today we are going to discuss the cell
cycle mitosis and cytokinesis. So what is
the cell cycle? It's actually the cycle
from one cell division to the next cell
division. We're going to look here. So
this cell has just divided into two new
cells. There's going to be a cycle where
this cell is going to go through some
phases until it is ready to divide once
again. So this is the cell cycle.
What are those phases? We are going to
look at them in detail. But to start with
there's the G1, the first gap. The S, the
synthesis. G2, the second gap. And then M
is actually the mitosis where the actual
division takes place. So we're going to
look at each one of these and learn the
most important event. We are looking at
G1. We call it Gap 1 or the First Gap
Phase. This is the interval between the
cell division and the DNA
replication. So we haven't replicated the
DNA yet so what is the cell doing?
It's actually growing, maturing and
multiplying all its contents. So now the
cell is going to get ready to divide
into two cells. It has to double up its
cytoplasm. It has to double up all its
organelles, the endoplasmic reticulum, the
Golgi and all of those organelles except
the chromosomes. They will replicate
later on. When it comes to the synthesis
phase or the S phase this is where the
centrioles will replicate. I will make
another copy and the chromosomes will
replicate. They will make another copy.
Let's look at our figure again. This is
the G1 phase. When we come to the S phase,
the synthesis, each chromosome right here
made another copy of itself. This
chromosome made another copy of itself.
So the original 46 chromosomes we're
going to have them each one of it
replicating so that when we divide at
the end we have 46 for each new cell. The
third phase is the G2 phase. This is the
second gap. This is after the DNA has
replicated and the cell is not dividing
yet. So it's really getting ready and
it's making all the enzymes important
for cell division. The M phase is the
actual mitotic phase. This is where the
cell will divide into two. Instead of one
nucleus we will have two nuclei. The
cytoplasm will divide and we will have
two daughter cells that are going to be
formed. So we're going to look at the M
phase, the mitosis phase. We're going to
look again. It is in different stages. We
are going to look at what happens in
each of those stages. So mitosis is the
cell division. It's actually the division
of the nucleus -- we're going to look at
this in a minute -- and cell division will
occur in all body cells except the
reproductive cells. So if we're talking
about muscle cells, liver cells, bone cells,
skin cells, all of those cells in the body
are going to divide by mitosis except
the sperms and the eggs. They're going to
divide by a completely different process.
This is called meiosis and we will look
at it in a different presentation. Now
why does mitosis occur? First,
because of growth. So if there is a baby
or a young child, this child has to grow
to mature size. So the baby liver has to
grow to an adult liver. How does this
happen?
One liver cell will divide to make two
new ones. Each of those two new ones will
divide again. So for growth the same for
the bone is going to divide by mitosis
to reach an adult size and so forth. For
replacement. So cells die out. The skin
is made up of several layers and the
topmost layer eventually dies and we
don't know it but every time we sleep
and we rub against this those bed covers
some of those dead cells slough away and
we need to make new cells. So the deeper
layers of the skin will divide by
mitosis for the skin to grow back. In
repair, so if there is a cut or if there
is a surgical incision, how do cells re --
how do tissues repair themselves? By a
process of mitosis where they grow back.
So mitosis is extremely important for
our body. These are the phases of mitosis.
We are going to look at them. Prophase.
Metaphase. Anaphase and Telophase. Let's
look at what happens in each of those
stages. We need to memorize the different
events and these are the most important.
In prophase, the chromatin is no longer
in the shape of fibers. It's going to
start to condense and we're going to see
actual chromosomes. The centrioles are
going to move to opposite poles and the
centrioles are going to create the
mitotic spindle. The nuclear membrane
is going to start to disappear. In
metaphase, the chromosomes that formed
over here are now going to line up along
the cell's equator. We're going to look
at the diagram and come back to this. So
this is where prophase starts. Let me
take you through here. This is the
interface. This is still at the second
gap stage. If I look at this cell we have
duplicated the number of chromosomes but
they are in the shape of chromatin
fibers. We have two of the centrioles.
In prophase, the chromatin will start to
condense and take the specific shape of
chromosomes. The centrioles are going to
start to move away to different poles
of the cell and they will create the
mitotic spindle. In metaphase those
chromosomes are going to align to the
middle of the cell. We call this the cell
equator. We can see already the
centrioles have moved to the opposite
poles and the centrioles have created
the mitotic spindle which helps the
alignment of those chromosomes. In
anaphase, the centromeres which are
holding one pair of chromosomes together --
are going to break and split and now
those chromosomes are going to move to
opposite sides of the cell. In telophase
and cytokinesis, the cytoplasm is going
to divide. So cytokinesis is division of
the cytoplasm. How does this happen? We're
going to see a cleavage furrow starting
to appear and two new cells are going to
start forming. Each one is going to have
it's nuclear membrane. So this is
anaphase. The two chromosomes are going
to move to different poles. They split --
the centromeres have split-- this is the
telophase and the cytokinesis. This is
the furrow, the cleavage furrow, it
indicates where the cytoplasm is
actually going to divide. We can see that
a new nuclear membrane is forming for
this cell, another for that cell and
eventually, two new daughter cells have
been produced. A very important concept
to remember in mitosis. We started with a
parent cell. This cell is going to divide
through those four stages and form two
new daughter cells. These two daughter
cells are identical to the parent cell
in every aspect. So I have 46 chromosomes
over here. Each of these will have 46
chromosomes. They will be totally and
completely identical. As I said a liver
cell wants to make two more liver cells,
a bone cell will make two bone cells and
so forth. We're going to end up by
talking about a few facts on mitosis. Now
remember, this occurs in all body cells
except reproductive gamete cells. But I
have one cell in the body that does not
have a nucleus and this cell is the
mature red blood cell. So mature RBC's
cannot undergo mitosis. They don't have a
nucleus. They will age and die away and
then the bone marrow is going to produce
newer cells.
How many times does a cell divide? Most
body cells divide between 40 and 60
times before they eventually die. How
long is the cell cycle? Most body cells
on average take about 24 hours to divide.
There are few very rapidly dividing
cells that might have a shorter time
like 10 hours or so, but this is the
normal average of a cell cycle. And again
thank you for joining us in the
presentation of cell cycles mitosis and
cytokinesis.