Video Transcript
In this video, we will learn how to
describe the different stages of mitosis as well as interphase. We will explain the importance of
mitosis in living organisms, and we will discuss the relationship between mitosis
and cancer.
So what exactly is mitosis? You may be surprised to learn that
you’re experiencing lots of mitosis right now. Some scientists have estimated that
around 300 billion cells die and are replaced in the human body every day. And all of these cells are being
replaced by mitosis. Mitosis is a process that’s mainly
used for the growth, maintenance, and repair of multicellular organisms like you and
me.
First, let’s talk a little bit
about growth. Years ago, you started off as a
single cell called a zygote. That zygote divided into two cells,
and then those two cells divided into four. Four became eight, eight became 16,
and so on. This process continued until you
had all of the cells that you needed to be born. Over the years, that same process
has allowed you to grow into who you are today. The trillions of cells that you’re
made of right now all came from that one cell that you started out as. And all of these cells were made by
mitosis.
We typically call the type of cell
division used for growth and repair of organisms mitosis. However, it’s useful to know that
mitosis is specifically the name of the set of steps that allows the nucleus of a
eukaryotic cell to make an exact copy of itself. You will recall that eukaryotic
cells are cells that have a nucleus. All multicellular organisms are
made of eukaryotic cells. Multicellular organisms include
animals, plants, some fungi, and even some protists.
While the cells of these different
types of organisms have different features, one thing that they all have in common
is that their genetic material called DNA is stored and protected inside of an
organelle called a nucleus. The genetic material, or DNA, works
the same way in all living things. DNA holds the code that tells the
cell what to do. DNA is responsible for determining
all of the traits of different organisms.
The DNA inside of the nucleus is
arranged in chromosomes. In sexually reproducing organisms,
the chromosomes usually occur in pairs. This means that the cell actually
has two sets of similar chromosomes, one set that came from one parent and a second
set that came from the other. We call these kinds of cells that
have two sets of chromosomes diploid. Di- is a prefix that means two. Human diploid cells have 46
chromosomes, or 23 pairs. And those chromosomes are
illustrated here.
Your typical house cat is made of
cells with 38 chromosomes, or 19 pairs. The cells of a chicken have 78
chromosomes, or 39 pairs. A common species of tomato plant
has 24 chromosomes within each diploid cell, or 12 pairs. The cells of a common fungus that
we typically call bread mold have 14 chromosomes, or just seven pairs. Organisms that belong to the same
species usually have the same number of chromosomes within the nucleus of each of
their cells. In order to make new cells for
growth or repair, these cells use mitosis to copy their nucleus and the chromosomes
inside so that the new daughter cells are exactly the same or identical to the cells
that they came from.
Mitosis occurs as a series of four
steps. These steps are also known as the
phases of mitosis. We call these four phases prophase,
metaphase, anaphase, and telophase. However, cells are not always
dividing. Most cells spend the majority of
their life cycle in what we call interphase. Inter- is a prefix that means in
between. Interphase is how the cell lives in
between mitosis cycles. During interphase, the cell carries
out its normal functions. Then at a certain point in
interphase, the cell begins to prepare for mitosis. This preparation mainly occurs
inside of the nucleus.
Instead of a human cell that has 46
chromosomes, let’s illustrate this process with the cell of a fruit fly that has
eight chromosomes. In order to prepare for mitosis,
during interphase the chromosomes of the cell make exact copies of themselves. This is called DNA replication. The cell will continue to exist in
interphase with these replicated chromosomes until mitosis begins.
Mitosis begins with prophase. During prophase, several changes
occur within the cell. The chromosomes become more
visible. During interphase, the chromosomes
are stretched out in long, thin strands and are not typically visible under a light
microscope. These long strands are called
chromatin. During prophase, the chromosomes
coil into tightly condensed structures. This makes them easier to see. Each chromosome will be roughly X
shaped.
Each half of the replicated
chromosome is called a sister chromatid. The sister chromatids are exact
copies of each other. In fact, when they separate from
each other, each sister chromatid will be considered an individual chromosome. The sister chromatids were made
when the chromosome was replicated during interphase. The place where the sister
chromatids connect, which makes them look like an X, is called a centromere.
Also during prophase, the nucleus
begins to break down. The nuclear envelope, which is the
membrane surrounding the nucleus, begins to break apart and dissolve. In animal cells like our fruit fly
cell, a special structure called a centrosome begins to form what we call a mitotic
spindle. The mitotic spindle helps to keep
everything organized during mitosis. It’s very important in the phases
to come. Note that not all cells have a
centrosome. In fact, plant cells do not have a
centrosome. Those cells use other types of
structures to move their chromosomes around during mitosis.
The second phase of mitosis is
metaphase. During metaphase, the mitotic
spindle attaches to the centromere of each of the chromosomes. The mitotic spindle helps the
chromosomes to line up along the middle of the cell. We call this place where the
chromosomes line up the metaphasic plate. We also sometimes refer to this
location as the equator of the cell. The word equator means middle line,
just like the equator that’s the imaginary middle line around our planet.
The third phase of mitosis is
called anaphase. During anaphase, the sister
chromatids separate from each other. The mitotic spindle pulls the
chromatids towards opposite ends of the cell. Similar to how we describe the cell
as having an equator, we sometimes refer to the opposite ends of the cell as poles,
sort of like the North and South Pole on our planet. Remember that each of the sister
chromatids is an exact copy of the other. So at the end of this step, each
side of the cell has a full identical set of chromosomes.
The fourth and last phase of
mitosis is called telophase. During telophase, two new nuclei
begin to form. The chromosomes unwind and become
less visible. The nucleus of the cell has
successfully made an exact copy of itself. Now the cytoplasm is ready to
divide in two. After mitosis, the cell and its
contents split into two new cells. The phases of mitosis describe how
one cell can split into two identical daughter cells.
Mitosis is generally used for the
growth, maintenance, and repair of multicellular organisms. As organisms become larger and
develop over their lifespan, mitosis creates the new cells that they need. When old cells wear out and die,
mitosis replaces them with new ones. And when organisms get hurt,
mitosis creates new cells to repair the damage.
Another function of mitosis is a
certain kind of reproduction. Mitosis is also used for asexual
reproduction in some simple multicellular organisms. For example, a hydra is a type of
very small, simple animal that lives in fresh water. During reproduction by budding, a
smaller hydra grows from the side of its parent organism. It starts off as a small clump of
cells. And over time, it develops into a
new individual. Once the offspring is large enough
to survive on its own, it separates from its parent. And the offspring begins its life
elsewhere. Because this offspring is the
result of mitosis, it will be genetically identical to the parent that it came
from.
Mitosis can be used in asexual
reproduction, but be careful not to confuse it with binary fission. Mitosis primarily occurs in
multicellular organisms. Although some unicellular
organisms, like yeast, also reproduce by mitosis. Binary fission is the term we
generally use to describe asexual reproduction in unicellular organisms.
Before we wrap up our lesson, let’s
examine how mitosis plays a role in a common type of disease.
We mentioned earlier that most
cells typically spend the majority of their life cycle in interphase. Interphase occurs between mitosis
cycles, and it’s when the cells carry out their normal function. Sometimes something will happen to
a cell that will make it skip over many of the important parts of interphase. In these cells, the cell cycle has
been broken. This causes these cells to repeat
mitosis without stopping. And as a result, they grow and
multiply rapidly. When this occurs, the illness that
it causes is called cancer.
Cancer cells grow and divide much
more rapidly than normal cells. They usually first form a mass that
we call a tumor. Sometimes, these rapidly dividing
cells break away from the tumor and move around the body. When this happens, we say that the
cancer has metastasized. The moving cancer cells can grow
into new tumors in other places in the body and cause serious health issues. Since cancer is caused by the
body’s own cells multiplying out of control, it can be very difficult to treat.
Let’s wrap up our lesson by taking
a moment to review what we’ve learned about mitosis. In this video, we learned about
mitosis. We learned that mitosis produces
genetically identical daughter cells. We learned that mitosis functions
in the growth, maintenance, and repair of multicellular eukaryotic organisms. And we learned that mitosis also
plays a role in some types of asexual reproduction. We learned about the four phases of
mitosis. Those phases are prophase,
metaphase, anaphase, and telophase.
