The Cell Cycle
Cells are created, they grow, they perform their
specific jobs, and they multiply. These events are
called the “cell cycle.” The cell cycle is simply the
events that take place in the life of a cell. In the picture
to the right, you can see the cell cycle is composed of
five stages: interphase, prophase, metaphase,
anaphase, and telophase. Notice in the picture to the
right that mitosis is only composed of four stages, as
interphase is not a stage of mitosis. So what is mitosis?
Mitosis is the process of
how diploid cells divide. Recall that diploid cells are
cells with paired sets of chromosomes. Humans have
23 pairs of chromosomes, so the human diploid
chromosome number is 46. In the picture to the left,
you can see a karyotype of a diploid cell with 46
chromosomes. Do you notice how the chromosomes
are arranged in pairs? That’s because these
chromosomes come from a diploid cell.
All cells will eventually get old and die. Therefore, cells
must divide in order to replace old and dying cells. In
fact, we can see evidence of cell death if we just look in
the family room of our home. We have all seen dust
accumulate on our coffee table, on shelves, and on our
television. So what is dust? We all lose skin cells over time. And when our skin cells
die, the cells simply fall off our bodies and eventually settle on the ground, or the
coffee table, or the television. Dust is dead
skin cells.
So what is the purpose of mitosis? Mitosis
makes new diploid cells to replace the cells
that die. Understanding mitosis is very
important in our studies of cells and cell
biology. This packet will discuss the various
stages of the cell cycle and mitosis.
Interphase
During interphase, cells perform their specific functions. Red
blood cells carry and transport oxygen. Nerve cells send electrical
signals to and from the brain. Stomach cells release digestive
enzymes to break down food you have eaten. Intestine cells
absorb nutrients into the blood. Cells spend the majority of their
lives in the interphase stage. That means that cells spend the
majority of their lives doing their specific jobs. However, cells will
also begin to prepare for mitosis during the interphase stage.
Interphase can be sub-divided into three stages:
The first stage of interphase is called the G1 stage. The “G” stands for “growth”
because the cell will increase in size during the G1 stage. The cell grows because it is
getting ready for mitosis. The purpose of mitosis is to split one cell into two cells.
Therefore it should make sense for the cell to grow larger before it splits in half.
During the G1 stage, the cell will also begin to create more organelles. More
lysosomes are created. More vacuoles. More smooth ER’s. More ribosomes. More
Golgi bodies. Keep in mind the cell will eventually split into two cells. Therefore it
should make sense for the cell to create more parts. Both cells will need their own
supply of parts.
The second stage of interphase is called
the S stage. The “S” stands for “synthesis”.
The word synthesis means “to make.”
Two pieces of
During the S stage of interphase, the cell
DNA forming
makes extra amounts of DNA. During this
process the DNA double helix is separated
and two new pieces of DNA are created. In
the picture to the left, you can see the
DNA splitting apart and two pieces of DNA forming. Eventually one cell will split into
two cells, so it should make sense to make more DNA. Each cell will need a copy of
DNA for itself.
The third stage of interphase is called the G2 stage. The “G”
again stands for “growth.” For the second time, the cell will
grow in size and become larger. This is one of the final
preparations made by the cell prior to the start of mitosis. The
picture to the right shows a cell in the interphase stage.
Prophase
Metaphase
Now that the cell has prepped for mitosis, mitosis can begin. Remember that the point of
mitosis is to split one cell into two cells. The first stage of mitosis is called prophase. Several
key events take place during prophase.
In the previous stage of the cell cycle called prophase, spindle fibers were created.
Spindle fibers are long, thin threads of protein that grow from organelles called
centrioles. In the picture below, you can see a diagram of a cell with the centrioles
highlighted.
During metaphase, those spindle fibers will attach
themselves to the centromere location on every
chromosome. If you recall, humans have 46
chromosomes in every cell. So the spindle fibers
will attach themselves to the centromere location
on all 46 chromosomes. In the picture below you
can see a diagram of a chromosome. If you recall,
the centromere is the location where the two
halves of a
chromosome overlap one another. The two
halves of a chromosome are called
“chromatids.”
At the start of prophase, the cell’s DNA is in a long, loose form
called chromatin. An analogy would be a long strand of
spaghetti. During prophase, the loose chromatin begins to coil
and tighten into easily movable structures called chromosomes.
In the picture to the right, you can see the X shaped
chromosome is made from a long, loose strand of DNA called
chromatin. So what is the point of condensing
the DNA into chromosomes? During mitosis,
the cell must move its DNA from one cell to
another. As an analogy, while packing clothing for a vacation, it makes
sense to condense all your clothing into one easily moveable suitcase.
When cells divide, they will package their DNA into easily movable
packages called chromosomes. During prophase, human DNA condenses
into 46 moveable chromosomes.
The purpose of the nucleus is to store and protect DNA.
But during prophase, the nucleus is simply in the way.
During prophase, the nucleus of the cell will dissolve.
There are enzymes within the cytoplasm that actively
break apart the nucleus. In the picture to the right you can
see the nucleus of the cell is breaking apart.
Nucleus breaking apart
centrioles
centrioles
Spindle
fibers
Spindle
fibers
Another key step of
prophase is the formation of spindle fibers. Spindle fibers
are created by organelles called centrioles. As we
progress through the stages of mitosis, we will learn the
function of spindle fibers. In the picture to the left, you
can see the centrioles at
opposite ends of the cell
and spindle fibers
growing from the
centrioles. The picture
to the right shows a cell
in the prophase stage of
mitosis.
Once the spindle fibers attach to the chromosomes, the spindle fibers engage in a
“tug-of-war” contest. During metaphase, the spindle fibers pull and tug on the
chromosomes until all chromosomes are aligned in the middle of the cell, called the
equator. In the drawing below, you can see the X
shaped chromosomes are aligned in the middle, or
equator, of the cell. This is a drawing of a cell in the
metaphase stage.
Chromosomes aligned in the middle of the cell
Chromosomes aligned in the middle of the cell
Anaphase
Telophase
In the previous stage, Metaphase, the chromosomes were pulled to the center
(equator) of the cell by the spindle fibers. In human cells, there will be 46
chromosomes aligned along the cell’s equator. In the picture below, the cell appears
to be in the metaphase stage.
Before we proceed, let’s remember,
chromosomes are tightly coiled pieces
of DNA. In the diagram to the right, you
can see that every chromosome is
made from two halves called
chromatids.
Now that the chromosomes are aligned at the cell’s equator, the
spindle fibers are going to continue their tug-of-war battle. During anaphase, the
spindle fibers are going to pull at the chromosomes in
opposite directions. In the picture to the left, you can see this
cell has 4 chromosomes aligned in
the cell’s equator. But in the picture
to the right, you can see the 4
chromosomes are being ripped
apart as the spindle fibers continue
their tug-of-war contest. In the
picture to the right, you can see 4 chromatids being pulled
to the left of the cell and 4 chromatids pulled to the right of
the cell. In human cells, 46 chromosomes are aligned in the
middle of cells during metaphase. Therefore during anaphase in human cells, 46
chromatids would be pulled to the left of the cell, while 46 chromatids would be
pulled to the right of the cell.
Anaphase is an easy stage to recognize under the
microscope. In the picture to the right, you can see a
cell in the process of anaphase. In the picture to the
right, you can see half the chromatids being pulled to
the left and the other half being pulled to the right.
Chromatids separating
In the previous stage of mitosis called anaphase, the
chromosomes of a cell are ripped apart. Half the chromatids are
pulled to the left and the other half are pulled to the right. The
picture to the right shows anaphase.
Now that anaphase is complete, the cell will enter the final stage of mitosis called
telophase. Remember the purpose of mitosis is to split
one cell into two cells. Therefore, by the end of telophase,
two complete cells should exist. There are many events
that happen during telophase. One key moment of
telophase is the return of the nucleus. The nucleus’ job is
to protect the DNA. Now that mitosis is nearly finished,
the nucleus will reform in order to protect the DNA of the
Nuclei reforming
two cells. In the picture to the left, you can see two nuclei
forming. Eventually, the cell will split in half and each cell will contain one nucleus.
Another major event that happens during telophase is
the division of the cytoplasm. This process is called
cytokinesis. In the picture to the right, you can see the
cell is pinching inward. Cytokinesis helps to physically
break apart the left cell from the right cell.
One of the final events of telophase is the return of DNA back into the loose strands
called chromatin. If you recall during anaphase, 46 chromatids were pulled to the left
of the cell. Those 46 chromatids will unwind into
46 loose strands of chromatin. If you recall, 46
chromatids were pulled to the right of the cell.
Those 46 chromatids will unwind into 46 loose
strands of chromatin. In the picture to the left,
you can see that a
chromatid is unwinding
into a long, loose strand of chromatin. In the picture to the
right, you can see a cell in the telophase stage. Notice how the
cell is virtually split into two complete cells. This is the final
stage of the cell cycle.
Why should we care about Mitosis?
The #2 cause of human death on Earth is cancer. Cancer is a disease where cells are
constantly dividing by the process of mitosis. Normally, when a cell goes through
mitosis to make new cells the process will end with telophase. But in cancer cells
there are errors that prevent the shutoff of mitosis. Cancer cells are basically stuck in
mitosis. Therefore our understanding of mitosis is
important in the battle to save millions of lives per
year. Chances are that you personally know
someone who has battled cancer in their lives.
If a person has cancer, the cancer cells are
constantly dividing by mitosis. As the cancer cells
multiply and multiply and multiply, eventually a
lump of cancer cells begins to form. This lump of
cancer cells is called a tumor. Tumors can be
hazardous for a variety of reasons. One reason tumors can be hazardous is because
often cancer cells can metastasize. This simply means that a few cancer cells can
break away from the tumor and travel in your blood stream to another part of your
body. The spreading of cancer cells from one area to another can spread the cancer
cells around the body. This makes treatment very difficult.
Treating cancer can be very difficult
because cells are microscopic. The
average human body is made from
approximately 100 trillion cells.
There is no easy way to find and kill
all the cancer cells. When cancer
cells are found, they can be
targeted with X-ray radiation as a
way to kill them. However,
radiation also kills normal cells as
well as cancer cells. If you have ever had a relative undergo radiation treatment, you
may know this causes the person to get very sick. This is because the radiation
treatment is killing the person’s healthy cells as well as the cancer cells. In the picture
to the left, you can see radiation is being applied to a specific area of a person who
has cancer. The X-ray radiation will hopefully kill the cancer cells and shrink the
tumor. As our knowledge of cells and mitosis grows, this may lead to new treatments
in the battle of cancer. Knowledge of mitosis may one day save your life.