The cell is the fundamental unit of
life. It is the smallest structure of the body capable
of performing all of the processes that define life.
Each of the organs in the body, such as the lung, breast,
colon, and brain, consists of specialized cells that
carry out the organ's functions, for example, the transportation
of oxygen, digestion of nutrients, excretion of waste
materials, locomotion, reproduction, thinking, etc.
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To assure the proper performance of each organ, worn out
or injured cells must be replaced, and particular types of
cells must increase in response to environmental changes.
For example, the bone marrow increases its production of oxygen-carrying
red blood cells sevenfold or greater in response to bleeding
or high altitude. Certain white blood cells are produced more
rapidly during an infection. Similarly, the liver or endocrine
organs frequently respond to injury by regenerating damaged
cells.
As stated in the previous section, reproduction of cells
is a process of cell division. The division of normal cells
is a highly regulated process. The cell growth, inheritance
and containment is controlled by its DNA (deoxyribonucleic
acid).
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DNA is a highly complex molecule manufactured in
the cell nucleus and serves as the cell's "brain."
DNA is the blueprint for everything the cell does.
In a human cell, the DNA is arranged in 46 distinct
sections called chromosomes. They are arranged in
pairs, 23 chromosomes from each biological parent.
Together, the 46 chromosomes contain more than 100,000
genes. A gene is a segment of DNA that determines
the structure of a protein, which is needed for development
and growth as well as carrying out vital chemical
functions in the body. Like the chromosomes, genes
are arranged in pairs - one gene from the mother and
one from the father.
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Each gene occupies a specific location on a chromosome. Through
a number of biochemical steps, each gene tells a cell to make
a different protein. Some genes instruct the cell to manufacture
structural proteins, which serve as building blocks. Other
genes tell the cell to produce hormones, growth factors or
cytokines, which exit the cell and communicate with other
cells. Still other genes tell the cell to produce regulatory
proteins that control the function of other proteins or tell
other genes when to turn "on" or "off." When a gene is turned
on, it manufactures another complex molecule called ribonucleic
acid (RNA), which contains all the information the cell needs
to make new proteins.
Cells divide only when they receive the proper signals from
growth factors that circulate in the bloodstream or from a
cell they directly contact. For example, if a person loses
blood, a growth factor called erythropoietin which is produced
in the kidneys circulates in the bloodstream and tells the
bone marrow to manufacture more blood cells.
When a cell receives the message to divide, it goes through
the
cell cycle (Get
Acrobat
Reader to view and print the cell cycle.), which
includes several phases for the division to be completed.
Checkpoints along each step of the process make sure that
everything goes the way it should.
Many processes are involved in cell reproduction and all
these processes have to take place correctly for a cell to
divide properly. If anything goes wrong during this complicated
process, a cell may become cancerous.
A cancer cell is a cell that grows out
of control. Unlike normal cells, cancer cells ignore
signals to stop dividing, to specialize, or to die and
be shed. Growing in an uncontrollable manner and unable
to recognize its own natural boundary, the cancer cells
may spread to areas of the body where they do not belong.
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In a cancer cell, several genes change (mutate) and the cell
becomes defective. There are two general types of gene mutations.
One type, dominant mutation, is caused by an abnormality in
one gene in a pair. An example is a mutated gene that produces
a defective protein that causes the growth-factor receptor
on a cell's surface to be constantly "on" when, in fact, no
growth factor is present. The result is that the cell receives
a constant message to divide. This dominant "gain of
function gene" is often called an oncogene (onco = cancer).
The second general type of mutation, recessive mutation,
is characterized by both genes in the pair being damaged.
For example, a normal gene called p53 produces a protein that
turns "off" the cell cycle and thus helps to control cell
growth. The primary function of the p53 gene is to repair
or destroy defective cells, thereby controlling potential
cancerous cells. This type of gene is called an anti-oncogene
or tumor suppressor gene. If only one p53 gene in the pair
is mutated, the other gene will still be able to control the
cell cycle. However, if both genes are mutated, the "off"
switch is lost, and the cell division is no longer under control.
Abnormal cell division, can occur either when active oncogenes
are expressed or when tumor suppressor genes are lost. In
fact, for a cell to become malignant, numerous mutations are
necessary. In some cases, both types of mutations - dominant
and recessive - may occur.
A gene mutation may allow an already abnormal cell to invade
the normal tissue where the cancer started, or to travel in
the bloodstream (metastasize) to remote parts of the body,
where it continues to divide.
A normal cell can become damaged in different ways. A cell
can become abnormal when part of a gene is lost (deleted),
when part of a chromosome is rearranged and ends up in the
wrong place (translocation), or when an extremely small defect
occurs in the DNA, which results in an abnormal DNA "blueprint"
and production of a defective protein occurs.
Abnormal cell division can also be caused by viruses. In
this case, genes may be normal, but the protein may not function
normally because the cell contains a cancer-producing virus.
How a specific cancer cell behaves depends on which processes
are not functioning properly. Some cancer cells simply divide
and produce more cancer cells, and the tumor mass stays where
it began. Other cancer cells are able to invade normal tissue,
enter the bloodstream, and metastasize to a remote site in
the body.
In summary, cancer cells have defects in normal cellular
functions that allow them to divide, invade the surrounding
tissue, and spread by way of vascular and/or lymphatic systems.
These defects are the result of gene mutations sometimes caused
by infectious viruses.
To learn more about the cell biology of cancer, please visit
the CancerQuest Web
site.
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