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When Control Is Lost
Cancer
When U.S. President Richard
Nixon recruited biologists to
join a “War on Cancer” in 1972,
we did not know very much
about the causes of cancer,
although many Americans were
dying of it. In the 30 years since
then, biologists have learned a
lot. Cancer is a growth defect in
cells, a breakdown of the
mechanism that controls cell
division.
Cell Cycle Regulation
Certain genes contain the information necessary
to make the proteins that regulate cell growth
and division. If one of these genes is mutated,
the protein may not function, and regulation of
cell growth and division can be disrupted. , the
uncontrolled growth of cells, may result. Cancer
is essentially a disorder of cell division. Cancer
cells do not respond normally to the body’s
control mechanisms.
Some mutations cause cancer by overproducing
growth-promoting molecules, thus speeding up
the cell cycle. Others cause cancer by
inactivating the control proteins that normally
act to slow or stop the cell cycle.
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Although all cancers are not curable, great progress has been
made in cancer research over the last 30 years.
Cell Quality Control
The cell cycle has built-in quality
control checkpoints that monitor
the cell cycle and can stop it if
something goes wrong.
For instance, near the end of the
G1 stage, the cell monitors its
DNA for damage and can stop the
cell cycle before entering the
S
stage of interphase if something is
wrong. There are other quality
control checkpoints during the S
Cancer is caused by
mutations, or changes, in
segments of DNA that code
for production of proteins,
including those that regulate
the cell cycle. Often, cells
can fix mutations in DNA.
If the repair system fails,
cancer can result.
Environmental factors can
increase the risk of cancer.
stage and after DNA duplication
in the G2 stage. During mitosis,
the cell checks the spindle fibers
before it undergoes cytokinesis. If
the cell detects a failure, the cell
cycle stops.
Sometimes control of the cell
cycle fails. When cells do not
respond to control mechanisms,
cancer results. Cancer is the
uncontrolled growth and division
of cells.
Cancer cells grow and divide as
long as they receive nutrients.
They crowd normal cells causing
tissues and organs to stop
working. Cancer can kill an
organism.
Substances that are known to
cause cancer are caled
carcinogens (kar SIH nuh
junz).
Tobacco, tobacco smoke,
alcohol, some viruses, and
radiation are examples of
carcinogens.
Avoiding
carcinogens can help reduce
the risk of cancer. Federal
laws protect people from
exposure to carcinogens in
the workplace and in the
food supply. People can
reduce their risk of cancer
by avoiding all tobacco
(including
secondhand
smoke
and
smokeless
tobacco) and by using
sunscreen to protect their
skin
from
ultraviolet
radiation from the Sun.
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years indicates
that mutation of only a few genes can transform
normal cells into cancerous ones. All of these
cancer-causing genes are involved with regulating
how fast cells grow and divide. How is cell division
regulated? As a crude analogy, imagine a car
parked on the side of a road. To get it going, you
Mutations and
Cancer
must step on the accelerator and
release the brake.
How many mutations are
required to produce cancer?
Research in the last several
years indicates that mutation
of only a few genes can
transform normal cells into
cancerous ones. All of these
cancer-causing genes are
involved with regulating
how fast cells grow and
divide. How is cell division
regulated? As a crude
analogy, imagine a car
parked on the side of a road.
To get it going, you must
step on the accelerator and
release the brake.
Stepping on the
Accelerator
A cell divides when it
receives a signal to do so. A
“divide” signal is usually in
the form of a chemical
substance
released
by
another cell. A protein on
the surface of the receiving
cell binds the substance.
This binding activates a
second protein inside the
cell—relaying the signal
from the outside of the cell
to the inside. Here, a family
of proteins then relays the
signal inward to the nucleus.
One protein molecule passes
the signal to the next like a
baton in a relay race. The
genes for these signalcarrying proteins are called
oncogenes (onco is Greek
for “mass” or “tumor.”). If
oncogenes are changed by
mutation to become more
active, cancer can result.
Like stepping on the
accelerator of a car, an
increase in the activity of
these proteins amplifies the
“divide” signal. This causes
the cell to divide more often.
Releasing the Brakes
At the nucleus, the divide
signal overrides a set of
genes that act as “brakes.”
These braking genes—called
tumor suppressor genes—
prevent cell division from
occurring too often. In
cancer,
these
tumor
suppressor
genes
are
damaged. Like removing
pressure from the brakes of a
car increases a car’s speed,
decreasing the activity of
tumor suppressors speeds up
cell division. Cells have
three kinds of tumor
suppressors, all of which
must be disabled before
cancer can occur.
The genes for these
signal-carrying proteins are
called oncogenes.
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Why does cancer run in families?
Cancer runs in some families. People
might inherit one or more DNA mutations
from their parents, increasing their risk of
developing cancer.
Cancer can occur in people of all ages,
but older people have a higher risk. This
might be because it takes more than one
DNA mutation to change an abnormal
cell into a cancer cell. Older cells have
had more time to accumulate the
mutations that lead to cancer.
We now know that many cancers can be
largely avoided. To sharply reduce your
risk of lung cancer, for example, don’t
smoke. Many other cancers can be
treated successfully when detected
early. Colon
cancer, for example,
develops slowly from
intestinal tissue growths called
polyps. A simple medical
examination enables the detection
and removal of the polyps.
Great progress is being made in
curing many cancers. Anticancer
drugs that stick to these extra cell
proteins, gumming them up so
they cannot promote excessive
cell division, appear to offer great
promise.