Understanding
Cancer
Do you know what cancer is
and what may cause it?
Bronchial tumor
Tumor cells have replaced the
normal, ciliated tissue in the bronchi.M
ost healthy cells of the body grow, divide
a certain number of times, age, and die.
Sometimes, this orderly process is dis-
rupted when cells lose the ability to limit
and direct their growth. Cells may divide too
often and produce an excess of cells, called a
tumor. A tumor can be benign or malignant.
Benign tumors do not spread to other parts of the
body, and can usually be surgically removed. A
malignant tumor—cancer—invades and destroys
nearby healthy tissues and organs. Cancerous
tumors can metastasize, that is, can spread to
other parts of the body and form new tumors.
There are probably at least 100 different kinds of
cancer, with each one affecting different kinds of
cells and having different characteristics.
Cancer cells do not respond normally to the
chemical signals that regulate the cell cycle. In
some cancer cells, the way cell division signals
are transmitted to the nucleus is abnormal. In
other cancer cells, the entire cell cycle control
system may be abnormal. If cancer cells stop
dividing, they do so at random points in the cell
cycle rather than at the normal checkpoints.
When cultured in the laboratory with adequate
nutrients, cancer cells can divide indefinitely and
are referred to as “immortal.” Most mammalian
cells grown in culture divide only 20 to 50 times
before they stop dividing, age, and die.How Cancer Begins
Changes in the genes that influence the cell cycle
can cause the transformation of a normal cell into
a cancer cell. There are two types of these genes.
The first type codes for proteins that stimulate
cell division. Genes of this type are normally
turned off in cells that are not dividing. This type
of gene can be converted to an oncogene, that is,
a “cancer gene,” by mutation. One common onco-
gene, a gene called called ras, is present in
mutated form in about 30 percent of human can-
cers and in some forms of leukemia.
The second type of gene associated with
cancer is a tumor suppressor gene. Tumor sup-
pressor genes code for proteins that normally
restrain cell division. In many cancers, tumor
suppressor genes have been inactivated by muta-
tion. An inherited mutation in one copy of a
tumor suppressor gene results in higher risk of
cancer. However, cancer does not occur unless
and until the remaining, healthy copy of the gene
is also inactivated by mutation. If a person is
born with two normal copies of a tumor suppres-
sor gene, both must be inactivated before cancer
can develop.
Almost 50 percent of human cancers are asso-
ciated with a mutation in the tumor suppressor
gene p53. These cancers include many breast,
colon, lung, prostate, and skin cancers. The
protein produced by the p53gene normally acts
as an emergency brake in the cell cycle. The p53
protein also induces the death of damaged cells.
To perform these functions, p53 protein must138138 UNIT 2UNIT 2
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