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The cells that make up all
living things continually
renew themselves through the
process of cell division.
When a cell divides into two
new cells, the DNA that
makes up the genes of the
original cell is copied.
Normally, the two new cells
are formed, each with a
complete and exact copy of
that DNA, but mistakes in
copying can occur. Most are
corrected naturally or have
no ill effect. However, if
many small mutations
accumulate over time they
can significantly alter the
way a cell behaves.
Mutations can occur due
to ageing, and environmental
factors such as UV radiation
from sunlight, poor diet,
tobacco smoking, and
exposure to some chemicals.
The types of mutations
most likely to lead to
cancer are those which occur
in genes that regulate the
process of cell division and
gene copying. If a mutation
occurs in one of these
control genes, cell division
in subsequent generations of
cells is likely to be more
loosely regulated. This
could lead to more
mutations, eventually
leading to cells growing and
dividing abnormally. Then
they become one of the more
than a hundred kinds of
cancer which share one
trait: cells multiplying out
of control to supplant or
destroy healthy tissue.
Often, these cancer cells
become “immortal” – they
multiply out of control,
lose their programming to
die like normal cells, or
both.
There are three types of
genes which are collectively
known as cancer control
genes. When these genes are
mutated, they may no longer
be able to prevent the
development of cancer, or
they may actively promote
it.
- Tumour suppressor
genes act as brakes on
uncontrolled cell growth.
If mutations render these
genes inactive, a tumour
may result.
- Oncogenes, when
mutated, can accelerate
cell division to the point
where a tumour results.
- Mismatch repair genes
find and repair mistakes
that occur when genes are
copied during cell
division. If these genes
are mutated so that they
no longer perform their
corrective function,
further mutations in genes
may result, leading to
tumour formation.
It is important to note
that cells do not usually
become cancerous due to a
single mutation or loss of
function in one control
gene. Cancer is due to the
build-up of mutations in a
number of different control
genes. This usually makes
cancer development slow and
complex. |
