Cambridge International A Level Biology Revision Guide

Chapter 5: The mitotic cell cycle
Cancers are thought to start when changes occur in the
genes that control cell division. A change in any gene
is called a mutation. The particular term for a mutated
gene that causes cancer is an oncogene, after the Greek
word ‘onkos’, meaning bulk or mass. Mutations are not
unusual events, and most of the time they don’t lead to
cancer. Most mutated cells are either affected in some
way that results in their early death or are destroyed
by the body’s immune system. Since most cells can be
replaced, mutation usually has no harmful effect on the
body. Cancerous cells, however, manage to escape both
possible fates, so, although the mutation may originally
occur in only one cell, it is passed on to all that cell’s
descendants. By the time it is detected, a typical tumour
usually contains about a thousand million cancerous cells.
Any agent that causes cancer is called a carcinogen and is
described as carcinogenic.
Although you do not need to know about different
types of tumours, you may be interested to know that
not all tumours are cancerous. Some tumours do not
spread from their site of origin, and are known as benign
tumours; warts are a good example. It is only tumours
that spread through the body, invade other tissues and
destroy them that cause cancer, and these are known as
malignant tumours. Malignant tumours interfere with
the normal functioning of the area where they have started
to grow. They may block the intestines, lungs or blood
vessels. Cells can break off and spread through the blood
and lymphatic system to other parts of the body to form
secondary growths. The spread of cancers in this way is
called metastasis. It is the most dangerous characteristic of
cancer, since it can be very hard to find secondary cancers
and remove them.
The steps involved in the development of cancer are
shown in Figure 5.15.
Carcinogens cause mutations.
e.g. UV light
tar in tobacco
smoke
asbestos
X-rays
1 Oncogenes
transformed
by carcinogens.
2 Cancerous cell
does not respond
to signals from
other cells so
continues to divide.
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3 Mitosis
6 Tumour gets bigger.
Cells change their
characteristics and
look different under
the microscope.
5 Rapid mitosis
4 Cancerous cells
not removed by
immune system.
absorption of
nutrients
7 Tumour supplied
with blood and
lymph vessels.
Tumour cells spread
in blood and lymph
to other parts of the
body.
8 Metastasis. Tumour
cells invade other
tissues. Secondary
cancers form
throughout the body.
Figure 5.15 Stages in the development of cancer.