Chapter 10.3
REGULATING THE CELL CYCLE

Not all cells move through the cell cycle at the same rate. In humans
nerve and muscle cells do not divide once they develop. Skin cells and
cells in the digestive tract grow and divide rapidly.
Controls on Cell Division

Controls of cell growth and division can be turned on and off.

For example if cells get too crowded they stop growing. If there is a cut in
the skin of a break in the bone cells at the injury site will start growing
rapidly until the injury is healed.
Cell Cycle Regulators

Cyclin (a protein in the cell) regulates
the cell cycle.

There are 2 types of proteins that
regulate cell cycle: those inside the
cell and those outside the cell.
Internal Regulators

Internal regulators are found inside the
cell. These allow the cell cycle to
continue only if certain processes
have taken place.

Example: all chromosomes have to be
regulated for the cell cycle to
continue.
External Regulators

External regulators are outside the cell.
They tell the cell to speed up or slow
down the cell cycle.

Growth factors would be an example
of an external regulator. They
stimulate growth and division. There
are also molecules on the surface of a
cell that tell it to stop growing or
dividing if they come in contact with
one another.
Uncontrolled Cell Growth

Cancer is when cell’s lose the ability to
control growth. Cancer cells do not
respond to signals that regulate
growth of most cells. These cells will
continue to divide and form masses or
tumors.

Most cancer cells have a damaged
p53 gene – this gene is responsible for
stopping the cell cycle if
chromosomes have not be correctly
duplicated.