Knowing When to Stop

Suppose you had a paper cut on your
finger. Although the cut may have bled
and stung a little, after a few days, it
will have disappeared, and your finger
would be as good as new.
Bell Work
1. How do you think the body repairs an
injury, such as a cut on a finger?
2. How long do you think this repair
process continues?
3. What do you think causes the cells to
stop the repair process?
Regulating the Cell Cycle
Regulating the Cell Cycle


Cell Growth and Cell Division are
carefully controlled
Not all cells move through the cell
cycle at the same rate

Example: Most muscle cells and nerve
cells do not divide once they have
developed; other cells, such as skin cells,
divide rapidly throughout an organism’s
life
Figure 10-7: Contact Inhibition

The controls on cell growth can be turned
on and off
Cell Cycle Regulators


The cell cycle is regulated by a specific
protein
o The amount of this protein in the cell
rises and falls in time with the cell
cycle
Cyclins are proteins that regulate the
timing of the cell cycle in eukaryotic
organisms.
Figure 10-8: Regulating the Cell Cycle
A sample of cytoplasm
is removed from a cell
in mitosis.
The sample is injected
into a second cell in
G2 of interphase.
As result, the second
cell enters mitosis.
Internal Regulators


Proteins that respond to events inside
the cell are called internal regulators
Internal regulators allow the cell cycle
to proceed only when certain
processes have happened inside the
cell

Example: all chromosomes must be
replicated
External Regulators


Proteins that respond to events
outside the cell are called external
regulators
External regulators direct cells to
speed up or slow down the cell cycle

Example: healing wounds
Uncontrolled Cell Growth

Why is cell growth regulated so
carefully?


The consequences of uncontrolled cell
growth in an organism are severe → it
disrupts the normal functions of the body
Cancer is a disorder in which some of
the body’s own cells lose the ability to
control growth
Cancer

Cancer cells do not respond to the
signals that regulate the growth of
most cells

they divide uncontrollably and form
masses of cells called tumors that can
damage the surrounding tissues
Cancer

Cancer is a disease that can be
caused by: environmental factors,
genetics (inherited), or viruses

they all have one thing in common: the
loss of control over regulating the cell
cycle