1. 10.3 Regulating the Cell Cycle
Lesson Overview
10.3 Regulating the Cell Cycle

2. THINK ABOUT IT
How do cells know when to divide? How is the cell cycle regulated?

3. Controls on Cell Division
The cell cycle is controlled by
regulatory proteins both inside
and outside the cell.

4. Controls on Cell Division
When grown in a laboratory, most cells will divide until they come into contact with
one another. Once a cell comes into contact with another cell, it will stop dividing and
growing. This is called contact inhibition.
If some cells were removed, the remaining cells will begin dividing again until they
once again come into contact with other cells.
This shows that controls on cell growth and division can be turned on and off.

5. The controls on cell growth and division can be turned on and off.
For example, when an injury such as a broken bone occurs, cells are stimulated to divide rapidly and start the healing process. The rate of cell division slows when the healing process nears completion.

6. The Discovery of Cyclins
Cyclins are a family of proteins that regulate the timing of the cell cycle in eukaryotic cells.
This graph shows how cyclin levels change throughout the cell cycle in fertilized clam eggs.

7. Regulatory Proteins
Internal regulators are proteins that respond to events inside a cell. They
allow the cell cycle to proceed only once certain processes have happened
inside the cell.
For example,
cell can’t enter mitosis until its chromosomes have replicated
cell can’t enter anaphase until spindle fibers have attached to the chromosomes

8. Regulatory Proteins
External regulators are proteins that respond to events outside the cell.
They direct cells to speed up or slow down the cell cycle.
Growth factors are external regulators that stimulate the growth and division of cells. They are important during embryonic development and wound healing.
Other external regulatory proteins on the surface of neighboring cells often have the opposite effect of growth factors. They cause cells to slow down or stop their cell cycles to prevent excessive cells growth and to keep body tissues from disrupting one another.

9. Apoptosis
Just as new cells are produced every day in a multicellular organism, many other cells die. Cells end their life cycle in one of two ways:
1. due to damage or injury
2. “programmed to die”
Apoptosis is a process of programmed cell death.
Once apoptosis is triggered, the cell undergoes a series of controlled steps leading to self-destruction.

10. Apoptosis
Apoptosis plays a role in development by shaping the structure of tissues
and organs in plants and animals. For example, the foot of a mouse is
shaped the way it is partly because the toes undergo apoptosis during
tissue development.
This picture shows a mouse foot
in utero. The red cells located
(where the black arrow is pointed)
are apoptotic.

11. Cancer: Uncontrolled Cell Growth
Cell growth is carefully regulated because the consequences of uncontrolled
cell growth in a multicellular organism are very severe.
Cancer is a disorder in which body cells lose the ability to control cell growth.
How do cancer cells differ from other cells?
Prostate cancer cells

12. Cancer: Uncontrolled Cell Growth
Cancer cells do not respond to the
signals that regulate the growth of
most cells. As a result, the cells
divide uncontrollably.
Cancer cells divide uncontrollably
to form a mass of cells called a tumor.
However, not all tumors are
cancerous.

13. A benign tumor is noncancerous. It does not
spread to surrounding healthy tissue.
A malignant tumor is cancerous. It invades and
destroys surrounding healthy tissue and can
spread to other parts of the body.
The spread of cancer cells is called metastasis.
As cancer cells spread, they absorb nutrients
needed by other cells, block nerve connections,
and prevent organs from functioning. Soon,
homeostasis is disrupted and life-threatening
illness results.

14. What Causes Cancer?
Cancers are caused by defects in genes that regulate cell growth and division.
Some sources of gene defects are smoking tobacco, radiation exposure, defective
genes, and viral infection.
Regardless of the source of the gene defect, all cancers have one thing in common:
the control over the cell cycle has broken down – no response to external growth
regulators or failure to produce internal regulators that ensure orderly growth.

15. What Causes Cancer?
A damaged or defective p53 gene is common in cancer cells.
It causes cells to lose the information needed to respond to growth signals.

16. Treatments for Cancer
Some localized tumors can be removed by surgery. Skin cancer can
usually be treated this way.
Many tumors can be treated with targeted radiation. This treatment makes
use of the fact that cancer cells grow rapidly and, therefore, need to copy
their DNA more quickly than normal cell. This makes these cancer cells
vulnerable to damage from radiation.

17. Treatments for Cancer
Chemotherapy is the use of chemical compounds that kill or slow the growth of cancer cells. However, because most chemotherapy compounds target rapidly dividing cells, they also interfere with cell division in normal, healthy cells. This produces serious side effects – hair loss, nausea, anemia, etc.