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Regulation of the Cell Cycle
How does a cell know when to
divide and when not to divide?
Regulation of the Cell Cycle
• Internal and external factors regulate cell
– External factors – factors or signals that come
from the outside of the cell that help to control
the division process
– Internal factors – factors or signals from the
inside of the cell that control the division
External Factors
• Cell to Cell contact
• Growth Factors
– Platelets
– Erythropoeitin
– Hormones
Cell to Cell Contact
• Once a cell touches other cells it stops
• The exact reason is considered a
phenomenon and is unknown.
• One hypothesis suggests that receptors
on neighboring cells bind to each other
and trigger the cytoskeleton to block
signals that will trigger growth.
Growth Factors
• Growth factors are a broad group of
proteins that act as chemical signals
released from other cells to tell other cells
to divide.
• Growth factors bind to receptors that
activate specific genes to trigger cell
• Cells will usually respond to a combination
of several growth factors not just one.
– Platelets-form clots to
stop bleeding and
carry growth factors
that helps your body
repair wounds by
signaling certain cells
to divide.
– Erythropoietin-stimulates the production of red
blood cells and becomes present when
oxygen levels are depleted.
• Hormones stimulate the growth of certain
cell types.
– Ex. Growth hormone results in bone growth
and also affects metabolism.
Internal Factors
• Enzymes
– Kinases
• Cyclins
Kinases and Cyclins
• Kinases are enzymes that when activated
transfer a phosphate group from one
molecule to a target molecule. This gives
energy or changes the shape of the target
• Some kinases help control the cell cycle.
• Cyclins are proteins that are rapidly made
and broken down that activate kinases.
• Just as some cells need to divide, some
cells need to die.
• Apoptosis is programmed cell death.
• Internal or external factors stimulate genes
that activate self destructive enzymes.
• The cell will then be recognized and
ingested by immune system cells.
What happens when cell division
becomes uncontrolled?
• Cancer is a common or generic name
given to diseases that are characterized
by uncontrolled cell division.
• Cancer cell will continue to divide without
the presence of growth factors and will
also continue to divide when in contact
with other cells.
• Cancer cells divide much more than
healthy cells.
• When clumps of cancer cells form they are
called tumors.
• Two types:
– Benign
– Malignant
Benign Tumors
• Benign tumors usually remain localized
and do not spread.
• Usually harmless and may be cured
simply by removal.
Benign Tumor Image
Malignant Tumor
• In a malignant tumor certain cells may
break away from the tumor and spread.
• When cancerous cells spread, we say that
they metastasize.
– Done by entering the blood and lymp vessels
of the circulatory system where they invade
other areas and form new tumors.
• Tumors are difficult to eradicate once they
• Right lung is
healthy, the left
lung is not.
Skin Cancer
Why are tumors harmful?
Cancer Cells
– Cancer cells don’t perform the specialized
functions needed by the body.
– The body has large volumes of rapidly
dividing cells that require lots of blood and
energy but pose no usefulness to the body.
– Tumors can exert pressure on vital organs
when they become too large.
Where do cancer cells come
• Cancer cells will usually come from normal
cells that have suffered damage to their
genes (mutations) that are associated with
making proteins that regulate the cell
• Oncogenes are one type that will
accelerate cell growth.
• Other genes act as cell cycle brakes.
• Mutations in various genes can be
inherited (e.g. breast cancer).
Other mutations can also be caused by
exposure to different chemicals or
radiation (e.g skin cancer).
UV Radiation
Carcinogens are substances that
may cause mutation.
Carcinogens cont…tobacco
cinogens are substances that may cause mutation (e.g. tobacco smoke, air pollutants, asbes
Carcinogens….air pollutants.
Some oncogenes may be carried by a virus
and cause cancer (e.g. cervical cancer
often results from the Human papilloma
virus - HPV).
• The HIV virus leaves the body more
susceptible to cancer.
Localized treatment that damages a cell’s DNA
and prevents it from dividing
– Drugs that kill actively
dividing cells.
– May kill healthy cells.
Cancer Treatment
More effective treatments may include a
combination of both chemotherapy and