Download Cell Cycle Control System Mechanism and Cancer

Cell Cycle Control System Mechanism and Cancer
The regulatory molecules are proteins which are made of two types:
Kinases – enzymes that activate or inactivate other proteins by phosphorylating
them. They are present at a constant concentration in a growing cell but much of
the time they are inactive.
To be active, a kinase must be attached to a cyclin (a protein that “cycles”
fluctuating concentration). These kinases are called “cyclin-dependent kinases” or
Cdks. The activity of Cdks rise & fall with changes in the concentration of its cyclin
partner.
A cyclin-Cdk complex called “maturation promoting factor” or MPF acts at the G2
checkpoint to trigger mitosis. The level of MPF fluctuates with the levels in cyclin in
use. The cyclin levels rise throughout interphase then falls abruptly during mitosis.
The peaks of MPF activity and cylcin concentration correspond. The Cdk itself is
present at a constant level.
MPF promotes mitosis by phosphorylating various proteins, including other
enzymes. One effect of MPF is the initiation of a sequence of events leading to the
breakdown of its own cyclin. The Cdk component of MPF is recycled. Its kinase
activity will be restored by association with new cyclin that accumulates during
interphase.
G1
S
G2
M
G1
S
G2
M
G1
S
G2
M
cyclin
MPF activity
MPF activity
MPF activity
What is CANCER?
Cancer is a continuous cell division process that creates a cluster of cells called a
TUMOR. When the cells that make up a tumor break off and flow into the blood stream
then it is said to be METASTASIZED. This occurs because damage has occurred with
genes that control the above mentioned proteins. Cancer cells are virtually immortal.
Tumors that arise from connective tissue, bone or muscle are known as SARCOMAS
while those that originate in epithelial tissue such as skin are called CARCINOMAS.
LYMPHOMA is cancer of the blood, blood producing tissues and lymph nodes. A
CARCINOGEN is cancer caused by mutagens.
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Cancer and the Cell Cycle
Oncogene – genes that can mutate and cause cancer by inappropriately altering the cell
cycle.
Tumor suppressor genes – produce proteins that can block the bonding of cyclins to
CDK’s (cyclin dependent kinases). The CDK’s are enzymes and cause the construction
of proteins that allow mitosis and the cell cycle to continue from G1  S  G2
checkpoints.
P53 is a tumor suppressing gene. Over 45% of all cancers involve mutations to
this gene.
Often times a “point mutation” in which one base is changed will code for a faulty
suppressor protein and allow cancer to start.
Cancer is caused by mutations in 30 known genes. Usually four or five genes
must mutate before an individual gets cancer. Cancer mutations are either:
a. mutations to genes that produce proteins that suppress cell division.
b. mutations to genes causing them to overproduce cell cycle activating
proteins.
Cancer occurs more often in the elderly because multiple mutations must occur. *1/3 of
all females will die of some sort of cancer and ¼ of all males eventually die of cancer.
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