Cell Cycle
Cell Cycle
• Ordered sequence of events a
cell goes through to grow and
cell division.
• Regulation of the cell cycle is
critical for the normal
development of multicullular
organisms.
Cell Cycle
Gap 1 (G1)- growth
normal cell metabolism
Synthesis of enzymes needed for
DNA replication.
S- synthesis- DNA replicatedsister chromatids
Gap 2 (G2)- metabolic activity,
production of microtubules
M- Mitosis- cell division
Go- Cell is quiescent/senescent
Some cells stay in “time out”
Most neurons
Mature muscle cells
Interphase
Inter = between
Mitosis
• Karyokinesisdivision of the
chromosomes
• Cytokinesis- division
of the cytoplasm.
mito= thread
kine= move
Cell cycle
Is it regulated?
• When do you turn off the
Cell Cycle?
• How does the cell know
when to leave G1  S
All the time- skin, nails, intestinal
lining
Rarely- brain and CNS cells
Only if damaged- liver
Checkpoints
Kinases- control movement
Stop and go signals
3 major checkpoints
G1 Checkpoint
G1 Checkpoint- most important“Go ahead”- will usually complete the cell cycle
If it does not get the Go ahead- it will move to Go.
G2
Checkpoint – verifies integrity of DNA
M Checkpoint
Prior to anaphase
Checks the condition of the mitotic spindle
Hayflick Limit
• 1961- Hayflick and Moorhead
Embryonic stem cells will divide approx. 50 times.
Different cells different rates
All the time- skin, nails, intestinal lining
Rarely- brain and CNS cells
Only when damaged- liver
Telomeres and Aging
Telomers
Telomeres are the tips of the
chromosomes.
Everytime the cell divides it loses
a little bit of the telomere.
How are telomers analogous to
the aglet on your shoe laces?
Telomerase
Telomerase is the enzyme that
builds and maintains the
telomers.
For our cells, the gene that codes
for telomerase is turned off.
Are Cells Immortal?
Cancer cell are Immortal
Cancer results from the loss of control of the cell
cycle.
Mutation(s) in one or more of the proteins
involved in the checkpoint system.
Telomerase- the enzyme that puts the telomers
back on the chromosome is turned on.
Cancer
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Disease of the cell cycle
90% of the cancers are in somatic cells
Cancer will occur in 1: 5 people
Disease of the aging –
Cancer cells divide constantly and invade other
tissues.
Why is cancer a genetic disorder of the
elderly?
Carcinogens
Cancer causing agent (mutagens)
1. Environmental exposure
2. Viruses (15 – 20 %)
3. Inheritance ( 5 -10 %)
Contact Inhibition
Most cells are attached to other
cells, bathed in extracellular
fluid and is nourished by
blood.
When signaled, cells will divide
until they come into contact
with other cells – then they
will stop.
Not Cancer
Tumors
A tumor is a mass of dividing cells.
• Benign- not cancer, not spreading
• Malignant- cancerous mass of dividing cells
• Metastasis - the spread of cancer cells via the
circulatory system.
Cancer Types
Named by the organ/tissue in which they
originate.
• Sarcoma- connective tissue – bones & muscles
Carcinoma- (external/internal coverings )- skin & lining of
intestines
Leukemia & lymphoma- (blood forming tissues)- bone
marrow, spleen, lymph nodes
Chemotherapy
Toxic Drugs that interfere with cell division.
Taxol- freezes the spindle stopping mitosis.
Vinblastin- stops the spindle from forming
Tumor Suppressor Genes
Genes that are involved cell cycle
regulation- they inhibit the growth of
tumors.
1. p21- slows or stops cell division
2. P53- triggers apoptosis when the cell
cannot be repaired.
Mutations to Tumor Suppressor Genes
1. Alcohol
1. tobacco