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Regulation of the Cell Cycle
Since the cell cycle is known, now we must ask what controls it
Noted that healthy cells in contact will not divide◦ Essentially, this is how our skin heals
But what drives the cycle? Sequential control? Experiments have shown that proteins
control it
Molecular Control
The Rao Johnson Experiment Experiment 1 Experiment 2
S
S S
G1 G1M
M M
When a cell in the M phase was fused with a cell in G1, the G1 cell immediately beganmitosis—a spindle formed and chromatin condensed, even though the chromosome had not been duplicated. Something in M phase induced interphase cells to divide.
When a cell in the S phase was fused with a cell in G1, the G1 cell immediately entered the S phase—DNA was synthesized. S cells contained something that induced regulation in G1
cells.
Conclusion: the S phase proteins work on G1 nuclei, M phase works on everything
Biologists named these proteins “cyclins” These are used in all three portions of
interphase Each is referred to as a “checkpoint” Collectively, they are called growth factors The cell has mechanisms that ensure each
phase is complete before moving onto the next
We have seen this already with the possibility of a cell entering G0
LE 12-15
G1
G1 checkpoint
G1
G0
If a cell receives a go-ahead signal at the G1 checkpoint, the cell continues on in the cell cycle.
If a cell does not receive a go-ahead signal at the G1 checkpoint, the cell exits the cell cycle and goes into G0, a nondividing state.
Overall, it appears as though there are many factors, internal and external that control the cell cycle
Ex. Kinetochores not attached to microtubules
Ex. Density dependent inhibition Ex. Anchorage dependence, where cells
have to be attached to a substratum (solid surface) to divide
Internal and External Factors
Cells anchor to dish surface anddivide (anchorage dependence).
When cells have formed a completesingle layer, they stop dividing(density-dependent inhibition).
If some cells are scraped away, theremaining cells divide to fill the gap andthen stop (density-dependent inhibition).
25 µmNormal mammalian cells
Cancer can be thought of as uncontrolled growth of cells◦ They can lack the mechanisms, do not respond
properly to them, or have an error in the pathways
If they stop, it is at a random point
Cancer
Cancer cells do not exhibit anchorage dependenceor density-dependent inhibition.
Cancer cells25 µm
A mass of cancer cells is known as a tumor◦ If this tumor stays in the original site – benign◦ If it spreads to surrounding tissue - malignant
A individual tumor cell can break free from the group and invade other organs◦ It is said to have metastasized◦ http://www.youtube.com/watch?v=rrMq8uA_6iA
Unfortunately, these cells can continue to grow if supplied with nutrients◦ Normal cells divide 25-50 before dying
Cancer cell
Bloodvessel
LymphvesselTumor
Glandulartissue
Metastatictumor
A tumor grows from asingle cancer cell.
Cancer cells invadeneighboring tissue.
Cancer cells spreadthrough lymph andblood vessels toother parts of thebody.
A small percentageof cancer cells maysurvive and establisha new tumor in anotherpart of the body.
Cancer
The two most common treatments we have are chemotherapy and radiation treatments
Chemotherapy By definition, treatment of any condition via
chemicals Our focus is on antineoplastic drugs –
cancer fighters Act by targeting rapidly dividing cells
◦ However, this also includes healthy cells such as bone marrow (blood cells), hair follicles, and sex cells
Treatments
Common side effects – hair loss and an compromised immune system
Essentially theory is the drugs will kill the cancer faster than the patient
If not discovered soon enough, treatment would take too long
Radiation Therapy Beam of subatomic particles that damages
DNA Has to focus on a tumor
With more knowledge on mechanisms of cancer, new treatments are being proposed
Viruses are being designed to specifically target cancer cells
Alternatives