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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

Experiment 1
S G
1
Experiment 2
M G
1
S S
When a cell in the S phase was fused with a cell in G
1
, the G
1 cell immediately entered the S phase—DNA was synthesized. S cells contained something that induced regulation in G
1 cells.
M M
When a cell in the M phase was fused with a cell in G
1
, the G
1 cell immediately began mitosis—a spindle formed and chromatin condensed, even though the chromosome had not been duplicated. Something in M phase induced interphase cells to divide.

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Conclusion: the S phase proteins work on G
1 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 G
0

G
0
G
1 checkpoint
G
1
If a cell receives a goahead signal at the G
1 checkpoint, the cell continues on in the cell cycle.
G
1
If a cell does not receive a go-ahead signal at the G
1 checkpoint, the cell exits the cell cycle and goes into
G
0
, a nondividing state.

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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

Cells anchor to dish surface and divide (anchorage dependence).
When cells have formed a complete single layer, they stop dividing
(density-dependent inhibition).
If some cells are scraped away, the remaining cells divide to fill the gap and then stop (density-dependent inhibition).
Normal mammalian cells
25 µm

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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 cells do not exhibit anchorage dependence or density-dependent inhibition.
25 µm
Cancer cells

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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

Tumor
Glandular tissue
A tumor grows from a single cancer cell.
Cancer cells invade neighboring tissue.
Lymph vessel
Cancer cell
Cancer cells spread through lymph and blood vessels to other parts of the body.
Blood vessel
Metastatic tumor
A small percentage of cancer cells may survive and establish a new tumor in another part of the body.

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The two most common treatments we have are chemotherapy and radiation treatments
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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

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Common side effects – hair loss and an compromised immune system
Essentially theory is the drugs will kill the cancer faster than the patient
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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

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With more knowledge on mechanisms of cancer, new treatments are being proposed
Viruses are being designed to specifically target cancer cells