Chapter 10 Notes
Regulating the Cell Cycle
Not all cells in the body go through the cell cycle at the same rate.
Ø For example: muscle and nerve cell never go through mitosis after they mature.
Ø Liver cells go through mitosis about once a year.
Ø Skin and intestinal cells go through mitosis every few hours.
What controls the rate of cell division?
1. Contact inhibition: chemicals released by cells so they stop dividing as they become crowded.
Ø Contact inhibition can be turned on or off as needed.
Ø For example, if a bone is broken, the bone cells near the break will start to divide until the break is filled in.
2. Cyclins: a family of proteins that regulate the timing of the cell cycle. These proteins trigger cell division.
Ø Internal regulators: several proteins make sure the cell cycle does not proceed unless certain processes have happened inside the cell.
Ø For example: one protein makes sure the cell does not go into mitosis until all of the DNA has replicated.
Ø External regulators: several proteins that respond to events outside the cell and direct the cell to either speed up or slow down the cell cycle. Growth factors are among the most important external regulators.
Ø Growth factors are proteins that bind to receptors on the cell surface, with the primary result of activating cellular division.
Ø For example: growth factors stimulate mitosis for wound healing.
3. Apoptosis: programmed cell death or cell suicide.
First the cell shrinks and pulls away from its neighbors (top right). Then blebs (pink spheres) appear on the surface (making the cell appear to boil), and the chromatin (black portion of larger inner cell), condenses at the edges of the nucleus. Soon the nucleus, and then the cell itself, breaks up, and the cell fragments are quickly ingested by other cells.
What happens if the rate of the cell cycle is not controlled?
What is cancer? When some of the body’s own cells lose the ability to control growth and division and start to divide uncontrollably.
What causes cancer? A mutation to the DNA that controls the cell cycle.
The accumulation of multiple mutations transform a normal cell into a cancer cell. The average cancer cell contains between six and eight different mutations. These mutations affect
• genes that control the cell cycle
• genes that control cell death
• genes that allow the cell to obtain nutrients and migrate through out the body.
CANCER CELLS IGNORE THE SIGNALS THAT REGULATE THE CELL CYCLE
The cell cycle is controlled by the products of two classes of genes, proto-oncogenes and tumor suppressor genes. Normally the expression of these genes is balanced.
1. Proto-oncogenes encode proteins that promote cell division (These proteins act like the gas pedal on a car. They say "go".)
2. Tumor-suppressor genes encode proteins that inhibit cell division (These proteins act like the brakes on a car. They say "stop".)
In cancer cells, the proto-oncogene(s) and tumor suppressor gene(s) are mutated. This is like the gas pedal in the car being stuck in the down position and the brakes of the car not functioning. The cell divides when it shouldn't.
Ø DNA mutations can be caused by genetic factors such as a defect in gene p53 (tumor suppressor gene) which normally stops the cell cycle until all chromosomes have been replicated.
Ø DNA mutations can be caused by environmental factors (also called mutagenic agents) such as:
o UV radiation
o Toxic chemicals (cigarettes)
How does cancer progress?
1. Several mutations to the DNA that controls the cell cycle occurs.
2. The p 53 gene does not find the mutations and destroy the cell.
3. Cells start to divide uncontrollably.
4. The new cells formed are also cancerous.
5. Large #’s of cancerous cells from clumps called tumors.
6. Tumors out-complete normal healthy cells for nutrients. The normal cells die.
7. Tumors break apart and spread cancer cells to other tissues and organs and out-compete the normal cells there killing them. This is called metastasis.