1. Regulating the Cell Cycle
Cells stop dividing when they come in contact with other cells.
Petri Dish Example and Healing example (cells near injury are stimulated to begin dividing, process slows as you are almost healed)

2. Cell Cycle Regulators Internal Regulators External Regulators
Proteins that respond to events INSIDE the cell.
Allow the cell cycle to proceed when certain processes have happened inside the cell (like chromosome replication)
Control the Checkpoints
External Regulators
Proteins that respond to events OUTSIDE the cell.
Direct cells to speed up or slow down.
Ex. Growth factors (stimulate growth)
A broken bone would trigger this.

3. Cell Cycle Regulators
Scientists discovered the protein CYCLIN (internal regulator)
The amount of cyclin in a cell fluctuates throughout the cell cycle.
Cyclins regulate the timing of the cell cycle in eukaryotic cells.

4. Cancer Cancer: uncontrolled cell growth.
Cancer cells do not respond to the signals that regulate the growth of most cells.
Divide uncontrollably and form tumors.
What causes cancer?
Mutations in specific genes.
Cell stops responding to external growth regulators.
Cells fail to produce internal regulators.

5. Cancer
P53 gene – a gene, which normally halts the cell cycle until all chromosomes have been replicated.
Most cancer cells have a defect in the p53 gene.

6. Cancer