1. Cell Cycle Regulation The cell cycle is driven by an internal clock regulated at certain checkpoints by both external (ex: faucets) and internal (ex: sensor) controls. Cell Cycle analogy to washer machine control

2. Major Checkpoints of the Cell Cycle Control System

3. Cell Cycle Clock: Cyclins and Cyclin- Dependent Kinases Cyclin fluctuates cyclically within the cell. Cyclin–dependent kinases (Cdks) such as MPF must be attached to cyclin to be active (MPF = mitosis promoting factor) Activity of Cdks rise and fall with changes in cyclin concentration. MPF complex initiates mitosis by phosphorylating a variety of proteins. MPF switches itself off by initiating a process that breaks down cyclin.

4. The effect of a Growth Factor on Cell Division A growth factor is a protein released by certain body cells that stimulates other cells to divide. PDGF = platelet- derived growth factor

5. Density-Dependent Inhibition Crowded cells stop dividing. Cultured cells normally divide until they form a monolayer. If some cells are removed, cell division begins in cells bordering the gap until it is filled. Most animal cells also exhibit anchorage dependence

6. Cancer cells have escaped from cell cycle controls Cancer cells: do not exhibit density-dependent inhibition or anchorage dependence. do not stop dividing when growth factors are depleted. can go on dividing indefinitely in culture if given a continual supply of nutrients (immortal). transformation is the process that converts a normal cell to a cancer cell.