Cell Cycle Control System

1. Cell Cycle Control System • Checkpoints stop the cycle until go-ahead signal is received. • There are 3 checkpoints (G1, G2, & M) that must be passed before Mitosis is completed.

2. Molecules associated with the cell-cycle clock • Cdks - cyclin-dependent kinase (remember that a kinase is an enzyme that activate or inactivate by phosphorylating) • cyclin is always present in the cell and is activated when phosphorylated • synthesized during the S-phase • MPF - M-phase promoting factor • partner molecule to Cdk • peaks during M phase and initiates Mitosis • aids in the disassembly of the nuclear lamina • switched off during Anaphase

3. Molecules associated with the cell-cycle clock • Growth factor • secreted by near-by cells • called a mitogen • many allow the cell to proceed past the G1 checkpoint • if the go-ahead signal is not reached it switches to the G0 phase • most adult cells are at this position

4. External controls • density dependent inhibition • cells stop dividing when overcrowded • anchorage dependence • to divide cells must be anchored

5. Cancer • Cancer cells lack the inhibition pathways • are immortal or stop at odd places in the cycle • HeLa cell line has been dividing since 1951 • usually 20-50 replications before cell death • normal cells are transformed to cancer cells through a transformation process • if the cells stay in the same location they are said to be benign • if the tumor invades an organ and impair its function, it is said to be malignant • when the cancer cells travel to different locations they are metastatic (process is metastasis)