1. Cell Cycle Control System Checkpoints stop the cycle until go-ahead signal is received. – There are 3 checkpoints (G 1, G 2, & 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 G 1 checkpoint if the go-ahead signal is not reached it switches to the G 0 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)