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Transcript
• 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. Cell Cycle Control System • Molecules associated with Cdks - cyclin-dependent kinase (kinase is an enzyme that activates or inactivates by the cell-cycle phosphorylating (adding or removing Pi)) – cyclin is always present in the cell and is clock 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 • Growth factor (external regulator) – 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 Molecules associated with the cell-cycle clock External controls • density dependent inhibition – cells stop dividing when overcrowded • anchorage dependence – to divide cells must be anchored 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 • Mutation in tumor suppressor genes – Must mutate both copies of the gene • Loss of function of p53 gene (apoptosis gene) – 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)