Transcript Cell Cycle Control System - Santa Susana High School

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.
Molecules associated with the cellcycle 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
Molecules associated with the cellcycle 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
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
– 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)