Concept 12.3: Regulation of Cell Cycle by a Molecular Control System Will S. Esther P. Michelle K. Alex G. Nick B.
Evidence Of Cytoplasmic Streaming Timing and the rate of cell division in plants and animals is crucial to normal growth, development, and maintenance Some of the most specialized cells do not divide in a mature human—these cell cycle differences are from regulation at the molecular level
What drives the cell cycle?One hypothesis—each event triggers the next;replication of chromosomes in S phase causes cellgrowth during G2 phase triggering onset of mitosis—not correctCell cycle is driven by specific molecular signalspresent in cytoplasmExample: mammalian cells grown in culture
ExampleS and G1 –G1 went tothe S phase and DNAwas synthesizedM and G1 –G1 beganmitosis- spindleformed andchromatin condensedeven thoughchromosomes werenot duplicated
The Cell Cycle Control SystemOperating set of molecules that triggers andcoordinates key events in the cell cycle.Driven by a built-on clockRegulated at checkpoints by internal and externalcontrols.Checkpoint = critical point where stop and go-aheadsignals regulate the cycle
ContinuedSignals come from surveillance inside the cellThree major checkpoints are G1, G2, and M phasesG1 is the most important and the cell will most likelycomplete the cycle if given go-ahead by G1Not given go-ahead then leaves into the G0
The Cell Cycle ClockRhythmic fluctuations in the abundance and activityof cell cycle control molecules pace the sequentialevents of the cell cycle.Protein kinases are enzymes that activate orinactivate other proteins by phosphorylating them.To be active, such a kinase must be attached to acyclin, a protein that gets its name form its cyclicallyfluctuating concentration in the cell.
Stop and Go SignalsEnsure mitosis properlycarried out“Stops” if parts not there
Cells examined for normalgrowth, space limitations,growth factor withdrawal,DNA damage, etc.
Look for DNAdamage, mismatchedbases that could havehappened during Sphase (DNA synthesis)
• metaphase ensures all chromosomes connected to mitotic spindle at kinetochore• then can go to anaphase
External FactorsInfluencesdivision Binding PDGF toPhysical or receptorchemical tyrosine kinases triggersGrowth Factors transduction Ex. PDGF pathway, allows passage of G1Density-DependentInhibitionAnchorageDependence
CancerWhen cells divide excessively and invade other tissuesand /or take up resources.Apoptosis: process of programmed cell deathNormal mammalian cells divide only about 20-50 times.Transformation: when normal cells transforms into acancer cell.Benign tumor: when cancer cells stay in original site.Malignant tumor (metastasis): when the cancer spreadsand proliferates in various parts of the body.
TreatmentTumors are treated with high powered.Chemotherapy: toxic drugs that impede the cell cycleand stop cells from dividing. There is a cell line in culture that has beenreproducing since 1951