Control of the Cell Cycle

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Mark Huyler

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Control of the Cell Cycle Cell cycle length can vary greatly among cell types variation in the length of G1 phase is responsible for these differences o G1 phase is essentially eliminated in rapidly dividing cells o nondividng cells get permanently stuck in G1 phase this arrested stage is called G zero stageThe rate of cell division can also respond to changes in environmental conditionsThese variations in cellcycle length suggest that the cell cycle is regulation varies among cells and organismsMitosispromoting factor MPF is present in the cytoplasm of Mphase cells and induces mitosis in all eukaryotesMPF is composed of two distinct subunits a protein kinase and a cyclin o The protein kinase is an enzyme that catalyzes the transfer of a phosphate group from ATP to a target protein phosphorylation o The cyclin subunit functions as a regulatory proteinThe concentration of MPF cyclin increases during interphase then peaks in M phase before decreasing againThe MPF protein kinase is a cyclindepenent kinase Cdk that is active only when bound to the cyclin subunit Thus when cyclin concentrations are high more MPF is active and the target proteins are phosphorylated inhibiting mitosisAfter it binds to cyclin MPFs Cdk subunit becomes phosphorylated at two sites rendering it inactiveLate in G2 phase enzymes cause one of the phosphate groups on the Cdk subunit to drop off o This dephosphorylation reaction changes MPFs shape activating itOnce MPF is activated it trigger a chain of event culminating in the condensation of chromosomes and formation of the mitotic spindle apparatusDuring anaphase an enzyme complex begins degrading MPFs cyclin subunit In this way MPF triggers a chain of events that leads to its own destructionMany other protein complexes are involved in regulating the cell cycle
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