Once a stage is complete there is no going back each

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Once a stage is complete, there is no going back Each stage is regulated by protein complexes Who are the regulators and how were they identified? 12
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Common protein modifications that occur during cell division 13
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MPF: maturation promoting factor Model system: Xenopus oocyte maturation Observation: Progesterone stimulates maturation of egg arrested in G2 to reenter cell cycle Fertilization of egg stimulates completion of cell cycle and first division 14
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MPF: maturation promoting factor What stimulates oocyte maturation and completion of first cycle? Observation: Soluble material in the cytoplasm of meiosis II-arrested eggs can induce entry into cell cycle similar to progesterone Identification of soluble factor regulating cell division Assay to test purified biochemical fractions for MPF activity 15
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MPF: Mitosis promoting factor MPF activity increases before cell division, peaks and abruptly drops Activity cycles with the cell cycle division What is the biochemical identity of MPF? 16 Assay for MPF activity
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Radiolabel sea urchin cells at fertilization Analyze newly synthesized proteins by SDS-PAGE/autoradiography over time Look for proteins that coordinately change with cell divisions Identification of cyclin 17
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18 Is cyclin MPF? Are the production and degradation of cyclin important for cell division?
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Cyclin synthesis and degradation regulate MPF activity Experiment: Determine MPF activity +/- cyclin (a) Positive control (b) Negative control (c) + cyclin-> sufficient to trigger cell division (d) + cyclin without degradation->destruction required for cell cycle progression Cyclin levels regulate MPF activity, but cyclin has no enzymatic function So, what is the enzymatic component of MPF? 19
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Yeast genetics identify regulators of cell division Model system: fission yeast (Schizosaccharamyces pombe) Phases of cell division the same as humans Size of yeast changes over cell cycle: Small at beginning/end Increases in size during mitosis, largest just before division 20
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Yeast cdc2 mutants are defective in cell division Too little cdc2- cells arrest Too much cdc2- cells divide prematurely 21 Cdc2 is the catalytic component of MPF and is a
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