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The Cell Cycle.doc

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Kathleen Gilmour

The Cell Cycle -the cell cycle involves a period of growth, followed by nuclear division and cytokinesis -the cell cycle allows for an increase in size of the organism, developmental complexity; the diversity of functioning multicellular eukaryotic organisms require strict control of cell division -the cell cycle ultimately results in a mature body composed of different subpopulations of cells Mitotic Cell Cycle Stages 1) Interphase -extends from the end of one mitosis to the beginning of the next mitosis -the cell grows at a stead rate through interphase G0: a resting phase where the cell has left the cycle and has stopped dividing; it is in a quiescent phase G1: the cell is metabolically active and continuously grows but does not replicate its DNA S: DNA replication takes place (scheduled DNA synthesis = doubling of the chromosomes) -DNA synthesis can occur when not in S-phase (unscheduled DNA synthesis) -this is seen in DNA repair, in mitochondria / chloroplast, etc., and in recombination during meiosis G2: cell growth continues and proteins are synthesized in preparation for mitosis 2) Mitosis a) Prophase b) Prometaphase c) Metaphase d) Anaphase e) Telophase 3) Cytokinesis -completes the cell division by dividing the cytoplasm between daughter cells -eukaryotic cytokinesis has some similarities with E.coli cytokinesis *-refer to lab manual Cell Cycle Regulation -discovered by Hartwell, Hunt, and Nurse -using genetic and biochemical methods, they identified the molecules CDK (cyclin dependent kinase; kinase is a protein that phosphorylates other proteins) and cyclin that control the cell cycle in eukaryotic organisms -CDK and cyclin are key molecules that control and coordinate DNA synthesis, chromosome separation, and cell division -CDK and cyclin together drive the cell from one cell cycle phase to the next -a combination of slightly different versions of CDK and cyclin are used to govern the transition from one phase to another -cyclins and CDKs are the internal controls that directly regulate cell division -maturation promoting factor (MPF) is a heterodimeric protein composed of cyclin B or cyclin A and cyclin-dependent kinase (CDK1) that stimulates the mitotic and meiotic cell cycles -internal checkpoints stop the cell cycle if the stages are incomplete -external controls coordinate the mitotic cell cycle of individual cells within the overall activities of the organism -complexes of cyclin and CDK directly control the cell cycle -these complexes are regulated by a variety of mechanisms that respond to the cell's environment (internal and external) -CDKs add phosphate groups to target proteins and are activated when combined with a cyclin -a dephosphorylase will remove the phosphate later one -different cyclin-CDK combinations regulate cell cycle transitions at different checkpoints Internal Controls -are important because they create checkpoints -ensure that reactions of one stage are complete before the cycle proceeds to the next stage External Controls -very numerous -based on surface receptors that recognize and bind signals -peptide hormones and growth factors -cell-surface molecules -molecules of the extracellular matrix -binding triggers internal reactions that speed up, slow down, or stop cell division Cell Cycle Control: Cyclin / CDK Control -entry into M (nuclear division) is initiated by MPF (maturation-promoting factor; the cyclin-CDK complex) which has kinase and regulatory subunits -the level of the regulatory subunit (cyclin) determines MPF activity -cyclin activity increases during interphase; appears to accumulate slowly during interphase stages -there are many cyclins and cyclin-dependent kinases -different variants, in combination, regulate the different transitions in the cell cycle -the complex is degraded through the degradation of cyclin -proteins are targeted to the proteosome in order for degradation to occur - If cyclins control the cell cycle (via MPF), what controls cyclins? 1) Cyclin Concentration -cyclin increases during the cell cycle 2) CDK phosphorylation state -cyclin targets proteins that can be phosphorylated -e.g.: the kinase Wee1 (the phosphorylator) and the phosphorylase cdc25 (the dephosphorylator) control phosphorylation by controlling the level of CDK -CDK phosphorylates, but these proteins control the level of CDK by phosphorylating CDK -the enzymatic activity of CDK itself is controlled by phosphorylation 3) CDK Inhibitors -in yeast Sic1 inhibits CDK activity 4) Controlled Proteolysis -uses a proteosome to target ubiquitin (a small protein that you use to bind to other proteins which you want to remove) in order to degrade / remove cyclin 5) Subcellular Localization -localization of the proteins in
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