Cell Cycle, Microtubules, Cell Cycle Control, Histones

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Biochemistry & Molecular Bio.
David Gross

Corey Reed February 18 • 4 phases of cell cycle o S-phase: DNAsynthesis o M-phase: mitosis o G-phase: growth & checkpoints • Mitosis o Prophase – DNAduplicated & condensed to sister chromatids o Metaphase – sisters align, attach to mitotic spindle o Anaphase – sisters separate, pulled apart by spindle o Telophase – chromosomes package into separate nuclei o Cytokinesis – cell separates into 2 • 3 different microtubules o astral microtubules – from the centrosome outwards, not connected to another microtubule o kinetochore microtubules – connect to the kinetochore of the sister chromatids o interpolar microtubules – extend from the centrosome, connect to another interpolar microtubule extending from opposite centrosome • Microtubules o grow by addition of tubulin monomers onto + end of filament o when not in cell cycle, animal cells have 1 centrosome, it nucleates most of cytoplasmic microtubules o G1/S-Cdk promotes centrosome duplication which forms mitotic spindle during mitosis • M-Cdk activity driving mitosis o causes phosphorylation of nuclear pore complexes & structural proteins of nuclear envelope o generates mitotic spindle by promoting separation of centrosome  G1-S-Cdk signals duplication of centrosome o increases microtubule dynamics so they can capture chromosomes • Chromosomes moving to midplane o prometaphase starts w/ nuclear envelope breakdown o chromosomes attach to spindle microtubules via kinetochores o during metaphase chromosomes aligned @ equator o kinetochore microtubules attach sisters to opposite spindle sides • Microtubule dynamics o dynamics increase when M-Cdk active (entering M-phase) o undergo search & capture for chromosomes o one astral microtubule attaches, chromosome slides toward that pole o astral microtubule converts to kinetochore microtubule o free microtubule from opposite spindle captures opposite side of kinetochore, chromosome moves toward equator • Mitotic spindle forces o kinesin-14, minus-end directed, pulls centrosomes together o kinesin-5, plus end directed, pushes centrosomes apart o M-Cdk phosphorylates kinesin-5 extends mitotic spindle o dynein motors anchor spindle to plasma membrane • kinetochore assembly o anchoring site for microtubules on centromere o plus end of microtubule inside kinetochore o shrinking microtubules pull kinetochore o microtubules move toward spindle poles b/c of depolymerization of plus end, subunits of kinetochore move toward minus end o polar ejection force motors push chromosomes away from spindle poles • unattached chromosomes o detected by tension loss o attachment of microtubules to one chromosome end rapidly release o components of outer kinetochore phosphorylated by kinases present closer to inner kinetochore  kinases only phosphorylate proteins when they’re very close  when chromosome stretched, outer kinetochore moves away from kinase, becomes less phosphorylated  phosphorylated outer kinetochores don’t bind microtubules well • APC/C activation o single unattached chromosome still activates spindle assembly checkpoint, preventsAPC/C activation o APC/C causes degradation of M-Cyclin • Cell Cycle Summary o biochemical switched control entry into diff. phases  upregulation of S-Cdkinitiates replication & M-Cdk activation to initiate mitosis  phosphorylation (cyclins/Cdk) & degradation (APC/C & SCF ubiquitin ligases) control diff. steps  mitogens & DNAdamage control cell cycle • Chromatin o complex of DNA& protein (half the proteins are histones & half not) o heterochromatin are compact w/ little gene expression o euchromatin are less compact w/ lots of gene expression o each DNAmolec. compacted into mitotic chromosome 10,
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