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Cell Division

Cell Division. Morphological changes in M-phase due to protein phosphorylation, dephosphorylation Chromosome condensation: histone NEBD: nuclear lamins Cytoskeletal rearrangement(spindle, contractile ring): caldesmon, c-src. Centrosome cycle. Formation of mitotic spindle pole

joelle-beasley
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Cell Division

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  1. Cell Division Morphological changes in M-phase due to protein phosphorylation, dephosphorylation Chromosome condensation: histone NEBD: nuclear lamins Cytoskeletal rearrangement(spindle, contractile ring): caldesmon, c-src

  2. Centrosome cycle • Formation of mitotic spindle pole • Independent to nuclear cell cycle • S-phase: centriol replicate • Prophase: centrosome split & move apart • Prometaphase: NEBD mt from each controsome to enter nucleus, & interact with chromosome  spindle pole

  3. Centriol replication

  4. The centrosome cycle Aster formation Polar MT formation

  5. Six steps in M-phase prophase prometaphase metaphase anaphase telophase cytokinesis

  6. Mitosis in an animal cell

  7. Time course for mitosis

  8. Prophase • Chromosome condensation: form 2 sister chromatids held together at centromere • Centrosome split & move apart • Dynamic microtubules: Half life of MT decreased 20X

  9. Prometaphase • Centrosome segregate to the pole • NEBD at early prometaphase • Enables mitotic spindle to interact with chromosome • Formation of mitotic spindle • Kinetochore MT: orientation and movement of chromosomes • Kinetochore act as cap that protect + end from depolymerizing • Centrosome at spindle pole protect – end from depolymerizing

  10. Mitotic spindle

  11. Formation of bipolar mitotic spindle Dynamically unstable + end + end overlap MAP (motors) stabilizes

  12. Separation of two spindle poles in prophase

  13. Kinetochore • Developed from centromere • MT attaches in metaphase • Consist of a specific DNA sequence • Large mutiprotein complex, platelike trilaminar structure • Human; 20-40 MT, yease; 1 MT • A puzzle: how MT & kinetochore connected to each other • * hold on to a MT end, • yet allow that end to add or loose subunits

  14. Centromere in the yeast

  15. Yeast kinetochore

  16. Metaphase • Kinetochore MT align chromosome in metaphase plate • MT are dynamic

  17. Aster exclusion force • The origin is not known • Aligning chromosomes at the spindle Evidence for an astral ejection force

  18. How to align the chromosomes in metaphase plate -> Balanced bipolar force

  19. A model for the centrosome-independent spindle assembly

  20. How MT & kinetochore connected to each other • Microinjection of labeled tubulin: • metaphase; incorporate tubulin near kinetochore • anaphase; reverse action at same site • Puzzle: • Hold on to a MT end, yet allow that end • to add or loose subunits • Sliding collar based model

  21. Microinjection of labeled tubulin: • - metaphase; incorporate tubulin near kinetochore • - anaphase; reverse action at same site

  22. Anaphase • Paired kinetochore separate –> separation & segregation of chromatid • Start abruptly by specific signal • Signal may be intracellular Ca2+ rise: • 1) Rapid, transient 10X increase Ca2+ at anaphase in some cells • 2) Injection of low level of Ca2+ into metaphase cell ->premature anaphase • 3) Accumulation of Ca2+ containing membrane vesicle at spindle pole • 4) Clamp Ca rise by EGTA, BAPTA -> arrest anaphase • **mechanism of Ca2+ rise during anaphase is a mystery • Anaphase A • shortening of kinetochore MT -> poleward movement of chromatids • no energy required for shrinking of kinetochore • Anaphase B • elongation of polar MT -> two spindle poles move further apart • ATP hydrolysis is required for elongation of polar MT; kinesin ATPase • drug chloral hydrate inhibits Anaphase B not A • pulling aster MT -> -end moter binds cell cortex & aster MT • -> pull spindle pole apart

  23. Chromatid separation at anaphase

  24. How kinetochore hold on to a MT end, • yet allow that end to add or loose subunits Motors as anchors

  25. Motor proteins in anaphase B

  26. A model for how motor proteins may act in anaphase B

  27. Telophase • Chromatids separate completely • Kinetochore MT disappears • Polar MT elongate still more • Nuclear envelope reassemble • Nucleoli reappear

  28. Cytokinesis • Begins at anaphase • Cleavage furrow occurs in the plane of metaphase plate, • right angle to the long axis of the mitotic spindle • Aster is responsible for cleavage furrow position • contractile ring: assembles in the early anaphase (actin & myosin II) • myosin dephosphorylation triggers cytokinesis • Midbody: bridge between two cells, contains polar MT • organelles partitioned with no special mechanisms • mitochondria, chloroplasts; grow, fission -> # doubles • ER, Golgi; fragmentation, vesiculation -> even distribution • unequal segregation of cell components • C. elegans “p-granules” to posterior -> form germ line cells • (independent to MT, but dependent on actin filament) • styela yellow cresent (myoplasm) to vegetal -> form muscle • (microfilament first phase, MT second phase)

  29. Asters signal to the cortex to initiate a cleavage furrow

  30. An asymmetric cell division of the nematode egg

  31. Spindle rotation Asymmetric cell division

  32. The contractile ring

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