Cell cycle and chromosome movement

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Cell cycle and chromosome movement
2013-12-10 00:58:14
Bio final

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  1. APC activation
    • -          APCCdc20  becomes activated prior to metaphase and ubiquitinates a key anaphase inhibitor called securin—so named because it secures the attachment between sister chromatids
    • o   the ubiquitination and destruction of securin at the end of metaphase reelase an active protease called separase
    • §  separase cleaves the Scc1 subunit of the cohesin molecule that holds sister chromatids together
  2. Cleavage
    • o   cleavage of cohesin triggers the separation of sister chromatids to mark the onset of anaphase
    • -          Near the end of mitosis, Cdc20 is inactivated, and the alternate adaptor, Cdh1, takes control of the APC’s substrate selection
    • o   When Cdh1 is associated with the APC, the enzyme completes the ubiquitination of cyclin B that was begun by APCCdc20
    • §  this destruction of cyclin leads to a drop in activity of the mitotic Cdk and progression of the cell out of mitosis and into the G1 phase of the next cell cycle
    • ·         if this destruction doesn’t occur, cells are arrested in the late stage of mitosis
  3. checkpoint
    • -          there is a checkpoint at the transition between metaphase and anaphase: spindle assembly checkpoint, which occurs when a chromosome fails to become aligned properly at the metaphase plate, causing delay of anaphase until proper positioning occurs
    • o   unattached kinetochores contain a complex of proteins, one being Mad2, that mediates the spindle assembly checkpoint
    • §  these proteins send a wait signal to prevent anaphase
    • §  only when properly attached does the wait signal turn off
  4. inhibition
    • -          Inhibition is achieved through direct interaction between Mad2 and the APC activator Cdc20.
    • o   when Cdc20 is bound to Mad2, APC complexes can’t ubiquitinate securing, keeping the sister chromatids together
    • -          Another signal may be the attachment of sister chromatids by microtubules from the same spindle pole, called syntelic attachment
    • o   this is resolved by Aurora B kinase which phosphorylates the protein substartes, destabilizing microtubule attachment and allowing the kinetochores to reattach 
  5. Chromosome movement
    __ of the chromosomal microtubules during anaphase was both a consequence and cause of chromosome movement; it involves two processes
  6. Microtubules undergo what...

    These combined activities lead to ...
    • -          The microtubules that comprise the chromosomal spindle fibers undergo depolymerization at both their minus and plus ends during anaphase
    • -          these combined activities lead to the movement of chromosomes toward the pole:
    • o   Depolymerization at the minus ends transports the chromosomes towards the poles due to poleward flux
    • o   depolymerization at the plus ends serves to “chew up” the fiber that is towing the chromosomes
  7. Force required
    • -          The force required for chromosome movement is provided by the release of strain energy as the microtubule depolymerizes
    • o   the released energy is utilized by the curled ends of the depolymerizing protofilaments to bias the movement of the bound heads of the Ndc80 complex
  8. Function of cell wall 
    Develop turgor pressure to provide shape and support
  9. Function of cell wall 
    Serves as a type of skeleton for the entire plant
  10. Function of cell wall 
    Protect the cell against damage from mechanical abrasion and pathogens, and they mediate cell-cell interactions
  11. Function of cell wall 
    Source of signals that alter the activities of the cells it contacts
  12. Types of macromolecules: hemicelluloses
    • Structure: Branched polysaccharides whose backbone consists of one sugar, such as glucose and side chains of other sugars, such as xylose
    • Function: bind to the surfaces of cellulose microfibrils, cross-linking them into a resilient structural network
  13. Types of macromolecules: pectin
    • Structure: heterogeneous class of negatively charged polysacchardies containing galacturonic acid
    • Function: hold water and form an extensive hydrated gel that fills in the spaces between the fibrous elements; upon release, they trigger a defensive response
  14. Types of macromolecules: proteins
    • Structure: there are numerous types
    • Function: mediate dynamic activities
  15. How Cell Walls arise
    • 1)      first, as a thin cell plate that forms between the plasma membranes of newly formed daughter cells following cell division
    • 2)      matures by incorporation of additional materials that are assembled inside the cell and secreted intot eh extracellular space
    • 3)      the primary walls are formed first and, as cellulose content increases, they become secondary walls composed of lignin