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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
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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
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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
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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
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Chromosome movement
__ of the chromosomal microtubules during anaphase was both a consequence and cause of chromosome movement; it involves two processes
Depolymerization
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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
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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
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Function of cell wall
1
Develop turgor pressure to provide shape and support
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Function of cell wall
2
Serves as a type of skeleton for the entire plant
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Function of cell wall
3
Protect the cell against damage from mechanical abrasion and pathogens, and they mediate cell-cell interactions
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Function of cell wall
4
Source of signals that alter the activities of the cells it contacts
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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
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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
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Types of macromolecules: proteins
- Structure: there are numerous types
- Function: mediate dynamic activities
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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
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