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Structure: wall
- Wall of each is composed of globular proteins arranged in longitudinal rows (protofilaments) that are aligned paraleel to the long axis of the tubule
- - actually contains 13 of these protofilaments side by side in a circular pattern held by noncovalent interactions
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Structure of protofilament
Protofilament: assembled from alpha and beta dimers, which together are a dimer and are arranged in a linear array along length of protofilament
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Structure of dimer
- - each dimer has polarity and each protofilament has the same polarity
- o one end is plus, which is terminated by a row of beta-tubulin subunits
- o the other is minus and is terminated by a row of alpha-tubulin subunits
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Stability of microtubules
- - some can be labile, like those of the mitotic spindle, or highly stable
- disassembly can be induced by cold temp, hydrostatic pressure, elevated calcium ion concentrations, and chemicals
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Function: 1``
- Structural support and organizer:
- - Can resist forces that might compress or bend the fiber; mechanical support; can determine shape of the cell
- - In plants: maintain cell shape through influence of formation of cell wall
- In animals: maintain internal organization of cells
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Function 2
- Agents of Intracellular Motility
- - serve as tracks for a variety of motor proteins that generate the forces required to move objects within a cell
- ex: axonal transport and neurotransmitter travel down the axon and RNA transport down stretch of extended cytoplasm
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MAPS
structure
- Heterogeneous class of proteins
- Structure: one domain attaches to side of microtubule and other projects outward as a tail from the microtubule surface; can be seen as cross-bridges connecting microtubule to each other, maintaining their parallel alignment
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MAPS
-function and control
- Function: increase stability of microtubules and promote their assembly
- How are they controlled? phosphorylation and dephosphorylation
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MTOCs
- Involved in nucleation (slow) and elongation
- - nucleation takes place rapidly inside a cell, where it occurs in association with the MTOC
- Centrosome: site of microtubule nucleation
- - contains two centrioles surrounded by PCM
- o microtubules don’t actually penetrate into this region
- o it is the PCM that initiates formation of microtubules
- § centrioles are not directly involved in microtubule nucleation, but recruit the PCM during assembly and in centrosome duplication
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Centrioles
- o contains nine evenly spaced fibrils, each which appears as a band of 3 microtubules, and it is connected to the center of the centrioles by a radial spoke; arranged in a pattern that gives a centrioles a pinwheel appearance
- o always found in pairs at right angles
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Polarity of microtubules
- - polarity of microtubules: minus end associated with the centrosome and the plus end at the opposite tip
- o microtubules are nucleated at the MTOC and elongated at the opposite end
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Centrosome of non-polarized versus polarized
- - the centrosome of a non-polarized cell is situated near the center of the cell and tends to remain associated with the minus ends of microtubuels
- - in polarized cells, the microtubules are anchored by their minus ends at dispersed sites near the apical end of the cell as their plus ends extend toward the basal surface
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Basal bodies and other MTOCs
- outer microtubules in a cilium or flagellum are generated as outgrowths from the microtubules in the basal body, which are identical in structure to centrioles and can give rise to one another
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Function of all MTOC
- - function: control the number of microtubules, their polarity, the number of protofilaments that make up their walls, and the time and location of their assembly
- all contain gamma tubulin
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