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  1. 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
  2. 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
  3. 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
  4. 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
  5. 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
  6. 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
  7. MAPS
    • 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
  8. MAPS
    -function and control
    • Function: increase stability of microtubules and promote their assembly
    • How are they controlled? phosphorylation and dephosphorylation 
  9. 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
  10. 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
  11. 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 
  12. 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
  13. 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
  14. 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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