Introduction to Molecular and Cell Biology, Biol. 220 Lecture 27: Actin
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Microtubule structures are responsible for various cell movements:

• Beating of cilia and flagella.
• Transport of membrane vesicles in the cytoplasm.
• Extension of surface membrane.
• Alignment and separation of chromosomes during meiosis and mitosis.

Microtubules are made up of polymerized alpha and beta tubulin (Fig. 19.2a).

Fig. 19.2a  Microtubule subunits.

• alpha tubulin binds GTP irreversibly.
• beta tubulin hydrolyses GTP and exchanges GDP for GTP.
• dimers of alpha and beta subunits bind head to tail (end to end) and associate 13 per single circumference of a microtubule (Fig. 19.2b).
  Fig. 19.2b  Microtubule organization.
  
• protofilaments can fuse 10 subunits laterally to form doublet or triplet filaments (Fig. 19.3).
  Fig. 19.3  Various structures of tubulin protofilaments.
  

Short lived and dynamic microtubules.

• cytosolic microtubule network -> spindle fibers during mitosis and miosis.

Stable microtubules.

• axoneme in the flagellum.
• axons in nerve cells.

Microtubules assemble from organizing centers and have a constant orientation relative to MTOCs.

• Flagellum or cilium (Fig. 19.7a) Crossection (Fig. 19.28a).
  Fig. 19.7a
  
• Fig. 19.28a  Crossection of Flagella.
  
  
• Mitotic organizing center (Fig. 19.7b).
  Fig. 19.7b
  
  
• Nerve cell axon (Fig. 19.7c).
  Fig. 19.7c
  
  
• Vesicle transport (Golgi oriented MTOC).

Gamma-tubulin complex nucleates polymerization of butulin subunits (Fig. 19.8b).
Fig. 19.8b  Initiation of tubulin polymerization begins at gamma tubulin.