ULTIMATE COMPUTING - Quantum Consciousness Studies

Cytoskeleton/Cytocomputer 89
Figure 5.7: Microtubule mitotic spindles (wispy filaments separating
chromosomes (heavy rods) establishing daughter cell polarity during cell division.
Centrioles and MTOC are at nidus of each array. With permission from Marc
DeBrabander (1982).
MT vary markedly in stability and function. Structurally similar MT may be
highly stable, or fleetingly transient. This variability can result from differences in
tubulin chemistry, from effects of secondary proteins attached at specific points
on MT surface lattices (microtubule associated proteins: MAPs), local
cytoplasmic influences, membrane interactions and many other factors. Highly
stable MT are found in cilia and flagella in which they are assembled into
complex patterns and carry out their functions without disassembly. The other
extreme are MT within mitotic spindles which separate chromosomes and
establish daughter cell shape in cell division (“mitosis”—Figure 5.7). These
“labile” MT not only assemble before mitosis and disassemble afterwards, but
apparently undergo localized disassembly/reassembly during mitosis (“dynamic
instability”). In some animal cells and during plant cell mitosis, assembly of MT
subunits into MT appears to take place in the cytoplasm without any relation to a
particular organelle. In other cells, structures including centrioles, basal bodies,
centrosomes, and kinetochores facilitate assembly of microtubules at a specific
site and orientation. These structures are called microtubule organizing centers
(MTOC), and they consist of a pair of centrioles and a dense granular material. In
general, negatively charged ends of MT attach to the MTOC, and positively
charged ends grow distal to the MTOC, establishing cell polarity, shape and
orientation.
The MT assembly process may be tracked by parameters that crudely reflect
the polymer level such as turbidity or viscosity. Generally, there is first a lag
phase when no microtubules form, then a phase of exponential growth and finally
a stable plateau. At low concentrations, no microtubules are formed. Above a