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Anatomy & Cell Biology
Course Code
ANAT 262
John Presley

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Example of dynamic process is complete remodeling of a microtubule array during a mi-
tosis (changes from a network radiating through out a cell to a compact bipolar mitotic
AFs and MTs are polar structures with a plus end and a minus end that polymer-
ize at different rates...
-polymerization is accompanied by NTP hydrolysis which renders AFs and MTs unsta-
ble (related but not interdependent on each other)
-this dynamic instability gives the ability to depolarize and polymerize rapidly which is a
key feature of AF and MT function
-ex: complete remodeling of MT network during mitosis
MTs are essential for a broad range of cell functions:
-mitosis - segregation of chromosomes
-transport - molecular motors (kinesins and dyenin)
-cell motility - cilia and flagella
-cell shape and polarity - axon in neurons
-positioning of membrane enclosed organelles (ex: Golgi and ER)
Beta III is only in neurons
These different isoforms of alpha and beta tubulins show differences mainly in a short
sequence of the 10-18 C terminal amino acids
-these stick out from the MT and can interact with different accessory proteins
-the differences are tailored to mediate functions in difference cell types or different cel-
lular domains

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- MT looks like a straw, it contains 13 protofilaments
-things do not go through the straw
-the microtubules constantly grow and shrink until they stabilize (cap or grab a chromo-
- microtubules are abundant in the brain - microtubules depolymerize at 4 degrees
A growth curve can be obtained by following the course of tubulin polymerization
in vitro
-the lag phase is the period during nucleation which promotes small oligomers
-this lag phase does not occur in the cell because nucleation sites are already
-the lag is followed by the growth phase (rapid polymerization)
- As the concentration of the free subunits declines equilibrium is reached
-so, the rate of addition of subunits to the polymer is equaled by the rate of dissociation
of subunits
-rate of addition involves only the triphosphate form or the T form of the subunit (GTP)
-concentration of free GTP tubulin limits the addition of GTP tubulin
-for MTs and AFs, polymerization rates and depolymerization rates are not the same
T = free GTP tubulin....concentration is moles/second

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ss = steady state (just as much tubulin is being added as taken off = treadmilling)
-in the cell the minus end of the MTs is attached to the MTOC (centrosome) so we have
only the + end of cellular MTs to consider when discussing MT dynamics in vivo
-in the cell, MTs exhibit a property known as dynamic instability
-when a MT is growing it has a cap of GTP tubulin at its plus end because hydrolysis of
GTP in the subunits at the growing end cannot keep up with the rate of polymerization
-when polymerization slows down or decreases, the GTP cap is lost and rapid depoly-
merization takes place
-this is called dynamic instability of MTs and is driven by cycles of GTP hy-
as you add GTP tubulin on you get hydrolosis (growing) but once you run out of a pool
of GTP tubulin the microtubule shrinks
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