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Anatomy & Cell Biology
Course Code
ANAT 262
John Presley

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MicrotubulesExample of dynamic process is complete remodeling of a microtubule array during a mitosis changes from a network radiating through out a cell to a compact bipolar mitotic spindleAFs and MTs are polar structures with a plus end and a minus end that polymerize at different ratespolymerization is accompanied by NTP hydrolysis which renders AFs and MTs unstable related but not interdependent on each otherthis dynamic instability gives the ability to depolarize and polymerize rapidly which is a key feature of AF and MT functionex complete remodeling of MT network during mitosisMTs are essential for a broad range of cell functionsmitosissegregation of chromosomestransportmolecular motors kinesins and dyenincell motilitycilia and flagellacell shape and polarityaxon in neuronspositioning of membrane enclosed organelles ex Golgi and ERBeta III is only in neuronsThese different isoforms of alpha and beta tubulins show differences mainly in a short sequence of the 1018 C terminal amino acidsthese stick out from the MT and can interact with different accessory proteinsthe differences are tailored to mediate functions in difference cell types or different cellular domains MT looks like a straw it contains 13 protofilaments things do not go through the strawthe microtubules constantly grow and shrink until they stabilize cap or grab a chromosome microtubules are abundant in the brainmicrotubules depolymerize at 4 degreesA growth curve can be obtained by following the course of tubulin polymerization in vitrothe lag phase is the period during nucleation which promotes small oligomersthis lag phase does not occur in the cell because nucleation sites are already formedthe lag is followed by the growth phase rapid polymerization As the concentration of the free subunits declines equilibrium is reachedso the rate of addition of subunits to the polymer is equaled by the rate of dissociation of subunitsrate of addition involves only the triphosphate form or the T form of the subunit GTPconcentration of free GTP tubulin limits the addition of GTP tubulinfor MTs and AFs polymerization rates and depolymerization rates are not the sameTfree GTP tubulinconcentration is molessecond
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