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Biological Sciences
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CYTOSKELETON: Taxol – inhibits cell division by slowing microtubule disassembly Cytoskeleton absolutely necessary for cell shape Made of microfilaments, intermediate filaments, and microtubules Intracellular Mechanical Environment: Newtonian irrelevant b/c of extreme viscosity/elasticity Viscosity: F=γv  v = F/γ (applying constant force results in constant speed) diffusion: larger proteins, organelles CANNOT diffuse due to random thermal motion due to presence of cytoskeleton sol’n: to use motor proteins to use cytoskeletal network! MICROTUBULES: A/B tubulin dimers long distance transport GTPase for polymerization MICROFILAMENTS: actin monomers support myosin-based motility/contractility ATPase structural/mechanical support INTERMEDIATE FILS: dimers non-polar  not used as tracks structural/mechanical function only most flexible PRINCIPLES OF POLYMERIZATION 1. NUCLEATION – RDS energetically unfavorable step 2. ELONGATION monomeric units are in tri-phospho form after addition, they are hydrolyzed to diphospho 3. TREADMILLING different critical concentrations (CC) at each end  allows depol. at one end, pol at other at intermediate concentration MICROTUBULES: protofilaments (linear) form rings brittle B always towards (+) end; A to (-) end B has GTPase activity GTP-binding will promote polymerization GDP binding: depol. (causes end of protofilament to “bow”)  splaying DYNAMIC INSTABILITY: alternating between GTP and GDP at plus end results in alternate growth (rescue) and shrinkage (catasprophe) rapid growth  A/B dimers used up  concentration dips below CC  slowed growth  (+) caps have enough time to hydrolyze GTP  CATASTROPHE (depol) concentration eventually regains above CC  RESCUE (pol.) (new cap fo
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