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Biology 2382B Lecture Notes - Colchicine, Paclitaxel, Microtubule Nucleation

Course Code
BIOL 2382B
Jessica Kelly

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Intro To The Cytoskeleton & Microtubules
What is The Cytoskeleton?
Cytoskeleton an intricate network of protein filaments that extend throughout the
Three Main Types:
Microtubules 25 nm diameter
Microfilaments 7-9 nm diameter
Intermediate filaments 10nm diameter
The functions of these cytoskeletal components often require energy…
Roles of The Cytoskeleton
Organelle/protein Trafficking
Cilia & Flagella Structure/Function
Mitosis & Cytokinesis
Muscle Contraction
Cell Adhesion
Cell Migration
Microtubules: Structure
A Polymer of
and b tubulin
Monomers 55kDa each
Have polarity (+/- ends)
Hollow tubes 25 nm diameter
and up to 100s of mm long (enough
to cross cell)

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b dimer is the subunit (8nm in length) polymerize end to end to form the
Dimer & protofilamet both have polarity ends are different (i.e. one ends in
the other ends in
+/- side refers to the polarity and also the speed at which these sides will grow
or polymerize at (+ end =
- end = a
13 protofilaments form a 25nm (diameter) hollow tube
8nm length of dimer allows prediction of how things are moving based on the
distance they move and the known size of the dimers
Not positive on how these tubes form but suspected that the protofilaments form a sheet
that rolls up on itself to form a tube but this is not certain
Dimeric Tubulin Subunit
Dimer is VERY stable remain as dimer once
formed whether in a microtubule or as
depolymerized as free dimers (i.e. extreme
conditions required to break dimer interaction)
a binds permanently to GTP can hydrolyze
GTP (i.e. can be bound to GTP or GDP)
subunits have a binding pocket for GTP
As polymer (protofilaments) grows…
’s GTP is
Polymerization energy is provided by this
subunit GTP hydrolysis…

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Arrangement of MT Protofilaments
Singlet MT most common & unstable (i.e. consistently grow/shrink) microtubule in
the cell & consists of 13 protofilaments in a hollow tube
Doublet MT addition of another 10 protofilaments (second tubule)…similarly a
Triplet MT can be formed with 20 protofilaments (2 new tubules) these structures are
much more stable
MT’s organize the interior of the cell…
Two Types:
Cytoplasmic MT’s that composes structure within the cytoplasm
Axonemal (i.e. cilia) MT’s that refer only to cilia and flagella (nothing to do with
Microtubule Organization
Centrosome - the major microtubule-organizing center (MTOC) in animal cells (not
found in plants)
Centrosomes contain centrioles
Centrioles made up of triplet microtubules and are part of the MTOC,
cylindrical structures oriented 90o from one another & surrounded by PCM
Pericentriolar Matrix/Material (PCM) an amorphous mass of protein which makes
up the part of the animal centrosome that surrounds the two centrioles
PCM contains proteins responsible for microtubule
nucleation and anchoring including γ-tubulin, pericentrin and ninein
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