2 - The Cytoskeleton

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Role of cytoskeletal proteins?
provide information to ensure things are located in the right place
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3 types of skeleton? thickness?
Microfilaments (actin - 9nm), microtubules (alpha-beta tubulin dimer - 25nm), Intermediate flaments (various - 10nm)
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Intermediate cytoskeleton role?
is linked to hemidesmosome and so provides structural support
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Structure of microtubules?
13 protofilaments each with an alpha and beta dimer (GTP bound) of globulin, with a lumen in centre.
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Difference between the 2 ends of a microtubule?
Positive end has alpha-tubu;ine, negatve end has beta-tubulin
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Describe alpha and beta tubulin
a/b heterodimers form microtubules. Major constituents of microtubules. Found in all eukaryotes.
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Describe gamma tubulin
Major component of g-tubulin ring complex (y-TURC) recruited to microtubule organizing centres (MTOCs) - usually the centrosome. Found in all eukaryotes.
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What is the critical concentration (Cc)?
the threshold above which beta-tubulin dimers can form a polymer
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How does critical concentration and the plus and minu end differ? effect?
lower at plus end than minus end so microtubules tend to grow at the plus end -> growth
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What happens to GTP bound tubulin?
undergoes slow hydrolysis to GDP so that GDP bound b-tubulin (D form) is the predominant form in the lattice (at plus end, polymerisation is much faster than hydrolysis polymerising microtubules have GTP cap, at minus end = slower -> always in Dform
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what is treadmilling?
When the rate of polymerisation at the plus end is equal to the rate of depolymerisation at the minus end the polymer stays the same length but microtubule flux occurs
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Whta drugs cause microtubule stability?
taxol, colchicine, colcemid and nocodazole
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What is the MTOC?
the microtubule organising centre. In animal cells this is the centrosome (minus end of microtubules near MTOC, plus end at cell periphery)
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Describe the structure of the centrosome?
composed of two centrioles surrounded by pericentriolar material (centrosome matrix) to which g-tubulin ring complexes (g-TURCs) are associated. The g-TURCs are responsible for nucleation of microtubules
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The g-tubulin ring complex (g-TURC) structure?
is composed of a ring of 13 subuntis of g-tubulin onto which a,b tubulin dimers bind. In addition g-TURC contains several other accessory proteins which are also highly conserved in eukaryotes
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What does electron microscopy reveal about microtubule plus ends?
They ends look different under growing and retracting conditions. Elongating filaments are straight, whereas shrinking filaments have curved or frayed ends (protofilaments splay outwards when depolymerising but are prevented when polymerising)
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When is microtubule assembly favoured?
when microtubule is straight. Hydrolysis of b-tubulin bound GTP to GDP causes a conformational change in the a,b tubulin dimer which tenses the MT lattice. MAPs may promote assembly
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How is disassembly promoted?
protofilaments are preented from splaying outwards when microtubule is polymerising by the presence of the GFP-tubulin cap. CAP dissociation promoted explosive (catastrophic) depolymerisation. MAPs may promote disassembly
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Role of The cycle of microtubule polymerisation and depolymerisation?
essential for the dynamic instability of microtubules observed in vivo
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Describe key concepts of dynamic instability of microtubuls?
Assembly: rapid growth with GTP capped end -> catastrophe: accidental loss of GTP cap -> disassembly: rapid shrinkage -> rescue:regain of GTP cap (curling out stops) -> rapid growth again etc.
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WHta does microtubule flux/treadmilling do?
aids the establishment of chromosome bi-orientation during prometaphase and the speed at which sister chromatids separate at anaphase
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How can microtubule flux/treadmilling be observed?
observed by fluorescence speckle microscopy (FSM) - using GFP, speckles of fluorescence can be seen to moving polewards during prometaphase, metaphase and anaphase. However spindle microtubule length remains constant
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How do interphase microtubules change during mitosis?
become more unstable during mitosis and are re-arranged to form a bi-polar spindle which segregates sister chromatids (nuclear division) and determines the formation and placement of the cleavage furrow
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Role of astral microtubules? interphase MTs? kinetochore MTs?
astral MTs - link the spindle pole to the cell cortex, interpolar MTs inter-digitate at the centre of the spindle and kinetochore MTs (k-fiber) connect the spindle poles to the chromosome at the kinetochore
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Other cards in this set

Card 2

Front

3 types of skeleton? thickness?

Back

Microfilaments (actin - 9nm), microtubules (alpha-beta tubulin dimer - 25nm), Intermediate flaments (various - 10nm)

Card 3

Front

Intermediate cytoskeleton role?

Back

Preview of the front of card 3

Card 4

Front

Structure of microtubules?

Back

Preview of the front of card 4

Card 5

Front

Difference between the 2 ends of a microtubule?

Back

Preview of the front of card 5
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