4.3 - The cytoskeleton

Question 1

importance of cytoskeleton in eukaryotic cells?

Answer 1

provides structural support for cytoplasm and nucleus, performs transport within the cell and motility of the cell

Question 2

3 major components of the cytoskeleton (3)

Answer 2

1. microtubule
2. intermediate filament
3. microfilament

Question 3

intermediate filaments (3)

Answer 3

1. strong/ropelike (primarily structural role)
2. strengthen cells against mechanical stress
3. nuclear envelope supported by meshwork of intermediate filaments

Question 4

what proteins connect intermediate filaments and bridge the nuclear envelope?

Answer 4

linker proteins

Question 5

structure of intermediate filaments (3)

Answer 5

1. like ropes, made of long twisted strands of proteins
2. can elongate by addition of tetramer arrays to either end
3. confers very high tensile strength

Question 6

actin filaments (microfilaments) (4)

Answer 6

1. thin/flexible - allow animal cell to adopt variety of shapes
2. cortex rich in actin filaments underlies plasma membrane of most eukaryotic cells
3. associates with myosin to form contractile structure (muscle)
4. actin forms contractile ring during cell division (cytokineses)

Question 7

actin monomer

Answer 7

actin subunit: has cleft containing binding site for ATP/ADP

Question 8

role of ATP in actin polymerisation (2)

Answer 8

1. ATP bound to actin monomer hydrolysed to ADP as monomers assemble into filament
2. ADP can only dissociate with the actin monomer

Question 9

how does tread-milling occur in actin filament formation?

Answer 9

when ATP-actin adds to the plus end of an actin filament at the same rate ADP-actin is lost from the minus end

Question 10

Listeria motility model (2)

Answer 10

1. assembling actin filaments propel the bacterium
2. mechanism has many similarities/ shares several proteins with the dense network of lamelipodial actin filaments that push the plasma membrane forwards at the leasing edge of a locomoting cell

Question 11

actin association with myosin I (transport) (3)

Answer 11

1. myosin I has singular globular head that attaches to actin filament and tail that attaches to another molecule/organelle in the cell
2. arrangement allows head domain to move a vesicle relative to an actin filament (eg anchored to plasma membrane)
3. myosin I can bind to actin filament in cell cortex, pulling plasma membrane into new shape
   (head group always walks towards plus end of actin filament)

Question 12

actin association with myosin II

Answer 12

muscle contraction

Question 13

microtubule assembly

Answer 13

assemble from microtubule organising centres (MTOCs), to generate diverse configurations

Question 14

microtubules in non-dividing cells

Answer 14

centrosome microtubule organising centres

Question 15

microtubules in mitotic cells

Answer 15

2 spindle pole centrosome microtubule organising centres organised into 2 microtubule arrays

Question 16

microtubules in cilium/flagellum

Answer 16

microtubules assembled on a basal body microtubule organising centre

Question 17

microtubule structure (tubulin heterodimers) (3)``

Answer 17

hollow cylinders made of alpha-beta tubulin heterodimers:
1. single = cytoplasm
2. doublet = cilia and flagella
3. triplet = basal bodies and centrioles

Question 18

what controls microtubule stability?

Answer 18

GTP hydrolysis

Question 19

different types of GTP hydrolysis (microtubule stability) (2)

Answer 19

1. rescue (disassembly -> assembly) = rate of GTP-tubulin addition greater that rate of GTP hydrolysis (constant)
2. catastrophe (assembly -> disassembly) = rate of GTP hydrolysis (constant) greater than rate of GTP-tubulin addition

Question 20

microtubule cell polarity (2)

Answer 20

1. microtubules dynamically polymerise/depolymerise from the microtubule organising centre
2. capping proteins stabilise the (+) ends and polarise the cell

Question 21

transport microtubules (2)

Answer 21

1. kinesins
2. dyneins

Question 22

kynesins

Answer 22

mediate anterograde (forwards) transport towards microtubule (+) ends at the periphery

Question 23

dyneins

Answer 23

mediate retrograde (backwards) transport of organelles towards microtubule (-) ends at the microtubule organising centre

Question 24

dynein/kinesin action on melanosomes at high/low cAMP (second messenger molecule involved in intracellular signalling) (2)

Answer 24

1. low cAMP = melanosomes aggregated by cytoplasmic dynein
2. high cAMP = melanosomes dispersed by kinesin-2

Question 25

cytoskeleton role in cell division (3)

Answer 25

1. microtubules attach to chromosome kinetochores during cell division
2. mitotic spindle responsible for aligning chromatids at metaphase and separating them at anaphase
3. actin filaments form contractile ring that helps separate cell during cytokinesis

Question 26

taxol

Answer 26

important chemotherapy medication, binds and stabilises microtubules - stopping cell division

Question 27

role of cytoskeleton in cilia and flagella (2)

Answer 27

1. sell surface projections with central pair of singlet microtubules and nine outer double microtubules
2. axonemal dynein motors attach to the A tubule on one doublet producing force on B tubule of another to bend cilia and flagella