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Molecular BIO > 19-21. Cytoskeleton > Flashcards

19-21. Cytoskeleton Flashcards

1
Q

What is the primary function of actin filaments?

A

determine cell shape, movement, secretion, and endocytosis

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2
Q

What is the primary function of microtubules?

A
  1. position membrane bound organelles
  2. intracellular transport direction
  3. form centrioles/mitotic spindle
  4. form cilia/flagella
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3
Q

What is the primary role of intermediate filaments?

A
  1. mechanical strength

2. resist mechanical stress

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4
Q

What is the structure of actin filaments?

A
  1. two-stranded helical polymer
  2. compact and globular
  3. flexible
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5
Q

What is the structure of microtubules?

A
  1. long hollow cylinder
  2. formed by tubulin subunits
  3. one end attached to microtubule organizing center (centrosome)
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6
Q

What is the structure of intermediate filaments?

A
  1. rope-like

2. span cytoplasm and cell-cell junction to provide support

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7
Q

Which surface of cells absorbs nutrients?

A

apical

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8
Q

What quadruples the surface area in intestinal cells to increase absorption rate?

A
  1. microvilli which form from actin units
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9
Q

What allows cytoskeletal filaments to re-arrange rapidly?

A
  1. non-covalent bonds between protein subunits
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10
Q

What is a protofilament?

A
  1. long linear string of proteins joined end-to-end.

must be stacked next to one another to increase strenght. 1 is weak

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11
Q

A stacked formation of protofilaments is most stable, what is the result of staggered protofilament assembly?

A
  1. flexible, bending and stretching intermediate filaments.

2. yarn-like properties

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12
Q

Polymerization of an actin filament refers to what?

A
  1. assembly of actin or tubulin monomers into a polymer
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13
Q

What is nucleation?

A
  1. the initial step required for polymer assembly to occur.
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14
Q

What units combine to form actin nucleation site?

A

alpha, beta, gamma

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15
Q

Describe lag phase of filament formation.

A
  1. filament nucleation

2. rate-limiting step and formation of aggregate

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16
Q

Describe the growth phase of filament formation.

A
  1. elongation of polymer

2. rapid addition of monomers to nucleated filament

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17
Q

Describe the steady equilibrium phase of filament formation?

A

1.addition of new subunits equals rate of subunit disassociation

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18
Q

What is the critical concentration, and when does it occur?

A
  1. occurs during the equilibrium phase (steady state)

2. refers to concentration of monomers in solution

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19
Q

Tubulin is a heterodimer structure of what?

A

alpha, beta units non-covalently linked

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20
Q

Does tubulin (microtubules) have polarity?

A

yes. due to alpha/beta subunit addition

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21
Q

Does actin have polarity?

A

yes, head to tail arrangement causes polarity.

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22
Q

What is the plus-end?

A
  1. fast-growing end

2. consists of beta-tubulin or barbed end

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23
Q

What is the minus end?

A
  1. slow-growing, shrinking end

2. pointed end on tubulin

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24
Q

What will cause treadmilling?

A
  1. plus end addition is faster than minus end.
  2. plus end will remain with a triphophate nucleotide and minus end will remain with diphosphate nucelotide
  3. polymer remains a constant length as the plus and minus ends change at the same rate.
  4. predominates in actin filaments
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25
Q

What type of filaments is treadmilling predominant in?

A

actin filaments.

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26
Q

What is catastrophe?

A
  1. microtubule begins to shrink
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27
Q

What is the cause of catastrophe?

A
  1. nucleotide hydrolysis more rapid than subunit addition, causing the cap to be lost and microtubule fall apart
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28
Q

What is rescue?

A
  1. GTP units add to shrinking end, and can form a cap
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29
Q

What does rescue of a microtubule refer to?

A
  1. returning microtubule back to growth.
30
Q

What does the loss of GTP on microtubule filaments produce?

A
  1. curving effect on confromation of subunits leading to shrinking
31
Q

Where is dynamic instability most often associated with?

A

microtubules

32
Q

Describe the building of intermediate filaments.

A
  1. antiparallel pairing of a coiled-coil dimer.

2. forms overall tetramer when two dimers combine

33
Q

How is an intermediate formed?

A
  1. lateral packing of 8 parallel tetramers
34
Q

What is keratin and where is it most likely to be found in cells?

A
  1. diverse group of intermediate filaments providing mechanical support
  2. found in hair nails
  3. at desmosomes, or hemodesmosomes
35
Q

What are the most important features of cytoskeleton

A
  1. form from protofilaments
  2. cytoskeleton filament formation has nucleation as rate limiting step
  3. subunits are polarized
  4. treadmilling and dynamic instability are results of nucleotide hydrolysis
36
Q

What is responsible for determining the shape and movement of cells?

A

actin filaments in the cell cortex

37
Q

What are lamellipodia?

A
  1. flat protrusive regions of the PM from actin filament rearrangment
38
Q

What are filopodia?

A
  1. spike-like protrusions from PM via actin filament rearrangment
39
Q

What is the role of ARP2, ARP3 or the ARP complex?

A
  1. nucleates actin filaments at the (-) end

2. but it requires activation factor (inactive proteins without)

40
Q

What is the benefit of having ARP complex and activating protein present?

A

allows growth/elongation by bypassing the normal nucleation step (rate-limiting)
– used by listeria for movement

41
Q

When is the ARP complex most efficient at elongation?

A
  • 70 degree angle from existing filament, forming cross-linking
42
Q

What is the specific role of formin with actin filament growth?

A
  1. facilitates straight, un-branched growth

2. bind to (+) end, provides growth from this region

43
Q

What is the role of thymosin in actin polymerization?

A
  1. binds to increase soluble monomers

2. unable to associate with actin filament, increases cytosolic concentration of actin monomers

44
Q

What is the specific role of profilin with actin polymerization?

A
  1. binds to actin monomer, causing reduced affinity for thymosin
  2. allows monomer assembly into an actin filament
45
Q

What two proteins competitvely compete for binding with actin monomers?

A
  1. thymosin (polymerization inhibitor)

2. profilin (polymerization activator)

46
Q

Both MAP2 and Tau help stabilize microtubules from degradation. What is the difference between the two?

A
  1. MAP2: only has one point of contact, which increases the space in between adjacent tubules.
  2. Tau: two tubule binding points that reduce amount of free protein, allowing for tighter packing.
47
Q

What is a key protein in erythrocyte membrane skeleton assembly that prevents actin filaments from interacting with other proteins?

A

tropomyosin

48
Q

Cofilin binds to ADP regions of actin filament to promote degradation how?

A
  1. induces twisting of filament to weaken interactions between actin subunits
  2. tropomyosin offers protection against cofilin
49
Q

What protein is able to increase the rate of microtubule shrinkage?

A
  1. kinesin-13
50
Q

What mechanism does kinesin-13 use to cause catastrophe in microtubules?

A
  1. lower activation energy barrier of the protofilaments, causing the protofilament to spring apart and curve, allowing it to fall away as a monomer
51
Q

Alpha-actinin and fimbrin are both actin filament cross-linking proteins. How do they differ from one another?

A
  1. alpha-actinin has loose bundle formation to allow for myosin II to form contactile fiber
  2. fimbrin: packs tightly together to exclude myosin II
52
Q

What do cells require in order to extend membrane projections allowing them to crawl across solid surfaces?

A
  1. filamin which forms an actin gel
53
Q

What proteins are required to enhance the attachment between actin and PM?

A

ERM family (ezrin, radixin, moesin)

54
Q

What is the purpose of the head and tail regions of motor proteins?

A
  1. head region: binds to certains tracks and moves along them
  2. tail region: binds cargo for transportation
55
Q

Where does ATP bind MyosinII?

A

N-terminal region followed by coiled-coil region for heavy chain dimerization

56
Q

Where can the large amount of myosin heads be found?

A

on the tail region where tail-tail overlapping occurs

57
Q

Myosin II uses ATP hydrolysis to produce what?

A
  1. movement toward (+) end, causing contraction
58
Q

Kinesin

A

Contain tail binding site for membrane enclosed organelles

- motor domain at N-terminus and migrate to (+) end of microtubules

59
Q

Dynein

A

motor domain migrates to (-) end of microbtubules

  1. two types
    - cytoplasmic dyneins
    - axonemal dynein
60
Q

What is cystoplasmic dyneins used for?

A
  1. vesicle trafficking and golgi apparatus localization

2. 2 motor domains

61
Q

What are axonemal dyneins used for?

A
  1. rapid microtubule movement of cilia/flagella.
  2. 2-3 motor domains
  3. faster than cytoplasmic dyneins
62
Q

What are the different stages of cell crawling?

A
  1. polarization and protrusion
  2. Adhesion and traction
  3. Re-traction
63
Q

What occurs in the polarization/protrusion phase of cell crawling?

A
  1. actin-rich regions extend in front of cell.

front and back differ in make-up

64
Q

What occurs in adhesion/traction phase of cell crawling?

A

1.adherence to ECM, providing traction for movement over material

65
Q

What occurs in re-traction phase of cell crawling?

A
  1. adhesions disassemble at rear. majority of cell is drawn forward
66
Q

Filopodia characteristics

A
  1. migrating fibroblasts
  2. 1-D
  3. long core of actin filament
67
Q

Lamellipodia

A
  1. epithelial cell, fibroblast, neurons
  2. 2-D, sheet
  3. cross-linked mesh of actin
68
Q

Psuedopodia

A
  1. amoeba and neutrophil formation

2. 3-D with actin filament gel protrusion

69
Q

Activation of Rac-GTP from PI3kinase acts on what messengers to induce formation for cell migration?

A
  1. activate WASP–> lamellipodia formation

2. activates PAK–> filamin but inhibiits myosin light chain kinase (reduce myosin activity)

70
Q

What does the activation of Rho-GDP to Rho-GTP form?

A
  1. activation of Rho kinase and formins which induce formation of contractile actin bundles.
  2. cofilin is also inhibited

Molecular BIO (24 decks)

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