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Biology 360 > Topic 6 - Cytoskeleton > Flashcards

Topic 6 - Cytoskeleton Flashcards

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1
Q

What are the three types of cytoskeletal filaments?

A
  1. Intermediate filaments
  2. Microtubules
  3. Actin filaments
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2
Q

What is the purpose of the intermediate filaments?

A

Provide strength for the cell

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3
Q

What is the function of microtubules in the cell? (2)

A

Determine the positioning of organelles

Associate with motor proteins

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4
Q

What are the functions of actin filaments in the cell? (2)

A

Determine the shape of the cell surface

Responsible for whole cell locomotion

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5
Q

Cytoskeletal filaments would not be able to function without (broad term)

A

Accessory proteins

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6
Q

Intermediate filaments are only present in… (phyla, 3)

A

Vertebrates, nematodes, mollusks

In other words, “squishy” animals

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7
Q

Microtubules and actin filaments are only present in (taxonomic group)

A

Eukaryotes

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8
Q

Rank the general size in width of microtubules, intermediate filaments, and actin filaments from largest to smallest

A

Microtubules > Intermediate > Actin

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9
Q

Describe what cytoskeletal elements are important in mitosis and cytokinesis

A

Microtubules: form the mitotic spindle
Actin: forms a contractile ring to cleave cell during cytokinesis

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10
Q

When a cell is in motion (like a neutrophil chasing a bacterium), what part of the cytoskeleton is responsible for leading the movement of the cell?

A

Actin filaments

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11
Q

What element of a cell is responsible for a cell’s polarity?

A

The cytoskeleton (actin and microtubules, intermediate filaments are nonpolar)

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12
Q

Intermediate filaments adhere to (2) on the periphery of the cell

A

Desmosomes and hemidesmosomes

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13
Q

In non-polar cells, microtubules are arranged like…

A

Stars

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14
Q

In polar cells, microtubules are often arranged in this way…

A

Linear - helps create cell polarity

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15
Q

In polar cells (like epithelial cells), actin-dense areas on neighbouring cells are attached to each other via…

A

Adherens junctions

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16
Q

Why does the cell disassemble filaments and reassemble them as opposed to moving assembled filaments?

A

It is much harder to move an entire filament, but individual monomers can diffuse easily through the cell to produce continuous movement

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17
Q

Describe the + end of an actin or tubulin polymer

A

The fast-growing end

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18
Q

Describe the - end of an actin or tubulin polymer

A

The slow-growing end

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19
Q

What happens to a monomer of actin or tubulin when it goes from being free to being a part of a polymer?

A

It undergoes a conformational change

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20
Q

What are the 2 forms of actin and tubulin monomers?

A

T-form and D-form

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21
Q

When actin is in its T-form, it is bound to…

A

ATP

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22
Q

When tubulin is in its T-form, it is bound to…

A

GTP

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23
Q

When actin is in its D-form, it is bound to…

A

ADP

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24
Q

When tubulin is in its D-form, it is bound to…

A

GDP

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25
Q

An ATP/GTP cap is formed when…

A

Addition of subunits is faster than ATP or GTP hydrolysis

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26
Q

Monomers of actin and tubulin are added in the (1) form

A

T-form

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27
Q

Monomers of actin and tubulin dissociate from the polymer in the (1) form

A

D-form

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28
Q

Define the critical concentration of actin and tubulin monomers

A

The concentration at which the rate of polymerization is equal to the rate of depolymerization (addition = subtraction)

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29
Q

Define persistence length

A

Measures the stiffness of a filament: The minimum length of a filament at which random thermal fluctuations are likely to result in bending of the monomer

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30
Q

A small persistence length means…

A

The substance is flexible, even short lengths will bend in an energetically favourable way

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31
Q

What is the persistence length of actin?

A

20-40 um

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32
Q

In situations where actin is kept from bending, what is required to do this?

A

Accessory proteins

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33
Q

Which end of an actin monomer (+ or -) binds ATP?

A
  • end
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34
Q

Describe what is happening to actin polymers during treadmilling

A

Net addition on one end of the polymer and net loss at the other end

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35
Q

What is profilin?

A

A protein which associates with actin to PROMOTE POLYMERIZATION

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36
Q

What is thymosin?

A

A protein which associates with actin monomers to INHIBIT POLYMERIZATION

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37
Q

Is it true that actin monomers can bind both profilin and thymosin?

A

No, can only bind one at a time

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38
Q

What is “nucleation”?

A

The initial process of actin polymerization in which 3 actin monomers come together to form the beginning of an actin filament

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39
Q

Actin filament nucleation occurs most commonly at or near…

A

The plasma membrane

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40
Q

Actin filament nucleation is regulated by…

A

External signals, i.e. cell signalling pathways

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41
Q

Actin filament nucleation is specifically regulated by (2)

A

ARP complex or formins

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42
Q

When active, ARP complexes…

A

Enhance polymerization of actin

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43
Q

Which end (+ or -) of the actin monomer does ARP bind to?

A
  • end
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44
Q

When ARP binds to the - end of an actin filament, (1) is prevented

A

Depolymerization

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45
Q

when ARP binds to existing actin filaments, what happens?

A

Typically creates a branch 70 degrees off from the original branch - creates a meshlike 2D network of actin

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46
Q

Formins produce (branched/unbranched) actin filaments

A

Unbranched

47
Q

Describe the “anatomy” of a formin

A

2 subunits (a dimer), each hold 1 actin monomer

48
Q

Formins associate with the (+/-) end of an actin filament

A

+ end

49
Q

How can the rate of polymerization or depolymerization of cytoskeletal filaments be altered?

A

With the addition of capping proteins

50
Q

The two general forms of actin arrays in a cell are…

A

Bundles and networks

51
Q

Actin bundles are made of (branched/unbranched) filaments

A

Unbranched

52
Q

Actin networks are made of (branched/unbranched) filaments

A

Branched

53
Q

How are actin bundles spaced out?

A

With the addition of alpha-actinin between the actin fibers to space them out

54
Q

The presence of alpha-actinin in cytoskeletal arrangements allows this to come between actin fibres

A

Myosin-II

55
Q

To prevent myosin-II from entering between contractile bundles, what protein holds actin filaments close to each other?

A

Fimbrin

56
Q

Fimbrin is a (-imer)

A

Monomer

57
Q

Alpha-actinin is a (-imer)

A

Dimer

58
Q

A bundle of actin fibers will have this mixture of accessory proteins to properly space the fibers together

A

Trick question: there will only be either alpha-actinin or fimbrin, not both in a given bundle

59
Q

What is filamin?

A

An accessory protein which associates with actin filaments to hold networks of actin filaments together

60
Q

Filamin is a (-imer)

A

Dimer

61
Q

Myosin is found in all members of this taxonomic grouping

A

All eukaryotes

62
Q

In humans, how many distinct myosin families are there?

A

37

63
Q

All (except 1) myosin type walk towards the (+/-) end of an actin filament

A

+ end

64
Q

In order to produce “walking” movement, myosin uses…

A

ATP hydrolysis

65
Q

Describe the anatomy of a myosin molecule

A

2 heavy chains (with globular heads, site of ATP hydrolysis)
2 light chains (associate with the head region)

The a-helix “tails” of myosin associate with other myosin dimers and form a bundle

66
Q

What does it mean for a myosin bundle to be “bipolar”?

A

Myosin heads are oriented in opposite directions on either side of the “bare zone”

67
Q

What causes rigor mortis?

A

In the absence of ATP upon death, the body tenses up due to the tight bond between actin and myosin

68
Q

What stops rigor mortis?

A

Decomposition/breakdown

69
Q

In the actin-myosin binding cycle, how often (in %) is myosin bound to actin?

A

Only 5% of the time, need to be able to synchronize this movement

70
Q

How long (in nanometers) is each “step” of myosin-II?

A

8.5 nm

71
Q

The power stroke of the actin-myosin binding cycle is caused by…

A

The removal of ADP from the N-terminal head group of myosin

72
Q

What are the 3 sites of the troponin complex?

A 
I = Inhibitory
C = calcium binding
T = tropomyosin binding
73
Q

When there is no Ca++ present, is tropomyosin in or out of its groove?

A

Out of its groove - preventing the interaction of actin and myosin

74
Q

When Ca++ is PRESENT, is tropomyosin in or out of its groove?

A

In its groove - out of the way, allows interaction of actin and myosin

75
Q

Where are troponin complexes located in comparison to tropomyosin?

A

Troponin complexes at the end of tropomyosin molecules

76
Q

How many Ca++ ions bind to the troponin complex?

A

4

77
Q

What form of myosin carries cargo (like vesicles and organelles) along actin filaments?

A

Myosin V

78
Q

Are there myosins in plants?

A

Yes! In all eukaryotes

79
Q

The lever arm swing in Myosin V is this many nanometers long

A

30-40 nm

80
Q

What are the 2 major groups of proteins that interact with actin filaments?

A
  1. Motor proteins

2. Accessory proteins

81
Q

Tubulin is a (-imer)

A

Heterodimer: contains alpha and beta subunits

82
Q

GTP can only be exchanged from (alpha/beta) subunits of microtubules

A

Beta

83
Q

One strand of a mictotubule is called a…

A

Protofilament

84
Q

The only exception where an alpha is not next to an alpha in a microtubule is…

A

At the “microtubule seam”

85
Q

How many protofilaments are in a microtubule?

A

13

86
Q

What protein is required for nucleation of microtubules?

A

gamma-tubulin

87
Q

Nucleation of a microtubule starts at…

A

The microtubule-organizing center (MTOC) also known as the centrosome

88
Q

What acts as the “template” which creates microtubules?

A

The gamma-tubulin ring complex

89
Q

What makes up the gamma-tubulin ring complex?

A

7 gamma tubulins (each with 2 heads) and accessory proteins

90
Q

What is a centriole?

A

A short cylinder of modified microtubules and accessory proteins

91
Q

Microtubule complexes are very stable when arranged in a (1) configuration

A

Star/astral

92
Q

In a case where microtubules are linear to make a polar cell, centrosomes will be…

A

At one end of the cell

93
Q

Describe the MTOCs of plants and fungi

A

Have centrosomes (the complex that microtubules stem from) but do not have centrioles

94
Q

If a cell is fragmented and microtubules are cut off from the MTOC, what will happen?

A

Microtubules will reform a MTOC, but it will not have any centrioles

95
Q

Kinesin-13 and XMAP215 are examples of…

A

Microtubule binding-proteins, or MAPs

96
Q

What end of a microtubule (+ or -) do dyneins move towards?

A

The - end

97
Q

What are the largest and fastest motor proteins? (excluding the special myosin)

A

Dyenins: move at 14 um/s

98
Q

What energy source do motor proteins use?

A

ATP (do NOT use GTP)

99
Q

The “motor” of motor proteins is contained within…

A

The head region

100
Q

When in dimer form, intermediate filaments are (polar/nonpolar)

A

Polar

101
Q

When in tetramer form, intermediate filaments are (polar/nonpolar)

A

Nonpolar

102
Q

How many tetramers associate to form a intermediate filament unit?

A

8 tetramers, can be joined to other groups end-to-end

103
Q

What is the persistence length of intermediate filaments?

A

less than 1 um - very bendy!

104
Q

Intermediate filaments are stabilized by…

A

Strong lateral hydrophobic interactions

105
Q

Keratin, desmin, and neurofilaments are all kinds of…

A

Intermediate filaments

106
Q

Keratin is found in this cell type

A

Epithelial cells

107
Q

Neurofilaments are found in this cell type

A

Axons

108
Q

Desmin is expressed in…

A

Muscle, all three types of it!

109
Q

Do bacterial cells have a cytoskeleton?

A

Yes - they just don’t have exact same cytoskeletal elements, no intermediate filaments and have homologs for actin and myosin

110
Q

What is special about fly embryonic development

A

Many nuclei form and then cellularization occurs

111
Q

Tissue with many nuclei (like the fly embryo before cellularization) is called…

A

Synctium

112
Q

Does the S phase take longer in humans or prokaryotes?

A

Longer in humans

113
Q

What is the difference between mitosis and cytokinesis?

A 
Mitosis = 2 nuclei
Cytokinesis = 2 cells

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