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Cytoskeleton Flashcards

1
Q

What is the cytoskeleton?

A

A network of protein filaments throughout the cytoplasm that is important for supporting a large volume of cytosol. It is highly dynamic and responsible for cell shape and movement.

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2
Q

What are some of the functions of the cytoskeleton?

A

Mitosis, cytokinesis, trafficking, support, allowing the sperm to swim, white blood cells to crawl, muscle contraction, formation of axons/dendrites, cell shape and growth of plant cell wall.

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3
Q

What are the three types of cytoskeletal filament?

A

Intermediate filaments, microfubules and actin filaments.

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4
Q

What are the size of intermediate filaments?

A

10nm.

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5
Q

What are the size of microtubules?

A

25nm.

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6
Q

What are the size of actin filaments?

A

7nm.

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7
Q

What are the roles of the intermediate fibres?

A

Provide tensile strength for cells.

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8
Q

What types of cells are intermediate fibres particularly abundant mean?

A

Cells that are subject to mechanical stress such as muscle cells or epithelial cells.

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9
Q

How do intermediate filaments form in cells?

A

They form a network throughout the cytoplasm, surround the nucleus and extend out to the cell periphery. They are often anchored at the plasma membrane cell junctions.

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10
Q

What are the three main classes that intermediate filaments can be grouped into?

A

Keratin filaments in epithelial cells, vimetin and vimetin related filaments in connective tissue cells, muscle cells and supporting cells of the nervous tissue (neuroglial cells) and neurofilaments in nerve cells.

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11
Q

How are intermediate filaments constructed?

A

They are made up of monomers with a central rod domain and a globular region at either end. The monomers dimerize.

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12
Q

How are all of the cytoskeletal structures constructed?

A

Smaller protein subunits oligomerise (join together) to form filaments.

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13
Q

What are the monomers that make up intermediate filaments?

A

Globular N and C termini with a long alpha-helical region in between.

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14
Q

What do the alpha helices in intermediate filaments made up of?

A

100’s of amino acids.

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15
Q

What is the name of the structure that is formed when the two monomers of intermediate filaments dimerize?

A

Coiled-coil dimers.

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16
Q

Where is the N terminus found and what is it made up of?

A

At the start of the polypeptide chain, it is an amino group.

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17
Q

Where is the C terminus found and what is it made up of?

A

At the end of the polypeptide chain. It is a carboxy group.

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18
Q

What happens after the dimers are formed in the formation of intermediate filaments?

A

Two line up to form a staggered tetramer. They line up with opposite ends closest to each other.

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19
Q

What happens after the tetramers are formed in the formation of intermediate fibres?

A

Tetramers can pack together end to end with the opposite termini interacting with each other.

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20
Q

What is the diameter of the rope structure that is formed by the tetramers in intermediate fibres?

A

10nm.

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21
Q

What are keratins and what do they do?

A

They are a type of intemediate filament found in the epithelia that span the interior from one side to the other. They indirectly connect to filaments of other cells through cell-cell junctions that are called desmosomes.

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22
Q

How do keratins connect to filaments of other cells?

A

Cadherins from one cell directly make contact with cadherins from another cell.

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23
Q

What are cadherins?

A

Transmembrane proteins that span the bilayer and interact with plaque proteins on the cytosolic side of membranes.

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24
Q

Why does keratin indirectly link cells together?

A

As plaque proteins interact with keratin filaments that interact with cadherins.

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25
What is an example of an intermediate filament disorder?
Epidermolysis bullosa simplex that is a rare genetic disorder where keratin cannot form normal filaments in the epidermis and the skin is very vulnerable to mechanical injury.
26
What benefits do intermediate filaments provide?
They provide strength to prevent rupturing of cells when stretched.
27
What is nuclear lamina?
Intermediate filaments that lie beneath the nuclear membrane. They are extracellular matrix proteins.
28
What provides the stability of intermediate filaments?
They have extensive protein-protein contacts. The individual contacts are not strong, but when all put together they are.
29
What cells are actin filaments found?
In all eukaryotic cells.
30
What is the diameter of actin filaments?
7nm.
31
What are actin filaments made up of?
Globular monomers that associate head to tail. They are unstable without associated proteins.
32
How are the actin filaments formed?
G-actin monomers form the filament (F-actin) in the presence of ATP, Mg and K (or calcium ? slides are unclear). The concentration of G-actin is important - needs to be above the critical concentration.
33
What is special about the growth of actin/polymerisation?
ATP is carried by actin monomers that is hydrolysed to ADP after assembly into the filament, but the monomer is less stable when ADP is bound. It will be released from the chain and can regain ATP and join again. The actin chain is continually being added to and degraded.
34
What is the rate of assembly when there is a high concentration of salts?
There is a high concentration of monomeric actin and the rate of assembly is greater than the rate of disassembly.
35
What happens when the concentration of the monomeric actin drops?
The rate of disassembly equals the rate of polymerisation. There is no net growth.
36
What is difference between actin filaments and intermediate filaments?
Actin filaments have polarity whereas intermediate filaments do not.
37
What are some of the proteins that might bind to actin to modify its properties?
Monomer binding proteins, nucleating proteins, cross-linking proteins, bundling proteins and motor proteins.
38
What are some of the drugs that act on the actin cytoskeleton and what they do?
Cytochalasin D binds to the positive end of F-actin and prevents further addition of G-actin. Phalloidin (from poisonous mushroom) binds to F-actin and prevents actin filaments from depolymerising.
39
What are the functions of actin?
Mechanical strength and cell shape, cell crawling, muscle contraction and organelle movement.
40
Where is actin usually found?
In a layer just below the plasma membrane.
41
How are actin filaments linked into a meshwork?
Actin binding proteins.
42
What is the function of cortical actin?
Provide mechanical strength and cell shape.
43
What is cell crawling and how does it work?
Cells use internal contractions to pull itself forward due to extensions of the plasma membrane called filopodia/lammelipodia.
44
What is the mechanism of cell crawling with lammelipodium?
There is actin polymerisation at the front of the cell, causing extension of the lammelipodium. There is attachment to the extracellular matrix that acts as an anchor point. Contraction at the opposite end of the cell pushes the cell forward.
45
What provides the energy to move actin filaments?
Myosin, which is an actin-dependent motor protein. It can bind and hydrolyze ATP to provide energy for the movement of the actin filament.
46
In what direction to myosins move?
From the negative end to the positive end of actin filaments.
47
What is the first stage in the cycle of myosin heads and actin filaments?
The myosin head laks ATP and is locked onto the actin filament.
48
What is the second stage in the cycle of the myosin head and actin filaments?
ATP binds and a conformational change occurs. The affinity of the myosin for the filament is reduced.
49
What is the third stage in the cycle of the myosin head and actin filaments?
ATP is hydrolysed to ADP and Pi and the head is displaced along the filament.
50
What is the final stage in the cycle of the myosin head and actin filaments?
There is binding to a new actin filament site that causes the release of inorganic phosphate and tight binding to the actin. There is a power stroke and the head regains the original conformation, and ADP is released. The myosin is now bound further away than it was originally.
51
What happens to the Z discs in muscle contraction?
They are pulled closer together.
52
What are microtubules?
Long, hollow cylinders made up of tubulin monomers. They are more rigid and straight than intermediate filaments or actin filaments.
53
How do microtubules form?
They grow out from a microtubule organising centre (MTOC).
54
What is an example of a MTOC?
Centrosomes in animal cells.
55
Do microtubules have structural polarity?
Yes.
56
What are the two types of tubulin?
Alpha and beta.
57
How does the tubulin arrange to form the microtubules?
The alpha and beta stack together to form a cylindrical microtubule.
58
How many subunits make up the circumference of the microtubule?
13.
59
What tubulin is exposed at the positive end of a microtubule?
Beta tubulin.
60
What is a microtubule subunit?
A subunit made up of one alpha tubulin and one beta tubulin.
61
How do the subunits add to the positive end of the microtubule?
The alpha unit binds to the exposed beta unit to create a structure with another exposed beta unit.
62
What is the difference between bound GTP in alpha and beta tubulin?
Alpha tubulin ATP provides a structural function to the molecule, whereas the beta tubulin can by hydrolysed after assembly.
63
What is dynamic instability?
When microtubule are constantly assembled and deassembled.
64
Where is the centrosome located?
The centre of cells.
65
What forms a centrosome?
A cylindrical arrangement of 9 microtubules (gamma tubulin) with 2 partial microtubules attached.
66
What determines the growth of microtubules?
GTP hydrolysis.
67
What happens if new subunits are faster than beta tubulin hydrolyses GTP?
A GTP cap will form which allows the microtubule to continue growing.
68
What happens if GTP is hydrolysed faster than new subunits are added?
The GTP cap is lost. GDP bound beta-tubulin has a different conformation to GTP bound and there are weaker interactions with the neighbouring beta tubulin. The ends disassemble and there is shrinking of the molecule.
69
What is the name for rapid shrinkage of a microtubule end?
Catastrophe.
70
How could a shrinking microtubule start to grow again?
GTP islands along the lengths of the microtubules could allow this.
71
What can prevent dynamic instability of microtubules?
Interacting proteins.
72
What are the functions of microtubules?
Cellular organisation, movement of organelles, cell polairty, cell divison, mitosis/meiosis, and cilia and flagella.
73
What are the motor proteins involved in microtubules?
Kinesins and dyneins.
74
What motor proteins move towards the positive end of microtubules?
Kinesins.
75
What proteins move towards the negative end of microtubules?
Dyneins.
76
What is the basic structure of the motor proteins associated with microtubules?
They both have two ATP binding heads and a tail, and have ATPase activity.
77
What drives the movement of motor proteins along microtubules?
ATP hydrolysis.
78
What drug interferes with polymerisation of microtubules and how?
Nocodazole. The golgi is pulled towards the centrosome and microtubules disassemble.
79
What direction do microtubules face in axons?
The negative ends are orientated towards the cell body.
80
What motor protein transport material to the termini in axons?
Kinesins.
81
What motor protein transports material to the cell body in axons?
Dyneins.
82
What happens to the centrosome before mitosis?
The centrosome duplicates and it moves to opposite sides of the nucleus.
83
What happens to the centrosomes as mitosis proceeds?
Microtubules grow out of the centrosome, with the positive ends growing towards the equatorial plate to form spindle fibres.
84
What drugs interfere with microtubule assembly?
Colchicine and taxol.
85
What do the drugs that interfere with microtubule assembly do?
Colchicine binds to free tubulin to prevent polymerisation into microtubules and Taxol binds to microtubules to prevent them from losing subunits. Both are anti-mitotic.
86
What is the primary function of cilia?
Move fluid over a cell or a cell through a fluid.
87
What is the difference between cilia and flagella?
They are similar in their internal structure, but flagella are usually much longer.
88
Where do cilia and flagella grow out from?
Basal bodies.
89
What is the cycle that cilia undergo?
Power stroke followed by recovery stroke.
90
What allows cilia and flagella to bend?
Nexin allows the bending rather than sliding and dynein walks along the microtubule.

BB10005 (22 decks)

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