11. Cytoskelton

Question 1

What is the Cytoskeleton (1pt)

Answer 1

The Cytoskeleton is the skeleton of the cell

Question 2

Describe the cytoskeleton (2pts)

Answer 2

The Cytoskeleton:

1. Helps the cell keep its shape and modifies it in response to environmental cues
2. Dynamic structure- responds quickly to environmental changes, interacts with other cells and so can change as a result.

Question 3

What is the cytoskeleton made up of (3pts)

Answer 3

Cytoskeleton is made up of 3 different polymers:

1. Microtubules- organelle positioning, intracellular transport
2. Intermediate filaments- gives the cell mechanical strength.
3. Actin filaments- cell shape, organelle shape, involved in cell migration from one point to another.

Question 4

Describe how the cytoskeleton is organised? (3pts)

Answer 4

1. The cytoskeleton is dynamic and is facilitated by its organisation.
2. The cytoskeleton is made from a balance of small soluble monomers or large filamentous polymers. This balance changes very quickly.
3. Dynamic does not mean it is not organised. It changes quickly and is regulated by accessory proteins.

Question 5

How does the constant change provide movement in the cytoskeleton? (3pts)

Answer 5

1. Cells react to external stimuli to know where to move.
2. When the signal is detected monomers form polymers and polymers dissociate to form monomers. This constant change provides movement.

Question 5

How does the constant change provide movement in the cytoskeleton? (3pts)

Answer 5

1. Cells react to external stimuli to know where to move.
2. When the signal is detected monomers form polymers and polymers dissociate to form monomers. This constant change provides movement.

Question 6

Describe Accessory proteins (3pts)

Answer 6

1. Control the site and rate of filament formation
2. Control the rate of polymerisation and depolymerisation
3. Control the function of polymerisation

Question 7

Describe Actin filaments (6pts)

Answer 7

1. Helical polymers made up of actin.
2. Made up of a series of units called G actin. The addition of G-actin forms F-actin.
3. Thinnest class of cytoskeleton filaments.
4. Presents structural polarity.
5. Associated with a large number of actin-binding proteins- filaments are maintained, polymerised and depolymerised by actin binding proteins.
6. There are 3 isoforms of G actin with different isoelectric points:
7. Alpha actin- found mainly in muscle cells.
8. B actin- found in non-muscle cells.
9. Y-actin- found in non-muscle cells.

Question 8

Describe actin Polymerisation (3pts)

Answer 8

1. Not random
2. The addition of G actin forms F actin. Actin joins at the plus end.
3. The length of the filament is determined by:
4. The concentration of G-actin
5. The presence of Actin-binding proteins.

Question 9

Describe how G actin levels are controlled? (2pts)

Answer 9

G actin levels are controlled by 2 actin binding proteins:

1. Profilin- facilitates actin polymerisation
2. Thymosin B4- prevents the addition of actin monomers to f actin.

Question 10

Describe how G actin levels are controlled? (2pts)

Answer 10

G actin levels are controlled by 2 actin binding proteins:

1. Profilin- facilitates actin polymerisation
2. Thymosin B4- prevents the addition of actin monomers to f actin.

Question 11

Describe actin binding proteins to filaments? (1pts)

Answer 11

Actin binding proteins keep F-actin in parallel bundles as in the microvilli observed in epithelial cells.

Question 12

Describe Cross linking proteins (1pt)

Answer 12

Cross linking proteins maintain F-actin in a gel like meshwork underneath the plasma membrane

Question 13

Describe F actin severing proteins (1pts)

Answer 13

Break F-actin into smaller filaments

Question 14

Describe motor proteins (Myosin) (1pt)

Answer 14

Transport of vesicles or organelles through actin filaments. Myosin are motor proteins that help vesicles to move and help sliding of actin filaments during muscle contraction

Question 15

Describe the function of the actin filament in the cell cortex? (1pts)

Answer 15

In the cell cortex actin filaments form a thin sheet between the plasma membrane

Question 16

Describe the function of the actin filament in the contractile bundles (1pts)

Answer 16

Allows parts of the cell to move

Question 17

Describe the function of the actin filament in the lamellipodia flopodia? (1pts)

Answer 17

Allows cell migration to occur

Question 18

Describe the function of the actin filament in the contractile ring? (1pts)

Answer 18

In cytokines the ring is made up of actin

Question 19

Describe cell migration in actin (3pts)

Answer 19

1. The cell pushes out protrusions at its front.
2. These protrusions adhere to the surface
3. Interaction between actin filaments and myosin occurs.

Question 20

Describe Intermediate filaments (5pts)

Answer 20

1. Gives the cell mechanical strength.
2. Toughest of the cytoskeletal filaments. it is resistant to detergents and high salts.
3. Rope like structure with many long strands twisted together and made up of different subunits.
4. Form a network throughout the cytoplasm joining up to cell-cell junctions (desmosomes).
5. Strengthens the nuclear envelope.

Question 21

Describe intermediate filament polymerisation (3pts)

Answer 21

Each unit is made up of:

1. N-terminal globular head.
2. C- terminal globular tail
3. Central elongated rod-like domain

• Units form stable dimers
• Every 2 dimers form a tetramer.
• Tetramers bind to each other and twist to constitute a rope-like filament .

Question 22

Describe intermediate filament binding proteins (IFBP)?

Answer 22

1. Main function is to give shape and strength
2. Mainly linkers of Intermediate filament structures
3. IFBP stabilise and reinforce intermediate filaments into 3D networks.
4. Examples:
5. Fillagrin- binds keratin filaments into bundles
6. Synamin and Plectin- bind desmin and vimetin. Link intermediate filaments to the other cytoskeleton compounds (i.e actin and microtubules) as well as to cell-cell contact structures (desosomes)
7. Plakins- keep the contact between desmosomes of epithelial cells.

Question 23

Describe the function of the intermediate filaments in the cytoplasm? (2pts)

Answer 23

In the cytoplasm they provide:

1. Tensile strength- this enables the cell to withstand mechanical stress.
2. Structural support by:
3. Creating a deformable 3D structural framework
4. Reinforcing cell shape and fix organelle localisation.

Question 24

Describe the function of the intermediate filaments in the nucleus (2pts)

Answer 24

1. In the nucleus intermediate filaments form a mesh along the inner surface of the nuclear envelope to strengthen it and provide attachment sites for chromatin. 2. Disassemble and reform at each cell division as the nuclear envelope disintegrates.

Question 25

Describe microtubules (4pts)

Answer 25

1. Hollow tubes made up from the protein tubulin. 2. Stiff ( 25nm)- thicker of the filaments 3. Each filament is polarised- i.e has direction 4. Dynamic structure

Question 26

Describe the polymerisation of microtubules (4pts)

Answer 26

1. Microtubule organising centre (MTOC) are specialised protein complexes from where the assembly of tubulin units start. 2. Centrosome is the MTOC in most of the cells. 3. Polarised growth- an end that grows faster on the + side rather than the - side. 4. There are 3 isomers of tubulin- Alpha, beta and gamma. 1. Alpha and beta= form heterodimers to constitute the microtubule 2. Gamma= responsible for forming ring complex for the starting point of polymerisation.

Question 27

What are the functions of microtubules? (3pts)

Answer 27

1. Intracellular transport 2. Organises position of organelles 3. Rythmic beating of cilia and flagella

Question 28

What is intracellular transport? (3pts)

Answer 28

1. Microtubules act like railway tracks on which molecular motors run 2. Directionality of filaments is vital- each motor only moves in one direction 3. Dynenin motor proteins moves cargo towards the minus ends of microtubules towards MTOC. Other side has kinesin which moves cargo towards the plus end, the growing end of microtubules.

Question 29

What is organising the position of organelles? (3pts)

Answer 29

1. Provides polarisation of cells. 2. Directionality of filaments is vital. 3. Microtubules allow vesicles to travel at very high speeds.

Question 30

What is the rhythmic beating of cilia and flagella? (5pts)

Answer 30

1. Motile processes with a highly organised microtubule core. 2. Core consists of 9 pairs of microtubules around 2 central microtubules. 3. Bending of the cilia and flagella is driven by the motor protein Dyenin. 4. The basal body controls the assembly of the axoneme. 5. Examples: 1. Cilia in the respiratory tract sweeping mucus and debris from lungs 2. Flagella on spermatoza