The cytockeleton

Question 1

Function of cytoskeleton

Answer 1

• Shape of cell
• Intracellular movement/location of organelles
• Modify cells in response to environmental cues
Cell movement

Question 2

Contents of cytoskeleton

Answer 2

• Actin filament
• Intermediate filaments

Micro tubules

Question 3

What does the cytoskeleton provide for?

Answer 3

• Shaping of the cell
• Intracellular movement of organelles

Cell movement

Question 4

Microtubules associated with the cytoskeleton

Answer 4

Organelle positioning

Intracellular transport

Question 5

Intermediate filaments associated with microtubules

Answer 5

Mechanical strength

Question 6

Actin filaments associated with microtubules

Answer 6

Cell shape
Organelle shape
Cell migration
Involved in cell migration
Helical polymers
Flexible structure: 2D networks, 3D gels

Question 7

How does the cytoskeleton work

Answer 7

• Cytoskeleton is dynamic
• The various filaments are made of monomers that continually polymerise and depolymerise
1. The cell receives a signal via receptors on the cell membrane
2. Existing filaments in the cell depolymerise to form free monomers
3. The monomers rapidly diffuse
4. The monomers reassemble at a new site

• Accessory protein regulate:
○ Nucleation: The site and rate of filament formation
○De/Polymerisation
○ Function

Question 8

Structure of actin filaments

Answer 8

○ Twisted chains of globular actin
○ When the G actin join together to form a filament, it is called F-actin
○ Thinnest form of cytoskeleton filaments(7nm)
○ Structural polarity
○ Large number of ABP(actin binding protein) in F-actin

Question 9

Three isomers of actin

Answer 9

○ α-actin found mainly in muscle cells
○ β-actin in non-muscle cells
γ-actin in non-muscle cells

Question 10

Polymerization of actin

Answer 10

○ G-actin added at either end of the chain
○ Length is determined by concentration of G actin or presence of ABP
Growth is faster at the barbed length

Question 11

ABP

Answer 11

Protein binding to monomers
○ They determine if G-actin joins together onto polymer chain
○ Two types:
§ Profilin: facilitates actin polymerization
Thymosin: prevents the addition of actin monomers to F actin.

Question 12

Proteins that bind to filaments

Answer 12

Actin bundling proteins: F-actin in parallel bundles
Cross linking proteins: F actin maintained in a gel like mesh work

F actin severing proteins: break F actin into smaller filaments
Motor proteins: transport of organelles and vesicles along actin filament

Question 13

Muscle

Answer 13

Interaction with Myosin allows for muscle contractions

Arranged in a paracrystalline structure and integrated with different ABP

Question 14

Non-muscle

Answer 14

Cell cortex: thin sheet under the plasma membrane

With myosin: cleavage of mitotic cells(D) (cytokinesis)

Question 15

Cell migration

Answer 15

1. The cell pushes out protrusions at its front
2. Actin polymerization
3. These protrusions adhere to the surface
4. Integrins( link the actin filaments to the extracellular matrix surrounding the cell
5. Cell contraction and refraction of the rear part of the cell
   Interaction between actin filaments and myosin.

Question 16

Structure of intermediate filaments

Answer 16

○ Each unit is made up of:
§ N terminal globular head
§ C terminal globular tail
§ Central elongated rod like domain
○ Units form stable dimers
○ Every 2 dimers form a staggered tetramear
○ Twists around each other to form rope structure

Question 17

list the IFBP

Answer 17

○ Fillagrin: Binds to Keratin filaments into bundles
○ Synamin and plectin: bind desmind and vimentin. This links IF to other compounds like actin and microtubules

Plakins: Keep contact between desmosomes of epithelial cells

Question 18

Function of intermediate filament in nucleus

Answer 18

○ Present in all nucleated eukaryotic cells
○ Form mesh structure
○ They line the inner face of nuclear envelope to strengthen it provide attachment site for chromatin
○ Disassemble and reform at each cell division as nuclear envelope disintegrates:
§ Very different from the stable cytoplasmic intermediate filaments

Process controlled by post transcriptional modifications

Question 19

Structure of Microtubules

Answer 19

○ Hollow tubes made up of a protein called tubulin
○ Stiff and thick
○ Ach filament polarized
○ Dynamic:
§ (Dis)assembles in response to cell needs

Question 20

Polymerization of microtubules

Answer 20

a. Microtubules organising centre: specialised protein complexes where microtubule assembly starts
b. Centrosome is the MTOC of most cells
i. Contains g-tubulin ring that initiates the microtubule growth
c. Heterodimers of a and b tubulin constitute the microtubule
d. It is polarized growth

Question 21

Functions of microtubules

Answer 21

· intracellular transport
· act like railway tracks on which molecular motors run
· different motors for different cargoes
· directionality of filaments is vital (each motor only moves in one direction)
· organises position of organelles
· hence, provides polarisation of cells
· directionality of filaments is vital