Cytoskeleton

Question 1

What is the cytoskeleton and why does the cell require it?

Answer 1

A complex network of protein filaments and tubules that extend throughout the cytoplasm to provide:
Structural integrity,
Organisation
And stability.

It keeps the cell in shape and also modifies it in response to the environmental cues.

Question 2

Is the cytoskeleton a dynamic structure or is it stationary?

Answer 2

It is a dynamic structure.

Question 3

What is the cytoskeleton made up of? Name the 3 different polymers? What are their individual functions?

Answer 3

Its a complex network of 3 polymers:

1. Actin
2. Intermediate Filaments
3. Microtubules

Question 4

Are the polymers which make up the cytoskeleton covalently linked?

Answer 4

NO.

Question 5

What 3 things do accessory proteins regulate?

Answer 5

1. Site and rate of filament formation (nucleation)
2. Polymerisation/depolymerisation
3. Function

Question 6

What’s the name of the monomer of actin?

Answer 6

G-actin

Question 7

What is F-actin?

Answer 7

A twisted chain of G-actin monomers

Question 8

Actin is the thinnest class of the cytoskeleton filaments. How thin are they (in nm)?

Answer 8

7nm.

Question 9

Actin chains are associated with many proteins (like how DNA is associated with histones). What are these proteins called? how many are there and what are they?

Answer 9

Actin-binding proteins (ABP) - 6.

1) Profilin
2) Thymosin beta 4
3) actin bundling protein
4) cross linking protein
5) F-actin severing protein
6) motor proteins, myosin.

Question 10

There are many forms of the actin monomer (from the first question on actin molecules). What are the 3 isoforms (with different isoelectric points) and where are they mainly found?

Answer 10

o Alpha-actin found mainly in muscle cells.

Beta-actin and gamma-actin found in non-muscle cells.

Question 11

How do actin filaments grow?

Answer 11

G-actin add onto the +ve end of the f-actin

Question 12

What 2 things determine the length of the actin filament?

Answer 12

1. [G-actin]

Presence of ABP (actin binding proteins)

Question 13

Monomer actin levels are controlled mainly by 2 proteins. Name the 2 proteins and their main functions.

Answer 13

1. Profilin - increases the rate polymerisation

Thymosin beta 4 –prevents the addition of G-actin to F-actin. (so slows the growth of actin filaments).

Question 14

What do Actin bundling proteins do??

Answer 14

keeps filamentous actin in parallel bundles

Question 15

What do Cross linking proteins do?

Answer 15

maintains polyamorous actin in a gel-like meshwork

Question 16

What do f-actin severing proteins do?

Answer 16

breaks polyamorous actin into smaller filaments

Question 17

What do Motor proteins, myosin do?

Answer 17

oversees transporting vesicles or/and organelles through actin filaments

Question 18

How are actin filaments arranged in non-muscular cells?

Answer 18

o Form thin sheath beneath plasma membrane.

o Associated with myosin.

Question 19

Actin-myosin rings are involved in which cell cycle process? What does it cause?

Answer 19

Cytokinesis
o Accumulates b/w the poles of the mitotic spindle beneath the plasma membrane.
o Ring contracts and forms an indentation/cleavage furrow, dividing the cell into 2 new daughter cells.

Question 20

Describe the process of cell migration – it is a multistep process – include all the proteins involved in this process.

Answer 20

1. Cell pushes protrusions at its front. Lamellipodia and filopodia (cytoplasmic projections) form, and Profilin is present here to increase the rate of polymerisation.
2. Protrusions adhere to surface. – Integrins link actin filaments to extracellular matrix surrounding cells.
3. Cell contraction and retraction of rear part of the cell. – Interaction b/w actin filaments and myosin.

Question 21

They surround the nucleus. How does this affect the strength of the nuclear envelope?

Answer 21

1. Strengthens it nuclear envelope.

2. Provides attachment site for chromatins.

Question 22

Structure of Intermediate Filaments

Answer 22

1. N-terminal globular head
2. C-terminal globular tail
3. Central elongated rod-like domain 48nm long
   These units form stable dimers.,
   8-12nm wide
   Toughest filament
   constitute “rope-like” structure.

Question 23

How many units in the rope like structure?

Answer 23

2 filaments= dimer
2 dimers= tetramer
8 tetramers= rope like structure

Question 24

Intermediate filaments can be found in 2 main areas. Name the 2 areas where they can be found and the associated filaments in the given areas.

Answer 24

1. CYTOPLASM ;
   I. Keratins – in epithelia
   II. Vimentin and vimentin-related – in connective tissue, muscle cells and neurological cells.
   III. Neurofilaments – in nerve cells
2. NUCLEUS
3. Nuclear lamins – in all nucleated cells (obviously)

Question 25

What does IFBP stand for? And what is its main function?

Answer 25

Intermediate filaments binding proteins (IFBP). They: • Are linkers of IF structures. • Stabilise and reinforce IF into 3D networks.

Question 26

What are the 3 IFBP?

Answer 26

o Fillagrin o Synamin and Plectin o Plakins

Question 27

Function of fillagrin

Answer 27

Binds keratin filaments into bundles

Question 28

Function of Synamin and Plectin

Answer 28

- Binds desmin and vimetin | - Links the IF to the other cytoskeleton compounds as well as to cell-cell structures.

Question 29

Function of Plakins

Answer 29

Keeps the contact between the desmosomes of the epithelial cells

Question 30

What are the functions of the IF in the cytoplasm?

Answer 30

1. Provides tensile strength • Allows the cells to withstand mechanical stress (to stretch!). 2. Provides structural support • Creating a deformable 3D structural framework. Reinforcing cell shape and fix organelle localisation.

Question 31

How can the IF support the structure of the cell? - remember SOS

Answer 31

Provides structural support • Creating a deformable 3D structural framework. • Reinforcing cell shape and fix organelle localisation. (Structure, Organisation, Strength)

Question 32

What are the functions of the IF in the nucleus?

Answer 32

1. To strengthen it | To provide an attachment site for chromatin.

Question 33

What is the difference in the shape and order of the IF structure when in the nucleus and when in the cytoplasm?

Answer 33

1. In the nucleus, they form a “mesh-like” structure, rather than the “rope-like” structure in the cytoplasm. 2. Line the inner face of the nuclear envelope, rather than being present throughout (in cytoplasm). Also, cytoplasmic IF join up cell-cell junctions (desmosomes). 3. In nucleus, they disassemble and reform at each cell division, as a nuclear envelope disintegrates. Process controlled by post-transcriptional modifications. – (mainly phosphorylation and dephosphorylation).

Question 34

Are microtubules dynamic? If yes, how so? If no, how does this affect the stability of the cell?

Answer 34

Yes, because they assemble and disassemble in response to cell needs.

Question 35

Is each microtubule polymer polarised or is it uncharged? If they are charged, where would the opposing charges be found on the chain?

Answer 35

Polarised – the opposing charges are found either end of the polymer.

Question 36

What is the ratio of the monomer to the chains in the cell?

Answer 36

50:50 monomer to chain in the cell.

Question 37

What is the centrosome?

Answer 37

(in the perinuclear region) – cytoplasmic region just around the nucleus. • Is the MTOC in most cells. • Contains gamma-tubulin ring – initiates the microtubule growth.

Question 38

Structure of Micrtotubules

Answer 38

>Hollow tubes made of tubulin >Polarized >Width 25nm (thickest of cytoskeleton structures >relatively stiff

Question 39

What are the functions of microtubules? Remember SOS!

Answer 39

1)Intracellular transport Like motorways for molecular motors Motors can only travel in 1 direction 2)Organises position of organelles causes polarisation of cells 3)Rhythmic beating of cilia 9 microtubule pairs around axenome(2 microtubules) Dynein is the motor protein that allows for bending

Question 40

There are 2 main molecular motors. In which they run?

Answer 40

Motor proteins; Dynein and Kinesin. • Dynein – moves cargo towards the –ve end of microtubules. • Kinesin – moves cargo towards the +ve ends of microtubules.

Question 41

Give 2 examples of where flagella and where cilia can be found.

Answer 41

1. Cilia in the respiratory tract, sweeping mucus and debris from lungs. 2. Flagella on spermatozoa

Question 42

Types of tubulin

Answer 42

alpha beta gamma

Question 43

Once growth is initiated, what process continues the growth?

Answer 43

Alternating addition of alpha and beta tubulin to the structure (mostly in the +ve) end.