cytoskeleton

Question 1

function of cytoskeleton

Answer 1

shape and modify response to environmental cues
intracellular and cell movement/ location of organelles
shape of cell

Question 2

what is the cytoskeleton made up of?

Answer 2

complex network made of microtubules, intermediate filaments and actin filaments

Question 3

function of microtubules associated with the cyotskeleton

Answer 3

organelle positioning

intracellular transport

Question 4

function of intermediate filaments

Answer 4

mechanical strength when cell is stretched

strengthens the nuclear envelope

Question 5

function of actin filaments

Answer 5

cell shape 
organelle shape 
involved in cell migration
helical polymers
flexible structure : 2D networks and 3D gels

Question 6

how does the cytoskeleton function

Answer 6

Cytoskeleton is dynamic

The various filaments are made of monomers that continually polymerise and depolymerize

Question 7

process of how the cytoskeleton works

Answer 7

The cell receives a signal (via receptors on cell membrane etc.)

Existing filaments in the cell depolymerize to form free monomers 

The monomers rapidly diffuse 

The monomers reassemble at a new site

Question 8

what do accessory proteins regulate?

Answer 8

nucleation: the site and rate of filament formation
de/polymerisation
function

Question 9

structure of actin filaments

Answer 9

Twisted chain of globular actin (G-actin, a protein monomer)

When the G-actin join together to form a filament/ polymer, it is called F-actin 

Thinnest form of cytoskeleton (7nm) 

Structural polarity 

Large number of ABP (actin binding proteins) in F-actin

Question 10

what are the three isomers of actin?

Answer 10

Alpha actin, mainly in muscle cells

Beta actin, mainly in non muscle cells 

 Gamma actin, mainly in non muscle cells

Question 11

what is the polymerisation of actin?

Answer 11

G-actin added at either end of chain

Length is determined by concentration of G-actin or presence of ABP

Question 12

ABP

proteins binding to monomers

Answer 12

Proteins that bind to monomers:

They determine if G-actin joins onto polymer chain  

Two types: 

Profilin – allows G-actin to join 

Thymosin – prevents addition of G-actin

Question 13

proteins that bind to filaments

Answer 13

o Actin bundling proteins: F-actin in parallel bundles (e.g. microvilli)
o Cross–linking proteins: F-actin maintained in a gel like mesh work
o F-actin severing proteins: break F-actin into smaller filaments
o Motor proteins: transport of organelles and vesicles along actin filament

Question 14

function of actin filament (muscle)

Answer 14

Interaction with myosin allows for muscle contractions

Arranged in paracrystaline integrated with different ABP

Question 15

non muscle cells containing actin filaments

Answer 15

Cell cortex: thin sheet under the plasma membrane

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

Question 16

cell migration

Answer 16

The cell pushes out protrusions at its front (lamellipodia & filopodia)

Actin polymerisation  

These protrusions adhere to the surface 

Integrins (link the actin filaments to the extracellular matrix surrounding the cell) 

Cell contraction and retraction of the rear part of the cell 

Interaction between actin filaments and myosin

Question 17

structure of intermediate filaments

Answer 17

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 tetramere 

Twists around each other to form rope structure

Question 18

IFBP

Answer 18

Fillagrin: binds to keratin filaments into bundles

Synamin and piectin: bind desmin and vimentin. This links IF to other compounds (e.g. actin and microtubules) 

Plakins: keep contact between desmosomes of epithelial cells

Question 19

function of intermediate filament in nucleus

Answer 19

Forms mesh structure

They line the inner face of nuclear envelope to strengthen it to provide attachment site for chromatin 

Disassemble and reform at each cell division as nuclear envelope disintegrates 

i.e. very different from the stable cytoplasmic intermediate filaments 

Process controlled by post translational modifications (mainly phosphorylation and dephosphorylation)

Question 20

Structure of microtubules

Answer 20

Hollow tubes made up of protein – tubulin

Stiff and thick 

Ach filament polarized (+ and – end) 

Dynamic  

(Dis) assembles in response to cell needs

Question 21

polymerisation of microtubules

Answer 21

Microtubules organizing centre (MTOC): specialized protein complexes where microtubule assembly starts

Centrosome: in perinuclear region: the MTOC of most cells 

Contains gamma-tubulin ring that initiates the microtubule growth 

Heterodimers of a and b tubulin constitute the microtubule 

It is a polarized growth (i.e. there is an end that grows faster (+end) than the other (-end)

Question 22

function of microtubules

Answer 22

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) 

Organizes position of organelles  

Hence, provides polarization of cells 

Directionality of filaments is vital