Cytoskeleton 2

Question 1

muscle contraction

Answer 1

filaments don’t shorten, myosin heads ‘walk’ towards the plus ends of the actin filament, pulling z discs closer

Question 2

microtubules

Answer 2

long, hollow cylinders 25nm diameter, made up of tubulin Monomers
Rigid and straight
Grow from microtubule organising centre
Have polarity- minus and plus end. One end has alpha subunit exposed and the other a beta subunit exposed

Question 3

Assembly of microtubules

Answer 3

13 protofilaments form a cylinder
heterodimer (see picture) of alpha and beta tubulin
subunits stack to form hollow cylindrical microtubule
With GTP bound, which is never hydrolysed

Question 4

What does it mean to say microtubules are dynamic (/dynamic instability)

Answer 4

balance between assembly and disassembly
Cell contains a mixture of microtubules and free tubulin
Microtubules unstable allowing them to undergo rapid remodelling

Question 5

Where do microtubules grow from

Answer 5

centrosome- microtubule organising centre

Question 6

Centrosome

Answer 6

in cytoplasm. just outside nuclear membrane

Question 7

centriole

Answer 7

each centrosome consists of 2 centrioles at right angles to one another
Centrioles built from cylindrical array of 9 microtubules each with 2 partial microtubules attached

Question 8

gamma-tubulin

Answer 8

rings of y-tubulin surround pair of centrioles, act as nucleating sites which microtubules grow out of

Question 9

Where are actin filaments nucleated vs microtubules

Answer 9

actin at pM

microtubules nucelated from centre of the cell

Question 10

what controls growth of microtubules (dynamic instability)

Answer 10

GTP hydrolysis
If new subunits (which have GTP bound beta tubulin) are added quicker than beta tubulin hydrolyses GTP, a GTP cap will form.
When a GTP cap is present the microtubule end is stable so continues to grow.
If GTP is hydrolysed faster than new subunits are added the GTP cap is lost.
GDP bound beta tubulin at the end of a microtubule has a different conformation to GTP bound beta tubulin, weaker interactions with neighbouring subunits in the filament, so ends start to disassemble and peel away causing rapid shrinkage of the microtubule

Question 11

how can microtubules be stabilised

Answer 11

interacting proteins on surface of organelles/PM

Question 12

Functions of microtubules

Answer 12

cellular organisation, movement of organelles (travel along microtubules), cell polarity (apical and baselateral membrane different)
Cell division incl mitosis and meiosis
Cilia and flagella

Question 13

Motor proteins associated with microtubules

Answer 13

Dyenins and Kinesins move along microtubules
Kinesins towards plus end and Dyneins towards the minus
Both have 2 binding heads and a tail, and ATPase activity

Question 14

How does movement of motor proteins along microtubules work

Answer 14

ATP hydrolysis provides energy for conformation changes in the head that allows it to move along the microtubule in a cycle of release and rebinding
Tail regions of kinesins and dyneins is where ATP hydrolysis occurs
Also, tails interact with cargo that the motor proteins carry

Question 15

Kinesins and ER

Answer 15

ER pulled outwards away from centre of cell along microtubules by kinesins

Question 16

Dyneins and Golgi

Answer 16

Pull golgi in opposite direction, towards the centrosome, which is why golgi is found near centre of the cell

Question 17

Mitosis

Answer 17

Centrosome duplicates, move apart until at opposite sides of nucleus, then nuclear envelope breaks down.
As mitosis proceeds, microtubules grow out from each centrosome with plus ends growing towards equatorial plate (midway between poles) forming spindle fibres.
Chromosomes pulled apart by spindles

Question 18

Colchicine drug

Answer 18

binds free tubulin and prevents polymerisation into microtubules

Question 19

Taxol drug

Answer 19

binds tightly to microtubules and prevents them from losing subunits
dynamics important so non-functional

Question 20

Cilia and flagella

Answer 20

Grow out of basal bodies ( different type of microtubule organising centre)
Very stable microtubules
Same internal structure but cilia shorter than flagella
cilia extend from surfaces of cells and move fluid over a cell/move a cell through fluid

Question 21

Arrangement of microtubules in cilia and flagella

Answer 21

Motor proteins associated with microtubules- dyenin
Walk along an adjacent microtubule doublet, this drives bending of cilia and flagella.
Stable connections between pair of microtubules - nexin

Question 22

If stable connections (nexin) between microtubules were not present..

Answer 22

walking of the dynein along (towards minus end) neighbouring microtubule would cause sliding rather than bending (see useful diagram)