Cytoskeleton Structure I

Question 1

List functions of the cytoskeleton.

Answer 1

Organize the cell-maintain correct shaped cells 
Move the cell
Rearrange the compartments
Support PM
Resist mechanical stress provide strength
Pull apart chromosomes
Split cells
Guide intracellular traffic

Question 2

Protofilaments are stable. TF

Answer 2

False they are thermally unstable because it is easy to remove a linear filament. Protofilaments bound side to side are stable.

Question 3

What filament is two stranded helical polymers, with compact and globular subunits, this is necessary for whole cell locomotion endocytosis and secretion.

Answer 3

Actin

Question 4

What forms a tube like structure and directs intracellular transport as well as determine the positions of membrane bound organelles?

Answer 4

Microtubules

Question 5

Where does one end of a microtubule always attach?

Answer 5

MTOC- Centrosome

Question 6

What provides mechanical strength, strongest filament, spans cell to cell junctions, rope like fibers staggered side to side?

Answer 6

Intermediate filaments

Question 7

Describe how actin is always changing?

Answer 7

Actin can help with crawling movement, pusing towards the leading edge to allow movement of whole cell. It also disassembles so the cell can change shape during cell division, the actin also forms a ring to split a cell in half during division, and then reforms.

Question 8

Describe how microtubules are changing?

Answer 8

They emanate from microtubule organizing center, but they can re arrange to form a mitotic spindle and separate chromosomes.

Question 9

What is an example of how actin forms stable structuers?

Answer 9

Actin forms the microvili in enterocytes- it is replaced every 48 hours, they also form part of the inner ear cells and last a life time.

Question 10

How is polarity maintained in intestinal cells by the cytoskeleton?

Answer 10

Microvilli, made of actin, project out on the apical surface to quadruple the surface area. Intermediate filaments attach to desmosomes and hemidesmosomes. Microtubules form tracks to get proteins to correct location. and the basolateral surface allows for transfer of nutrients to blood.

Question 11

What bond holds together cytoskeletal filaments/polymers?

Answer 11

Weak Noncovalent bonds

Question 12

How is actin formed?

Answer 12

Self assemble end to end and side to side.
Polymerization starts with Nucleation- the random collision of three G-Actin monomers. ATP actin monomers are added to both the slow growing and fast growing ends. T form actin stays bound to the plus end and hydrolysis lags. D form actin is ADP bound at the minus end and hydrolysis catches up.

Question 13

What is treadmilling?

Answer 13

Nucleoside hydrolysis leads to this. This occurs when sub units are added as quickly as they are removed. This can move an organelle down the filament.

Question 14

How are microtubules formed?

Answer 14

Tubulin subunits. Self association end to end and side to side. Polymerization occurs. The Beta tubulin on top binds to the bottom alpha tubulin. Also alpha alpha and beta beta side interactions. also has T and D form. T orm is GTP bound at the plus end and D form is GDP bound at minus end.

Question 15

What is catastrophe?

Answer 15

Microtubule going from growth to rapid shrinkage

Question 16

What is rescue?

Answer 16

Microtubule going from rapid shrinkage to growth

Question 17

What type of filament predominates dynamic instabiltiy?

Answer 17

Microtubule

Question 18

What causes bending in the microtubule?

Answer 18

Changing from GTP to GDP. tubulin GTP causes straight protofilaments to be produced but GDP causes a conformation change.

Question 19

Describe intermediate filament assembly?

Answer 19

Each monomer forms a coiled coil dimer with another. A pair of dimers associates antiparallel forming a staggered tetramer. NO ATP/GTP binding site. Two dimers are off set within each tetramer allowing for it to associate with another tetramer. Eight parallel tetramers form the filament.

Question 20

What is the rate limiting step of forming filaments?

Answer 20

Nucleation

Question 21

What are cytoskeletal filaments formed from?

Answer 21

protofilaments

Question 22

Define and differentiate nucleation, catastrophe and rescue?

Answer 22

Nucleation is the rate limiting step of forming actin and microtubule filaments. This is when a random collision of monomers needs to occur before polymerization can begin. Catastrophe predominates in microtubules and is when the filament goes from growth to rapid shrinkage. Rescue also predominates in microtubules and is when the filament goes from rapid shrinkage to growth.

Question 23

Differentiate actin filaments from microtubules and intermediate filaments.

Answer 23

Actin filaments are composed of G-actin, a compact globular monomer. They assemble end to end and side to side forming an alpha helical structure similar to mardi gras beads wrapping around each other. They help with movement of the cell. Microtubules are made from tubulin and form a hollow tube like structure composed of alpha and beta tubulin subunits. MT helps with intracellular trafficking. Intermediate filaments are formed from elongated fibrous subunits and composed in a staggered ropelike form which allows for bending. This provides mechanical strength.

Question 24

Describe treadmilling in actin filaments.

Answer 24

Treadmilling occurs when subunits are being added onto the plus end at the same rate they are being taken off at the minus end. An equilibrium has been met. This causes the subunits to shift down the filament similar to how and escalator works, an organelle can be placed on a subuint and carried down the filament.