Cytoskeleton 1

Question 1

Where do intermediate microfilaments extend from and to in a cell?

Answer 1

From the nuclear envelope to the periphery of the cell

Question 2

What do intermediate filaments do?

Answer 2

They are responsible for maintaining cell structure integrity.
They are intermediate in size between actin filaments and microtubules

Question 3

What are microtubules synthesized by?

Answer 3

Alpha and beta tubulin heterodimers that are always present in that state in the cytosol (combine in head to tail fashion). A string of alpha-beta heterodimers make up one protofilament. The polarity of microtubules arises due to an alpha subunit being exposed on one end and a beta subunit being exposed on the other

Question 4

How many protofilaments make up a single microtubule?

Answer 4

13 side by side

Question 5

In vitro, growth of a microtubule occurs on which end?

Answer 5

Both the positive and negative ends, but both growth and disassembly occur much more rapidly from the positive end

Question 6

In vivo, growth of a microtubules occurs on which end?

Answer 6

Only the positive end, due to the negative end attaching to the centrosome

Question 7

What are stable microtubules?

Answer 7

These form the structural basis of the cilia that are present on flagella

Question 8

Do cilia and flagella microtubules undergo transient assembly and disassembly?

Answer 8

No

Question 9

In cilia, what structure facilitates the assembly of microtubules by nucleation?

Answer 9

Basal bodies

Question 10

What drives nucleation of microtubules on a centrosome?

Answer 10

Gamma tubulin rings (these are the assembly sites)

Question 11

Why don’t tubulin dimers assemble without nucleation centers?

Answer 11

The concentration of tubular dimers in the cell is below a threshold concentration needed for spontaneous assembly of protofilaments/microtubules. If their conc. were above this threshold, they would assemble without a centrosome

Question 12

What is dynamic instability of microtubules?

Answer 12

At any given time, microtubules will either be growing or shrinking with random probability of either. This phenomenon only occurs in intact cells in vivo

Question 13

How does dynamic instability of microtubules work?

Answer 13

The beta subunit of alpha-beta tubulin heterodimer has a GTP binding site. Tubular GTP adds to the head of the growing microtubules. Once it’s incorporated, GTP is hydrolyzed to GDP. As long as the GTP tubulin is binding faster than it is being hydrolyzed, growth of the microtubule will occur. If addition is slower than GTP hydrolysis, GDP tubulin will ‘peel back’ and be released from the microtubule, only to be reconverted to GTP tubulin for re-addtion to the cell

Question 14

Is GTP hydrolysis necessary for assembly or disassembly of a microtubule?

Answer 14

Disassembly

Question 15

Why might the destabilize nature of microtubules be necessary?

Answer 15

It is possible that because the microtubule composition of the cell varies so drastically between interphase and a dividing cell, having the capability to activate/deactivate rapidly is advantageous

Question 16

What is anterograde transport?

Answer 16

Microtubule movement of cargo from a neuron cell body (soma) to dendrites

Question 17

Are dynein and kesine ATP dependent?

Answer 17

Yes, both have globular head domains that are ATP dependent. Dynein moves to the negative end, Kenesin to the positive end.

Question 18

How fast do kinesin motors move along the axon?

Answer 18

1 micron/second (some can move 5 microns/second)

Question 19

What does the percessive nature of dynein and kinesin mean?

Answer 19

It means once they bind to the microtubule, they don’t disassociate readily

Question 20

How does kinesin move along a microtubule?

Answer 20

“Walking” fashion by moving one dimer at a time at the expense of one ATP (hydrolyzed) per step

Question 21

What does ‘beating’ of cilia accomplish?

Answer 21

it prevents settling of large particles on the cell

Question 22

When does the nuclear envelope break down during mitosis?

Answer 22

Pro-metaphase

Question 23

What is a midbody in relation to mitosis?

Answer 23

It is the remnant of the mitotic spindle that it wedged in the middle of the two dividing cells during the final stages of cytokinesis

Question 24

When do centrosomes duplicate?

Answer 24

Prophase

Question 25

Approximately how long it a mitotic cycle?

Answer 25

30 minutes

Question 26

Are interloper microtubules stabilized?

Answer 26

Yes, they are no longer undergoing dynamic instability

Question 27

Sister chromatids are glued together along their length by which protein?

Answer 27

Cohesion

Question 28

What is the first step in anaphase?

Answer 28

A protein called seperase is activated and degrades cohesion proteins that hold the sister chromatids together

Question 29

How does kinectchore shortening occur during anaphase without causing disassociation from the chromosome?

Answer 29

There is a sheath that grabs on to the end of the chromosome and the microtubule that prevents the microtubule’s disassociation from the chromosome chromatid even as tubulin is lost from its positive end

Question 30

What is the sliding event of interpolar microtubules during Anaphase B dependent on?

Answer 30

Kinesin. There is also a pulling force on the centrosomes that is driven by dynein motors that pulls them toward opposite ends of the cell