Lecture 12

Question 1

Describe microtubules

Answer 1

Largest
Composed of tubulin subunits
About 25nm in diameter

Question 2

Describe microfilaments

Answer 2

Smallest
Composed of actin subunits
7 nm in diameter

Question 3

Describe intermediate filaments

Answer 3

In-between
vary in composition
8-12 nm in diameter

Question 4

True or false: The cytoskeleton is dynamically assembled and can change in response to different stimuli

Answer 4

True

Question 5

What are the functions of cytoplasmic microtubules

Answer 5

Maintaining axons
Formation of mitotic and meiotic spindles
Maintaining or altering cell shape
Placement and movement of vesicles

Question 6

Where are axonemal microtubules found?

Answer 6

In structures such as cilia, flagella, basal bodies

Question 7

What is an axoneme?

Answer 7

Central shaft of a cilium or flagellum, highly ordered bundle of microtubules

Question 8

What are protofilaments?

Answer 8

Longitudinal arrays of polymers that create the hollow cylinders of microtubules

Question 9

What is a heterodimer of tubulin made of?

Answer 9

One alpha tubulin
One beta tubulin
Does not dissociate

Question 10

True or false: all dimers in the microtubule are oriented in different directions

Answer 10

False

Question 11

Protofilaments have

Answer 11

an inherent polarity

Question 12

Define tubulin isoforms

Answer 12

Slight variants of alpha and beta tubulin

Question 13

Describe the structure of cytopasmic microtubules

Answer 13

simple tubes (singlets) with 13 protofilaments

Question 14

What are doublets and triplets?

Answer 14

One 13-protofilament tubule (A tubule) and one or two additional incomplete rings (B and C tubules) of 10 or 11 protofilaments

Question 15

How do microtubules form?

Answer 15

Reversible polymerization of tubulin dimers in presence of GTP and Mg2+

Question 16

Define elongation

Answer 16

addition of more subunits at either end is called elongation

Question 17

Define nucleation

Answer 17

Dimers aggregate into oligomers allowing new microtubules grow

Question 18

Define lag phase

Answer 18

Process of nucleation is slow causing the formation of microtubules to be slow

Question 19

Which is faster: lag phase or elongation phase?

Answer 19

Elongation phase

Question 20

Define plateau phase

Answer 20

When mass of MT’s reaches a point where free tubulin is diminished, assembly becomes balanced by disassembly

Question 21

Microtubule assembly in vitro depends on

Answer 21

concentration of tubulin dimers

Question 22

Define critical concentration

Answer 22

Tubulin concentration at which microtubule assembly is exactly balanced by disassembly

Question 23

Microtubules grow when tubulin concentration

Answer 23

exceeds the critical concentration

Question 24

Which is the rapidly growing end of a microtubule?

Answer 24

Plus end

Question 25

Treadmilling is

Answer 25

addition of subunits at the plus end and removal from the minus end

Question 26

Treadmilling will occur if

Answer 26

free tubulin concentration is above critical concentration for plus end, but below that of the minus end

Question 27

Binding of cochicine causes

Answer 27

inhibition of tubulin into microtubules | promotion of microtubules disassembly

Question 28

Define antimitotic drugs

Answer 28

drugs that interfere with spindle assembly and thus inhibit cell division Useful for cancer treatment

Question 29

Taxol

Answer 29

binds to microtubules, stabilizes them causes depletion of tubulin subunits Stops cells dividing during mitosis Used for breast cancer

Question 30

What is the dynamic instability model

Answer 30

One population of Microtubules grows by polymerization at plus end, and another shrinks via depolymerization

Question 31

Growing MTs have GTP at the

Answer 31

plus ends

Question 32

Shrinking MTs have

Answer 32

GDP

Question 33

What prevents subunit removal?

Answer 33

GTP cap at plus end