14.3 Microtubules

Question 1

microtubules are rigid hollow rods; composed of a single type of globular protein called ()

Answer 1

tubulin

Question 2

microtubules generally consist of ()

Answer 2

13 linear protofilaments assembled around a hollow core

Question 3

protofilaments are composed of () → microtubules are polar structures with distinct plus and minus ends

Answer 3

head-to-tail arrays of tubulin dimers (composed of ⍺ and β tubulin)

Question 4

while both ⍺ and β tubulins bind GTP, GTP bound to β-tubulin is ()

Answer 4

readily hydrolyzed

Question 5

with stabilized minus ends and rapid GTP hydrolysis of plus ends, microtubules go through alternate cycles of growth (rescue) and shrinkage (catastrophe) → a behavior known as ()

Answer 5

dynamic instability

Question 6

growth or shrinkage of microtubules is determined in part by (1) in relation to (2)

Answer 6

1. rate of tubulin addition
2. rate of GTP hydrolysis

Question 7

microtubule () occurs when new GTP-bound tubulin dimers are added more rapidly than GTP is hydrolyzed

Answer 7

growth (rescue)

Question 8

during microtubule growth (rescue), it retains a () at plus end from which growth continues

Answer 8

GTP cap

Question 9

microtubule () occurs if GTP hydrolysis is faster than polymerization

Answer 9

shrinkage (catastrophe)

Question 10

() results in rapid depolymerization of the microtubule

Answer 10

GDP-bound tubulin dissociation

Question 11

growth or shrinkage of the plus ends is regulated by () that favor either polymerization or depolymerization of tubulin dimers

Answer 11

microtubule-associated proteins (MAPs)

Question 12

what are the 3 classes of MAPs

Answer 12

1. polymerases
2. depolymerases
3. CLASP proteins

Question 13

MAPs that bind to microtubule plus end and stimulate growth

Answer 13

polymerases

Question 14

MAPs that are the opposite of polymerases and accelerate dissociation of GTP-tubulin from plus ends

Answer 14

depolymerases

Question 15

MAPs that suppress microtubule catastrophe and promote rescue by stopping disassembly of microtubules and restarting assembly

Answer 15

CLASP proteins

Question 16

in animal cells, most microtubules extend outward from the (), located adjacent to the nucleus in interphase

Answer 16

centrosome

Question 17

in animal cells, the centrosome plays a key role as a () in determining the intracellular distribution of microtubules in animal cells

Answer 17

microtubule-organizing center

Question 18

the centrosome serves as the initiation site for the () in animal cells

Answer 18

assembly of microtubules

Question 19

what is the key protein in the centrosome (1)? it forms the (2), which is a seed for rapid microtubule growth

Answer 19

1. γ-tubulin
2. γ-tubulin ring complex

Question 20

() are cylindrical structures containing 9 triplets of microtubules organized around a central cartwheel-like structure

Answer 20

centrioles

Question 21

centrosomes of most animal cells contain centrioles, oriented perpendicular to each other and surrounded by ()

Answer 21

pericentriolar material

Question 22

in nerve cells, both () extend from the cell body supported by microtubules

Answer 22

axons and dendrites

Question 23

how are microtubules in nerve cells different?

Answer 23

microtubules in nerve cells are released from the centrosome with both plus and minus ends terminating in the cytoplasm (stabilized by capping proteins)

Question 24

in axons, microtubules are oriented with their plus ends oriented (1) and are associated with the MAP (2)

Answer 24

1. away from the cell body (towards axon’s terminal)
2. tau

Question 25

in dendrites, microtubules are oriented (1) and are associated with the MAP (2)

Answer 25

1. in both directions
2. MAP2