Essential Cell Biology 5th edition

608 CHAPTER 17 Cytoskeleton
QUESTION 17–16
When cells enter mitosis, their existing array of cytoplasmic
microtubules has to be rapidly broken down and replaced
with the mitotic spindle that forms to pull the chromosomes
into the daughter cells. The enzyme katanin, named after
Japanese samurai swords, is activated during the onset of
mitosis, and chops microtubules into short pieces. What
do you suppose is the fate of the microtubule fragments
created by katanin? Explain your answer.
QUESTION 17–17
The drug Taxol, extracted from the bark of yew trees, has
an opposite effect to the drug colchicine, an alkaloid from
autumn crocus. Taxol binds tightly to microtubules and
stabilizes them; when added to cells, it causes much of the
free tubulin to assemble into microtubules. In contrast,
colchicine prevents microtubule formation. Taxol is just
as pernicious to dividing cells as colchicine, and both are
used as anticancer drugs. Based on your knowledge of
microtubule dynamics, suggest why both drugs are toxic to
dividing cells despite their opposite actions.
QUESTION 17–18
A useful technique for studying microtubule motors is to
attach them by their tails to a glass cover slip (which can
be accomplished quite easily because the tails stick avidly
to a clean glass surface) and then allow them to settle.
Microtubules may then be viewed in a light microscope as
they are propelled over the surface of the cover slip by the
heads of the motor proteins. Because the motor proteins
attach at random orientations to the cover slip, however,
how can they generate coordinated movement of individual
microtubules rather than engaging in a tug-of-war? In which
direction will microtubules crawl on a “bed” of kinesin
molecules (i.e., will they move plus-end first or minus-end
first)?
QUESTION 17–19
A typical time course of polymerization of purified tubulin to
form microtubules is shown in Figure Q17–19.
A. Explain the different parts of the curve (labeled A, B,
and C). Draw a diagram that shows the behavior of tubulin
subunits in each of the three phases.
B. How would the curve in the figure change if centrosomes
were added at the outset?
percentage of tubulin molecules
in microtubules
A
Figure Q17–19
B
C
time at 37°C
(A)
Figure Q17–20
QUESTION 17–20
The electron micrographs shown in Figure Q17–20A
were obtained from a population of microtubules that
were growing rapidly. Figure Q17–20B was obtained
from microtubules undergoing “catastrophic” shrinking.
Comment on any differences between A and B, and suggest
likely explanations for the differences that you observe.
QUESTION 17–21
The locomotion of fibroblasts in culture is immediately
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halted by the drug cytochalasin, whereas colchicine causes
fibroblasts to cease to move directionally and to begin
extending lamellipodia in seemingly random directions.
Injection of fibroblasts with antibodies to the intermediate
filament protein vimentin has no discernible effect on their
migration. What do these observations suggest to you
about the involvement of the three different cytoskeletal
filaments in fibroblast locomotion?
QUESTION 17–22
(B)
Complete the following sentence accurately, explaining your
reason for accepting or rejecting each of the four phrases
(more than one can be correct). The role of calcium in
muscle contraction is:
A. to detach myosin heads from actin.
B. to spread the action potential from the plasma
membrane to the contractile machinery.
C. to bind to troponin, cause it to move tropomyosin, and
thereby expose actin filaments to myosin heads.
D. to maintain the structure of the myosin filament.
QUESTION 17–23
Which of the following changes takes place when a skeletal
muscle contracts?
A. Z discs move farther apart.
B. Actin filaments contract.
C. Myosin filaments contract.
D. Sarcomeres become shorter.
(Micrographs courtesy of Eva Mandelkow.)
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