Essential Cell Biology 5th edition

A:46 Answers
the microtubule will always crawl minus-end first over the
cover slip.
ANSWER 17–19
A. Phase A corresponds to a lag phase, during which
tubulin dimers assemble to form nucleation centers
(Figure A17–19A). Nucleation is followed by a rapid
rise (phase B) to a plateau value as tubulin dimers add
to the ends of the elongating microtubules (Figure
A17–19B). At phase C, equilibrium is reached, with some
microtubules in the population growing while others are
rapidly shrinking (Figure A17–19C). The concentration
of free tubulin is constant at this point because
polymerization and depolymerization are balanced
(see also Question 17–3, p. 586).
B. The addition of centrosomes introduces nucleation
sites that eliminate the lag phase A, as shown by the
red curve in Figure A17–19D. The rate of microtubule
growth (i.e., the slope of the curve in the elongation
phase B) and the equilibrium level of free tubulin remain
unchanged, because the presence of centrosomes
does not affect the rates of polymerization and
depolymerization.
ANSWER 17–20 The ends of the shrinking microtubule
are visibly frayed, and the individual protofilaments appear
to come apart and curl as the end depolymerizes. This
micrograph therefore suggests that the GTP cap (which is
lost from shrinking microtubules) holds the protofilaments
properly aligned with each other, perhaps by strengthening
the side-to-side interactions between αβ-tubulin subunits
when they are in their GTP-bound form.
ANSWER 17–21 Cytochalasin interferes with actin filament
formation, and its effect on the cell demonstrates the
importance of actin to cell locomotion. The experiment with
colchicine shows that microtubules are required to give a
cell a polarity that then determines which end becomes
the leading edge (see Figure 17−15). In the absence of
microtubules, cells still go through the motions normally
associated with cell movement, such as the extension of
lamellipodia, but in the absence of cell polarity these are
futile exercises because they happen indiscriminately in all
directions. Antibodies bind tightly to the antigen (in this
case vimentin) to which they were raised (see Panel 4–2,
pp. 140–141). When bound, an antibody can interfere with
the function of the antigen by preventing it from interacting
properly with other cell components. The antibody injection
experiment therefore suggests that intermediate filaments
are not required for the maintenance of cell polarity or for
the motile machinery.
ANSWER 17–22 Either (B) or (C) would complete the
sentence correctly. The direct result of the action potential
in the plasma membrane is the release of Ca 2+ into the
cytosol from the sarcoplasmic reticulum; muscle cells are
triggered to contract by this rapid rise in cytosolic Ca 2+ .
Calcium ions at high concentrations bind to troponin, which
in turn causes tropomyosin to move to expose myosinbinding
sites on the actin filaments. (A) and (D) would be
wrong because Ca 2+ has no effect on the detachment
of the myosin head from actin, which is the result of ATP
hydrolysis. Nor does it have any role in maintaining the
structure of the myosin filament.
ANSWER 17–23 Only (D) is correct. Upon contraction, the
Z discs move closer together, and neither actin nor myosin
filaments contract (see Figures 17−41 and 17−42).
Chapter 18
ANSWER 18–1 Because all cells arise by division of another
cell, this statement is correct, assuming that “first cell
division” refers to the division of the successful founder cell
from which all life as we know it has derived. There were
probably many other unsuccessful attempts to start the
chain of life.
ANSWER 18–2 Cells in peak B contain twice as much DNA
as those in peak A, indicating that they contain replicated
DNA, whereas the cells in peak A contain unreplicated DNA.
Peak A therefore contains cells that are in G 1 , and peak B
contains cells that are in G 2 and mitosis. Cells in S phase
have begun but not finished DNA synthesis; they therefore
have various intermediate amounts of DNA and are found
in the region between the two peaks. Most cells are in G 1 ,
indicating that it is the longest phase of the cell cycle (see
Figure 18−2).
tubulin dimer
aggregate
of tubulin
(A) nucleation
(B) elongation
percentage of tubulin molecules
in microtubules
with centrosomes
added
elongation
equilibrium
nucleation
time at 37ºC
(C) equilibrium
(D)
Figure A17–19