Essential Cell Biology 5th edition

Answers A:49
overlapping interpolar microtubules from opposite poles
of the spindle have their plus ends pointing in opposite
directions. Plus-end directed motor proteins cross-link
adjacent, antiparallel microtubules together and tend to
move the microtubules in the direction that will push the
two poles of the spindle apart, as shown in the figure.
Minus-end directed motor proteins also cross-link adjacent,
antiparallel microtubules together but move in the opposite
direction, tending to pull the spindle poles together (not
shown).
ANSWER 18–20 The sister chromatid becomes committed
when a microtubule from one of the spindle poles
attaches to the kinetochore of the chromatid. Microtubule
attachment is still reversible until a second microtubule
from the other spindle pole attaches to the kinetochore
of its partner sister chromatid, so that the duplicated
chromosome is now put under mechanical tension by
pulling forces from both poles. The tension ensures that
both microtubules remain attached to the chromosome.
The position of a chromatid in the cell at the time that the
nuclear envelope breaks down will influence which spindle
pole it will be pulled to, as its kinetochore is most likely
to become attached to the spindle pole toward which it is
facing.
ANSWER 18–21 It is still not certain what drives the
poleward movement of chromosomes during anaphase.
In principle, two possible models could explain it
(Figure A18–21). In the model shown in (A), microtubule
motor proteins associated with the kinetochore dash
toward the minus end of the depolymerizing microtubule,
dragging the chromosome toward the pole. Although this
model is appealingly simple, there is little evidence that
motor proteins are required for chromosome movement
during anaphase. Instead, current experimental evidence
greatly supports the model outlined in (B). In this model,
chromosome movement is driven by kinetochore proteins
that cling to the sides of the depolymerizing microtubule
(see Figure 18–23). These proteins frequently detach from—
and reattach to—the kinetochore microtubule. As tubulin
subunits continue to dissociate, the kinetochore must slide
poleward to maintain its grip on the retreating end of the
shrinking microtubule.
ANSWER 18–22 Both sister chromatids could end up in the
same daughter cell for any of a number of reasons. (1) If the
microtubules or their connections with a kinetochore were
to break during anaphase, both sister chromatids could be
drawn to the same pole, and hence into the same daughter
cell. (2) If microtubules from the same spindle pole attached
to both kinetochores, the chromosome would be pulled to
the same pole. (3) If the cohesins that link sister chromatids
were not degraded, the pair of chromatids might be pulled
to the same pole. (4) If a duplicated chromosome never
engaged microtubules and was left out of the spindle, it
would also end up in one daughter cell.
Some of these errors in the mitotic process would be
expected to activate a checkpoint mechanism that delays
the onset of anaphase until all chromosomes are attached
properly to both poles of the spindle. This “spindle
assembly checkpoint” mechanism should allow most
chromosome-attachment errors to be corrected, which is
one reason why such errors are rare. The consequences of
both sister chromatids ending up in one daughter cell are
usually dire. One daughter cell would contain only one copy
of all the genes carried on that chromosome and the other
daughter cell would contain three copies. The altered gene
dosage, leading to correspondingly changed amounts of
the mRNAs and proteins produced, is often detrimental to
the cell. In addition, there is the possibility that the single
copy of the chromosome may contain a defective gene with
a critical function, which would normally be taken care of by
the good copy of the gene on the other chromosome that is
now missing.
ANSWER 18–23
A. True. Centrosomes replicate during interphase, before M
phase begins.
B. True. Sister chromatids separate completely only at the
start of anaphase.
C. False. The ends of interpolar microtubules overlap and
attach to one another via proteins (including motor
proteins) that bridge between the microtubules.
D. False. Microtubules and their motor proteins play no role
in DNA replication.
E. False. To be a correct statement, the terms
“centromere” and “centrosome” must be switched.
direction of
chromosome
movement
direction of
chromosome
movement
microtubule
motor protein
microtubule-binding
protein
kinetochore
microtubule
kinetochore
chromosome
kinetochore
microtubule
kinetochore
chromosome
Figure A18–21
DISFAVORED MODEL:
motor proteins drive chromosome movement
(A)
FAVORED MODEL:
microtubule-binding proteins drive
chromosome movement
(B)