Essential Cell Biology 5th edition

Questions
649
help determine the length of the spindle. How might each
type of motor protein contribute to the determination of
spindle length?
QUESTION 18–20
Sketch the principal stages of mitosis, using Panel 18−1
(pp. 628–629) as a guide. Color one sister chromatid and
follow it through mitosis and cytokinesis. What event
commits this chromatid to a particular daughter cell? Once
initially committed, can its fate be reversed? What may
influence this commitment?
QUESTION 18–21
The polar movement of chromosomes during anaphase
A is associated with microtubule shortening. In particular,
microtubules depolymerize at the ends at which they are
attached to the kinetochores. Sketch a model that explains
how a microtubule can shorten and generate force yet
remain firmly attached to the chromosome.
QUESTION 18–22
Rarely, both sister chromatids of a replicated chromosome
end up in one daughter cell. How might this happen? What
could be the consequences of such a mitotic error?
QUESTION 18–23
Which of the following statements are correct? Explain your
answers.
A. Centrosomes are replicated before M phase begins.
B. Two sister chromatids arise by replication of the DNA of
the same chromosome and remain paired as they line up on
the metaphase plate.
C. Interpolar microtubules attach end-to-end and are
therefore continuous from one spindle pole to the other.
D. Microtubule polymerization and depolymerization
and microtubule motor proteins are all required for DNA
replication.
E. Microtubules nucleate at the centromeres and then
connect to the kinetochores, which are structures at the
centrosome regions of chromosomes.
QUESTION 18–24
An antibody that binds to myosin prevents the movement
of myosin molecules along actin filaments (the interaction
between actin and myosin is described in Chapter 17).
How do you suppose the antibody exerts this effect? What
might be the result of injecting this antibody into cells
(A) on the movement of chromosomes at anaphase or
(B) on cytokinesis? Explain your answers.
QUESTION 18–25
Look carefully at the electron micrographs in Figure
18−38. Describe the differences between the cell that
died by necrosis and those that died by apoptosis. How
do the pictures confirm the differences between the two
processes? Explain your answer.
QUESTION 18–26
Which of the following statements are correct? Explain your
answers.
A. Cells do not pass from G 1 into M phase of the cell cycle
unless there are sufficient nutrients to complete an entire
cell cycle.
B. Apoptosis is mediated by special intracellular proteases,
one of which cleaves nuclear lamins.
C. Developing neurons compete for limited amounts of
survival factors.
D. Some vertebrate cell-cycle control proteins function
when expressed in yeast cells.
E. The enzymatic activity of a Cdk protein is determined
both by the presence of a bound cyclin and by the
phosphorylation state of the Cdk.
QUESTION 18–27
Compare the rules of cell behavior in an animal with the
rules that govern human behavior in society. What would
happen to an animal if its cells behaved as people normally
behave in our society? Could the rules that govern cell
behavior be applied to human societies?
QUESTION 18–28
In his highly classified research laboratory, Dr. Lawrence M.
is charged with the task of developing a strain of dog-sized
rats to be deployed behind enemy lines. In your opinion,
which of the following strategies should Dr. M. pursue to
increase the size of rats?
A. Block all apoptosis.
B. Block p53 function.
C. Overproduce growth factors, mitogens, or survival
factors.
Explain the likely consequences of each option.
QUESTION 18–29
PDGF is encoded by a gene that can cause cancer when
expressed inappropriately. Why do cancers not arise at
wounds in which PDGF is released from platelets?
QUESTION 18–30
What do you suppose happens in mutant cells that
A. cannot degrade M cyclin?
B. always express high levels of p21?
C. cannot phosphorylate Rb?
QUESTION 18–31
Liver cells proliferate excessively both in patients with
chronic alcoholism and in patients with liver cancer. What
are the differences in the mechanisms by which cell
proliferation is induced in these diseases?