Sample study questions on cell signaling.
1.
Succinylcholine is a chemical analog of acetylcholine. It is used by surgeons as a
muscle relaxant because it produces a type of flaccid paralysis by blocking the
nerve impulse at motor neuron end plates.
(a)
If succinylcholine is a chemical analog of acetylcholine, why do you think it
causes muscles to relax and not contract as acetylcholine does? In your answer,
you must relate your explanation specifically to the process of cell-cell signaling
that takes place at the motor neuron end plates. Also, suggest an experiment to test
your hypothesis. (A hint and note: this question has nothing to do with actin and
myosin. Keep it simple and don’t get sidetracked.) (5 pt)
(b)
Care must be taken in the use of succinylcholine because some individuals have
an adverse reaction. In most cases, the patient recovers from the succinylcholine
paralysis after a short time. In cases with sensitive patients, the individual
recovers abnormally slowly from the drug, with life-threatening consequences. It
is known that abnormal sensitivity to this drug can be inherited. Considering that
it is an analog of acetylcholine, suggest a molecular explanation of the sensitivity
to succinylcholine in sensitive patients. If your explanation is correct, then how
could a physician treat a sensitive patient to whom succinylcholine has been
accidentally administered? (5 pt)
2.
Briefly explain the location of the cellular receptor and the mechanism of action
of steroid hormones. (5 pt)
3.
(This is the one I mentioned in class. Adapted from Wilson and Hunt, Molecular
Biology of the Cell: The Problems Book)
Drosophila larvae molt in response to an increase in the concentration of a steroid
hormone called ecdysone. The polytene chromosomes of the Drosophila salivary
glands are an excellent experimental system in which to study the patterns of gene
activity initiated by the hormone, because active genes enlarge into puffs that are
visible under the light microscope. Furthermore, the size of a puff is proportional
to the rate at which it is being transcribed.
The diagrams show the results of three different experiments to investigate the
role of ecdysone in regulating transcription in Drosophila. In these experiments,
the salivary glands were dissected from live Drosophila larvae. At time 0,
ecdysone was added to the dissected glands, and changes in the pattern of puffing
were observed over a twelve hour period. The curves shows the sizes of three
different chromosomal puff regions (on different regions of the chromosome). The
three puff regions are called “intermolt,” “early,” and “late.” The differences
between the three experiments are as follows:
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Experiment (A) shows the normal response of Drosophila puffs
to ecdysone. At time 0, ecdysone was added to the dissected
salivary glands, and it was present at a constant concentration
throughout the experiment. Note that the intermolt puffs,
present at the beginning of the experiment, disappeared over
the first two hours. The early puffs began to appear after time 0,
reached their maximum size at about 4 hours, and then
regressed in size. The late puffs started to appear at about 5.5
hours, reached their maximum size at about 10 hours, and then
regressed. (Remember that all times are the number of hours
from the addition of ecdysone.)
Experiment (B) shows the response of Drosophila puffs to
ecdysone in the presence of cycloheximide. At time 0, both
ecdysone and cycloheximide were added to the dissected
salivary glands. Cycloheximide is an inhibitor of translation
(protein synthesis). This means that any protein present at the
beginning of the experiment is still active and that transcription
can still occur, but the cells cannot synthesize any new protein
during the course of the experiment. Note that there were no
changes in the intermolt puff response. The appearance of the
early puffs followed the same time course as in experiment (A);
however, the early puffs failed to regress. The late puffs never
appeared.
Experiment (C) shows the response of Drosophila puffs to a
brief exposure to ecdysone. At time 0, ecdysone was added to
the dissected salivary glands. After 2 hours, the ecdysone was
washed out of the salivary glands and removed, so it was no
longer present in the cells. Note that the early puffs
immediately began to regress in size after the ecdysone was
removed. Also note that late puffs were prematurely induced,
meaning that they began to appear after about 3 hours.
(a)
Briefly explain, in general, how the hormone ecdysone can cause changes in the
chromosomal puffing pattern. (5 pt)
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(b)
List three specific transcriptional processes that are regulated directly by the
ecdysone-receptor/ecdysone complex. Briefly explain how the data demonstrate
that these processes are regulated by the ecdysone-receptor/ecdysone complex. (15
pt; 2 pt for each process & 3 pt for each explanation)
(c)
Suggest an explanation for the observation that the early puffs do not regress and
the late puffs are not induced in the presence of cycloheximide. How could you
test your hypothesis? (5 pt)
4.
Distinguish between synaptic, paracrine, and endocrine mechanisms of cell
signaling. (5 pt)
5.
Cyclic AMP (cAMP) plays an important role as a second messenger in cell
signaling processes using G-protein-linked receptors. Therefore, it is important for
a cell to carefully regulate the intracellular concentration of cAMP. An interesting
illustration of the role that cAMP plays in the physiology of the whole organism
comes from studies of Drosophila melanogaster. There is a mutation in
Drosophila called dunce. Flies that are homozygous for dunce have a reduced
amount of an enzyme called cyclic AMP phosphodiesterase. In fact, dunce flies
have only about half the amount of cAMP phosphodiesterase as normal wild-type
flies. Cyclic AMP phosphodiesterase acts to break down cAMP, reducing its level
in the cells. Researchers developed a learning test in which flies were presented
two metallic grids, one of which was electrified. If the electrified grid was painted
with a strong-smelling chemical, normal flies learned quickly to avoid the grid
even when it was no longer electrified. Mutant dunce flies, on the other hand,
never learned to avoid the smelly grid.
(a)
Assume that Drosophila cells have a similar mechanism for the regulation of
glycogen metabolism as mammalian cells, and that Drosophila cells are capable
of responding to epinephrine in a manner similar to mammalian cells. In separate
experiments, cells from either wild-type or dunce flies were treated for a short
time with epinephrine; then, the epinephrine was removed. The rates of glycogen
synthesis and glycogen breakdown were measured at timed intervals after the
epinephrine treatment. How would dunce cells differ from wild-type cells? Briefly
explain your answer. (5 pt)
(b)
Caffeine is a phosphodiesterase inhibitor. What effect do you predict that caffeine
would have on the learning performance of normal, wild-type flies. Briefly
explain your answer. (5 pt)
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