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:
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 andcycloheximide 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)
(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 & 3pt 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. CyclicAMP 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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