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Protein synthesis begins when the gene encoding a protein is activated. The gene’s sequence of nucleotides is transcribed into a molecule of messenger RNA (mRNA), which reproduces the information contained in that sequence. Transported outside the nucleus to the cytoplasm, the mRNA is translated into the protein it encodes by an organelle known as a ribosome, which strings together amino acids in the order specified by the sequence of elements in the mRNA molecule. Since the amount of mRNA in a cell determines the amount of the corresponding protein, factors affecting the abundance of mRNA’s play a major part in the normal functioning of a cell by appropriately regulating protein synthesis. For example, an excess of certain proteins can cause cells to proliferate abnormally and become cancerous; a lack of the protein insulin results in diabetes.
Biologists once assumed that the variable rates at which cells synthesize different mRNA’s determine the quantities of mRNA’s and their corresponding proteins in a cell. However, recent investigations have shown that the concentrations of most mRNA’s correlate best, not with their synthesis rate, but rather with the equally variable rates at which cells degrade the different mRNA’s in their cytoplasm. If a cell degrades both a rapidly and a slowly synthesized mRNA slowly, both mRNA’s will accumulate to high levels.
An important example of this phenomenon is the development of red blood cells from their unspecialized parent cells in bone marrow (bone marrow: n. 骨髓). For red blood cells to accumulate sufficient concentrations of hemoglobin (hemoglobin: n.血色素) (which transports oxygen) to carry out their main function, the cells’ parent cells must simultaneously produce more of the constituent proteins of hemoglobin and less of most other proteins. To do this, the parent cells halt synthesis of non-hemoglobin mRNA’s in the nucleus and rapidly degrade copies of the non-hemoglobin mRNA’s remaining in the cytoplasm. Halting synthesis of mRNA alone would not affect the quantities of proteins synthesized by the mRNA’s still existing in the cytoplasm. Biologists now believe that most cells can regulate protein production most efficiently by varying both mRNA synthesis and degradation, as developing red cells do, rather than by just varying one or the other.
5. To begin to control a disease caused by a protein deficiency, the passage suggests that a promising experimental treatment would be to administer a drug that would reduce
(A) only the degradation rate for the mRNA of the protein involved
(B) only the synthesis rate for the mRNA of the protein involved
(C) both the synthesis and degradation rates for the mRNA of the protein involved
(D) the incidence of errors in the transcription of mRNA’s from genetic nucleotide sequences（A）
(E) the rate of activity of ribosomes in the cytoplasm of most cells
8. The passage suggests that a biologist who detected high levels of two proteins in a certain type of cell would be likely to consider which of the following as a possible explanation?
(A) The rate of mRNA degradation for one of the proteins increases as this type of cell develops a more specialized function.
(B) The two proteins are most likely constituents of a complex substance supporting the cells’ specialized function.
(C) The cells are likely to proliferate abnormally and possibly become cancerous due to the levels of these proteins.
(D) The mRNA’s for both proteins are being degraded at a low rate in that type of cell.（D）
(E) The mRNA’s for the two proteins are being synthesized at identical rates in that type of cell.
我理解文中先从DNA转录成mRNA，mRNA再被翻译成相应的蛋白质。文中讲到合成mRNA和分解mRNA的速度决定蛋白质的数量。
按照这个理解，应该分解得越快，蛋白质越多。为什么第5和第8的答案反而选降低分解速度能促进蛋白的生成？请问哪个地方我理解是错了？谢谢！ |
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发表于 2010-1-11 23:32:04
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