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  3. Protein synthesis begins when the gene encoding a protein is activated. The gene’ s sequence of nucleotides is transcribed into

Protein synthesis begins when the gene encoding a protein is activated. The gene’ s sequence of nucleotides is transcribed into

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问题     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 con- centrations 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 de- grades 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. For red blood cells to accumulate sufficient concentrations of hemoglobin(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.
According to the passage, which of the following best describes the current view on the relationship between the synthesis and the degradation of mRNA in regulating protein synthesis?
选项 A、Biologists have recently become convinced that the ribosome controls the rates of synthesis and degradation of mRNA.
B、There is no consensus among biologists as to the significance of mRNA degradation in regulating protein synthesis.
C、The concept of mRNA degradation is so new that most biologists still believe that the vital role in protein regulation belongs to mRNA synthesis.
D、Degradation of mRNA is now considered to be the key process and mRNA synthesis is no longer believed to play a significant role.
E、Degradation of mRNA is now considered to be as important as mRNA synthesis has been, and still is, believed to be.
答案E
解析 当前观点认为调节蛋白质合成的mRNA的合成与降解速率的关系如何?依然是全文结论最后一句的内容，即合成与降解同时起着重要作用。∴E正确。降解如今已被人们看作和合成同样重要了。A．“ribosome controls”和当前结论无关。B．“no consensus among biologists”与原文所说不符。见L56—57：biologists now believe。C．生物学家仍确信主要作用归于mRNA的合成。和结论句显然不符。D．只有降解起重要作用，合成不再被看作重要作用。不符原文结论。
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