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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.
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?
选项 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.
E、The mRNA’ s for the two proteins are being synthesized at identical rates in that type of cell.
答案D
解析 一个生物学家发现一细胞中两种蛋白质含量高，他会怎么解释：A．其中一个降解率升高。显然不合适，应对两种作解释。B．是否“supporting the cells’specialized function”和蛋白质高不高无关。C．这些蛋白质或许会异常增殖并造成癌变。这是结果，不是蛋白质含量高的原因。D．正确。两种蛋白质的mRNA的降解过程速率低。符合原文结论，并且文中有一例证与此相符。E．合成速度相同，不一定能从蛋白质最终含量高上推出。
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