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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 can happen when protein synthesis is not appropriately regulated?
选项 A、Diabetes can result from errors that occur when the ribosomes translate mRNA into protein.
B、Cancer can result from an excess of certain proteins and diabetes can result from an insulin deficiency.
C、A deficiency of red blood cells can occur if bone marrow cells produce too much hemoglobin.
D、Cancer can be caused by excessively rapid degradation of certain amino acids in the cytoplasm of cells.
E、Excessive synthesis of one protein can trigger increased degradation of mRNA’ s for other proteins and create severe protein imbalances.
答案B
解析 当蛋白质合成不能被有效控制时，会发生什么情况?A．糖尿病的发生是由于核糖体译解mRNA时出现错误。无。B．正确。癌症产生于特定蛋白质过多，糖尿病产生于胰岛素缺乏。C．骨髓细胞如造出过多血红蛋白，将会引起红细胞缺乏。荒谬。D．癌症和氨基酸降解过快有关。无。E．一种蛋白质合成过多会造成别的蛋白质的mRNA更多降解，从而引起严重的蛋白质不平衡。文中未提。
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