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  3. Protein synthesis begins when the gene encoding a protein is activated. The

Protein synthesis begins when the gene encoding a protein is activated. The

游客2024-01-13  59
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. [br] The passage suggests that a biologist who held the view described in the first sentence of the second paragraph would most probably also have believed which of the following?
选项 A、The rate of degradation of specific mRNA’s has little effect on protein concentrations.
B、The rate of degradation of specific mRNA’s should be studied intensively.
C、The rates of synthesis and degradation for any given mRNA are normally equal.
D、Different mRNA’s undergo degradation at widely varying rates.
E、Most mRNA’s degrade very rapidly.
答案 A
解析 文中指出，持第二段第一句话观点的生物学家将会同意什么主张?回原文，第二段第一句话：过去观点，即合成起决定性作用，未认识到降解的影响。∴A正确，特定mRNA的降解速度对蛋白质浓度影响很小。B、C、D、E都不符原文叙述。
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