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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2025-02-26  26
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] Which of the following best describes the relationship between the second and third paragraphs of the passage?
选项 A、The second paragraph presents arguments in support of a new theory and the third paragraph presents arguments against that same theory.
B、The second paragraph describes a traditional view and the third paragraph describes the view that has replaced it on the basis of recent investigations.
C、The third paragraph describes a specific case of a phenomenon that is described generally in the second paragraph.
D、The third paragraph describes an investigation that was undertaken to resolve problems raised by phenomena described in the second paragraph.
E、Both paragraphs describe in detail specific examples of the phenomenon that is introduced in the first paragraph.
答案 C
解析 哪一个说明了二、三两段的关系?A．“arguments against”不正确，第三段未反驳第二段观点。B．同上，不正确“replaced it on the basis of recent investigations”。C．正确。第三段描述了第二段所说结论的一个特例。符合原文，血红蛋白一例即为细胞合成和降解mRNA的速率同时起作用的说明。D．第三段解决第二段提的问题。无。E．两段都写细节。第二段不是。
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