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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  25
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 accumulation of concentrations of hemoglobin in red blood cells is mentioned in the passage as an example of which of the following?
选项 A、The effectiveness of simultaneous variation of the rates of synthesis and degradation of mRNA.
B、The role of the ribosome in enabling a parent cell to develop properly into a more specialized form.
C、The importance of activating the genes for particular proteins at the correct moment.
D、The abnormal proliferation of a protein that threatens to make the cell cancerous.
E、The kind of evidence that biologists relied on for support of a view of mRNA synthesis that is now considered obsolete.
答案 A
解析 文中提到红细胞血红蛋白的浓度是作为哪个观点的例证?A．正确。对mRNA的合成和降解速率同时变化的效果。见文中举过例证之后的总结。B．ribosomeC．activating the genesD．abnormal proliferation of a protein都和原文血红蛋白毫无关系。E．易混。生物学家用来支持关于mRNA合成的证据现已过时。据原文末句，现今生物学家并未完全否认合成作用，只是同时也强调降解的作用。
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