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Protein synthesis begins when the gene encoding aprotein is activated. The gene’s sequence of nucleotides is transcribed into a molecule of messenger RNA (mRNA), which reproducesthe information contained in that sequence. Transported outside the nucleus tothe cytoplasm, the mRNA is translated into the protein it encodes by anorganelle known as a ribosome, which strings together amino acids in the orderspecified by the sequence of elements in the mRNA molecule. Since the amount ofmRNA in a cell determines the amount of the corresponding protein, factorsaffecting the abundance of mRNA’s play a major part in the normal functioningof a cell by appropriately regulating protein synthesis. For example, an excessof certain proteins can cause cells to proliferate abnormally and becomecancerous; a lack of the protein insulin results in diabetes. Biologists once assumed that the variable rates atwhich cells synthesize different mRNA’s determine the quantities of mRNA’s andtheir corresponding proteins in acell. However, recentinvestigations have shown that the concentrations of most mRNA’s correlate best, not with their synthesis rate, but rather with the equally variablerates at which cells degrade the different mRNA’s in their cytoplasm. Ifa cell degrades both a rapidly and a slowly synthesized mRNA slowly, bothmRNA’s will accumulate to high levels. An important example of this phenomenon is the development of red bloodcells from their unspecialized parent cells in bonemarrow (bone marrow: n. 骨髓). For red blood cells to accumulate sufficientconcentrations of hemoglobin (which transports oxygen) to carry out their main function, the cells’parent cells must simultaneously produce more of the constituent proteins ofhemoglobin and less of most other proteins. To do this, the parent cells haltsynthesis of non-hemoglobin mRNA’s in the nucleus and rapidly degrade copies ofthe non-hemoglobin mRNA’s remaining in the cytoplasm. Halting synthesis of mRNAalone would not affect the quantities of proteins synthesized by the mRNA’sstill existing in the cytoplasm. Biologists now believe that most cells canregulate protein production most efficiently by varying both mRNA synthesis anddegradation, as developing red cells do, rather than by just varying one or theother.
5. To begin to control a diseasecaused by a protein deficiency, the passage suggests that a promisingexperimental treatment would be to administer a drug that would reduce (A) only the degradation rate forthe mRNA of the protein involved (B) only the synthesis rate for the mRNA of the protein involved (C) both thesynthesis and degradation rates for the mRNA of the protein involved (D) the incidence of errors in the transcription of mRNA’s from geneticnucleotide sequences（A） (E) the rate of activity of ribosomes in the cytoplasm of most cells
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发表于 2016-7-7 14:08:55
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楼主
这篇文章错了三个，读完已蒙，啥也找不着了，太感谢了！