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PASSAGE 36
* 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
5) sequence. Transported outside the nucleus to the cyto- plasm, 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
10) 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
15) 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
20) in a cell. However, recent investigations have shown that the concentrations of most mRNA's correlate best, not with their synthesis rate, but rather with the equally vari- able rates at which cells degrade the different mRNA's in their cytoplasm. If a cell degrades both a rapidly and
25) 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 accu-
30) mulate 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
35) of nonhemoglobin mRNA's in the nucleus and rapidly degrade copies of the nonhemoglobin 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
40) 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.
题目：
5. To begin to control a disease caused by a proteindeficiency, the passage suggests that a promisingexperimental treatment would be to administer a drugthat would reduce
(A) only the degradation rate for the mRNA of theprotein involved
(B) only the synthesis rate for the mRNA of the proteininvolved
(C) both the synthesis and degradation rates for themRNA of the protein involved
(D) the incidence of errors in the transcription ofmRNA's from genetic nucleotide sequences
(E) the rate of activity of ribosomes in the cytoplasm ofmost cells
这道题答案是A，而我选得是C，我想不通啊，请牛牛指教！
[此贴子已经被作者于2005-3-10 8:46:56编辑过] | 
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发表于 2005-3-10 08:41:00
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