XDF 36-5

vincent0330

Passage 36 (36/63)

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 concentrations 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 degrades 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.

5.     To begin to control a disease caused by a protein deficiency, the passage suggests that a promising experimental treatment would be to administer a drug that would reduce

(A) only the degradation rate for the mRNA of the protein involved

(B) only the synthesis rate for the mRNA of the protein involved

(C) both the synthesis and degradation rates for the mRNA of the protein involved

(D) the incidence of errors in the transcription of mRNA’s from genetic nucleotide sequences（A）

(E) the rate of activity of ribosomes in the cytoplasm of most cells

怎么我看都是(C)啊.....谁能讲讲啊!!

先谢过啦......

vincent0330

过了几天, 想想还是(A), 那个best可是key word......

paopao

vincent , pls look at the following sentence:

If a cell degrades both a rapidly and a slowly synthesized mRNA slowly, both mRNA’s will accumulate to high levels（不论合成快慢，只要分解慢，MRNA或P数量增加） 增加P—》减少分解

hope it can help

xjlv128

我也是做完了，错过了，才反应过来，文章的逻辑是：

原来生物学家认为只有合成速度影响蛋白质合成，后来发现分解速度也产生作用 the concentrations 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，甚至更有用。

所以，下了个迷惑大家的结论——分解和合成都产生作用。

于是，本来就迷迷糊糊的我们，就当成了  它们影响的地位是等同的！

rynnchan

8. The passage suggests that a biologist who detected highlevels of two proteins in a certain type of cell would belikely to consider which of the following as a possibleexplanation?
(A) The rate of mRNA degradation for one of theproteins increases as this type of cell develops amore specialized function.
(B) The two proteins are most likely constituents of acomplex substance supporting the cells' specializedfunction.
(C) The cells are likely to proliferate abnormally andpossibly become cancerous due to the levels of theseproteins.
(D) The mRNA's for both proteins are being degradedat a low rate in that type of cell.
(E) The mRNA's for the two proteins are beingsynthesized at identical rates in that type of cell.

请教第8题，为什么选D啊

meefoo

About question 8:

I'm not sure if I'm right. I chose C first, but I now think D is the correct answer.
My thought:
Read the question again, and focus on the "possible explanation". The question asks about the possible explanation, not the "consequence" of the circumstance.

What causes the high levels of two proteins? The low degradation rate!!
So (D) The mRNA's for both proteins are being degradedat a low rate in that type of cell.

dot_deng

我觉得此题还是选C

原文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.

fpig

第5题A比C更好，虽然我开始也选了C。一点见解：

为了增加protein,可以 synthesis 和degrade，但degrade 更有作用。在development of red blood cells这个例子中是最好的方式，因为它produce more of   the constituent proteins 的同时rapidly  degrade copies of the nonhemoglobin mRNA’s（所以也就有文章的结尾“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”）

其次才到A,再到C.

ssl507

To begin to control a disease caused by a protein deficiency, the passage suggests that a promising experimental treatment would be to administer a drug that would reduce

这题的关健在看清题目，题目是说 control a disease caused by a
            protein deficiency，要reduce什么，如果选C的话，reduce both，那不等于没用，还是不能增加protein.

Rayakawr

我选了C,因为"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."这是一个结论.

还是不理解为什么A优于C，麻烦高手解释清楚．谢谢！