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.

2. 对比取非题

The passage suggests that a biologist who held the view

described in the first sentence of the second paragraph

would most probably also have believed which of the

following?

(A) The rate of degradation of specific mRNA’s has

little effect on protein concentrations.

(B) The rate of degradation of specific mRNA’s should

be studied intensively.

(C) The rates of synthesis and degradation for any given

mRNA are normally equal.

(D) Different mRNA’s undergo degradation at widely

varying rates.

(E) Most mRNA’s degrade very rpaidly.

正确答案是A，而我选D.

观察第二段：对比为：

老观点：variable rate, synthesize, RNA quantity

新观点：equal rate, degrade, RNA

所以把新观点取非得：vary rate, degrade正好是D

那个A是怎么出来的？

这道题的选项问老观点的内容，但将新观点取非不一定就是老观点，不能由新观点取非得到。D的错误在于，老观点也可能认可它。A的正确在于如果对其取非，则直接与老观点矛盾。所以A正确。

A和B都是支持原文的推理的

C是无关选项

E跟原文是没有矛盾的

楼主再好好结合结论想一下。。。