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？ | 
