考生舉行GRE瀏覽測驗的備考，恰當的演習是必弗成少的，本日給考生分享的是GRE瀏覽真題200篇LEVEL1：Until about five years ago...，願望須要的考生可如下載下來然後模仿演習。
17. Until about five years ago,
Until about five years ago, the very idea that peptide hormones might be made anywhere in the brain besides the hypothalamus was astounding. Peptide hormones, scientists thought, were made by endocrine glands and the hypothalamus was thought to be the brains’ only endocrine gland. What is more, because peptide hormones cannot cross the blood-brain barrier, researchers believed that they never got to any part of the brain other than the hypothalamus, where they were simply produced and then released into the bloodstream.
But these beliefs about peptide hormones were questioned as laboratory after laboratory found that antiserums to peptide hormones, when injected into the brain, bind in places other than the hypothalamus, indicating that either the hormones or substances that cross-react with the antiserums are present. The immunological method of detecting peptide hormones by means of antiserums, however, is imprecise. Cross-reactions are possible and this method cannot determine whether the substances detected by the antiserums really are the hormones, or merely close relatives. Furthermore, this method cannot be used to determine the location in the body where the detected substances are actually produced.
New techniques of molecular biology, however, provide a way to answer these questions. It is possible to make specific complementary DNA’s (cDNA’s) that can serve as molecular probes to seek out the messenger RNA’s (mRNA’s) of the peptide hormones. If brain cells are making the hormones, the cells will contain these mRNA’s. If the products the brain cells make resemble the hormones but are not identical to them, then the cDNA’s should still bind to these mRNA’s, but should not bind as tightly as they would to mRNA’s for the true hormones. The cells containing these mRNA’s can then be isolated and their mRNA’s decoded to determine just what their protein products are and how closely the products resemble the true peptide hormones.
The molecular approach to detecting peptide hormones using cDNA probes should also be much faster than the immunological method because it can take years of tedious purifications to isolate peptide hormones and then develop antiserums to them. Roberts, expressing the sentiment of many researchers, states: “I was trained as an endocrinologist. But it became clear to me that the field of endocrinology needed molecular biology input. The process of grinding out protein purifications is just too slow.”
If, as the initial tests with cDNA probes suggest, peptide hormones really are made in the brain in areas other than the hypothalamus, a theory must be developed that explains their function in the brain. Some have suggested that the hormones are all growth regulators, but Rosen’s work on rat brains indicates that this cannot be true. A number of other researchers propose that they might be used for intercellular communication in the brain.
21. Which of the following titles best summarizes the passage?
(A) Is Molecular Biology the Key to Understanding Intercellular Communication in the Brain?
(B) Molecular Biology: Can Researchers Exploit Its Techniques to Synthesize Peptide Hormones?
(C) The Advantages and Disadvantages of the Immunological Approach to Detecting Peptide Hormones
(D) Peptide Hormones: How Scientists Are Attempting to Solve Problems of Their Detection and to Understand Their Function
(E) Peptide Hormones: The Role Played by Messenger RNA’s in Their Detection
22. The passage suggests that a substance detected in the brain by use of antiserums to peptide hormones may
(A) have been stored in the brain for a long period of time
(B) play no role in the functioning of the brain
(C) have been produced in some part of the body other than the brain
(D) have escaped detection by molecular methods
(E) play an important role in the functioning of the hypothalamus
23. According to the passage, confirmation of the belief that peptide hormones are made in the brain in areas other than the hypothalamus would force scientists to
(A) reject the theory that peptide hormones are made by endocrine glands
(B) revise their beliefs about the ability of antiserums to detect peptide hormones
(C) invent techniques that would allow them to locate accurately brain cells that produce peptide hormones
(D) search for techniques that would enable them to distinguish peptide hormones from their close relatives
(E) develop a theory that explains the role played by peptide hormones in the brain
24. Which of the following is mentioned in the passage as a drawback of the immunological method of detecting peptide hormones?
(A) It cannot be used to detect the presence of growth regulators in the brain.
(B) It cannot distinguish between the peptide hormones and substances that are very similar to them.
(C) It uses antiserums that are unable to cross the blood-brain barrier.
(D) It involves a purification process that requires extensive training in endocrinology.
(E) It involves injecting foreign substances directly into the bloodstream.
25. The passage implies that, in doing research on rat brains, Rosen discovered that
(A) peptide hormones are used for intercellular communication
(B) complementary DNA’s do not bind to cells producing peptide hormones
(C) products closely resembling peptide hormones are not identical to peptide hormones
(D) some peptide hormones do not function as growth regulators
(E) antiserums cross-react with substances that are not peptide hormones
26. Which of the following is a way in which the immunological method of detecting peptide hormones differs from the molecular method?
(A) The immunological method uses substances that react with products of hormone-producing cells, whereas the molecular method uses substances that react with a specific component of the cells themselves.
(B) The immunological method has produced results consistent with long-held beliefs about peptide hormones, whereas the molecular method has produced results that upset these beliefs.
(C) The immunological method requires a great deal of expertise, whereas the molecular method has been used successfully by nonspecialists.
(D) The immunological method can only be used to test for the presence of peptide hormones within the hypothalamus, whereas the molecular method can be used throughout the brain.
(E) The immunological method uses probes that can only bind with peptide hormones, whereas the molecular method uses probes that bind with peptide hormones and substances similar to them.
27. The idea that the field of endocrinology can gain from developments in molecular biology is regarded by Roberts with
(A) incredulity
(B) derision
(C) indifference
(D) pride
(E) enthusiasm
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