SAMPLE 13

　　[醫學類]

　　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 (c DNA’s) that can serve as molecular probes 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 c DNA’s should still bind to these mRNA’s, but should not bind as tightly as they would to m RNA’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 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.

　　1. Which of the following titles best summarizes the text?

　　[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?

　　2. The text 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.

　　3. According to the text, confirmation of the belief that peptide hormones are created 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] develop a theory that account for the role played by peptide hormones in the brain.

　　4. Which of the following is mentioned in the text 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.

　　5. 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] enthusiasm.

　　[答案與考點解析]

　　1. 【答案】D

　　【考點解析】這是一道中心主旨題。全文從頭至尾討論的是“peptide hormones”在人體內產生的部位，所以有關全文中心主旨內容的答案應該包含“peptide hormones”。從各段的主題句進行分析，第一至四段主要講如何“detect”(探測)肽激素(peptide hormones)所產生的位置，第五段主要講有關肽激素的“function”�？梢姳绢}的正確選項應該是D�？忌�在解題時一定要搞清楚原文所涉及的對象并且抓住每段的主題句以及它們之間的相互關系。

　　2. 【答案】C

　　【考點解析】這是一道審題定位題。根據題干中的“a substance detected”可把本題的正確選項迅速確定在第二段的尾句。該句所涉及的核心問題是“where”，所以本題的正確選項應該是C�？忌�在解題時一定要迅速而準確地進行審題定位。

　　3. 【答案】D

　　【考點解析】本題是一道細節推導題。通過本題題干中的“peptide hormones are created in the brain in areas other than the hypothalamus”可將本題的答案信息來源迅速確定在尾段的第一句。尾段第一句主要就肽激素的“function”進行論述，可見本題的正確選項應該是D，選項D中的“role”就等于原文中的“function”�？忌�在解題時一定要善于抓住主句中的重要信息。

　　4. 【答案】B

　　【考點解析】這是一道句間關系題。根據本題題干中的“the immunological method”可將本題的答案信息來源迅速確定在第二段的第二句，而本題的確切答題點在第二段的第三句的后半部分。從第二段第三句的后半部分可以推導出本題的正確答案是B�？忌�在解題時一定要注意一般概括句和具體陳述句之間的相互關系。

　　5. 【答案】D

　　【考點解析】本題是一道細節推導題。根據本題題干中的人名“Roberts”可將本題的答案迅速確定在倒數第二段引號部分的第二句話，即“But”一詞引導的句子。從該句中的“needed”一詞可以看出本題的正確選項應該是D�？忌�在解題時一定要學會深入理解原文的字面含義。

　　[參考譯文]

　　肽激素除了下丘腦能制造，在大腦中任何其它的地方都能夠制造。大約五年前僅這一想法本身就是令人驚詫的�？茖W家認為，肽激素是由內分泌腺制造的，而下丘腦被認為是大腦中唯一的內分泌腺。而且，由于肽激素無法穿過血腦障礙，研究人員認為它們從不曾到過除下丘腦以外的大腦任何其它部位，肽激素僅在下丘腦制造出來，然后被釋放到血管中。

　　但是關于肽激素的這種觀點已經遭到質疑。通過一次又一次的實驗發現，肽激素的抗血清一旦被注射到大腦中，它就會在下丘腦以外的地方粘接起來。這就說明這些地方或是有肽激素存在，或是有與抗血清發生交叉反應的其他物質存在。但是，通過抗血清來檢驗肽激素的免疫學方法是不精確的。交叉反應可能會發生，而且以這種手段無法確認用抗血清檢測的特質確實是肽激素還是僅是與其近似的親緣物質。另外，這種方法不能用來確定被測物質在人體內產生的部位。

　　然而，分子生物學的新技術為解決這些問題提供了一個新途徑�？茖W家可以制造出一種特別的互補DNA’S (cDNA’s)，作為分子探子查找出肽激素的信使RNA’S (mRNA’s)。如果腦細胞正在制造肽激素，那么它應該包含這些信使RNA’S。如果腦細胞制造的產品與肽激素相似但并不完全相同，那么這些互補cDAN’S仍然會和這些信使mRNA’S粘結，但不會象和真正肽激素的信使mRNA’S結合得那么緊密，這些包含信使mRNA’S的細胞能被分開。研究者可以將信使mRNA’S解碼以確定其蛋白質產品究竟是什么及這些產品在多大程度上類似于真正的肽激素。

　　采用cDAN探子這一分子生物學方法檢測肽激素同時也比免疫學方法快得多，因為如果用免疫學方法，分離肽激素需要幾年枯燥乏味的提純過程，然后還需培養出他們的抗血清。羅伯茨的一番話表達了許多研究人員的心聲，他說：“我是作為一名內分泌學家接受訓練的，但情況對我來說很清楚，內分泌學領域需要分子生物學的輸入，靠碾磨來制造蛋白質純化物的過程實在是太慢了。”

　　如果正如用cDNA探子所做的最初測試表明的那樣，肽激素確實是由大腦中下丘腦以外的部位制造出來的，則有必要建立一套理論來解釋它們在大腦中的作用。某些學者指出肽激素是生長調節劑，但羅森對老鼠大腦所作的實驗表明事實并非如此。很多其它的研究人員指出肽激素或許被用于大腦內細胞與細胞間的信息傳輸。