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Nearly a century ago, biologists found that if they separated an invertebrate animal embryo into two parts at an early stage of its life, it would survive and develop as two normal embryos. This led them to believe that the cells in the early embryo are undetermined in the sense that each cell has the potential to develop in a variety of different ways. Later biologists found that the situation was not so simple. It matters in which plane the embryo is cut. If it is cut in a plane different from the one used by the early investigators, it will not form two whole embryos. A debate arose over what exactly was happening. Which embryo cells are determined, just when do they become irreversibly committed to their fates, and what are the “morphogenetic determinants” that tell a cell what to become? But the debate could not be resolved because no one was able to ask the crucial questions in a form in which they could be pursued productively. Recent discoveries in molecular biology, however, have opened up prospects for a resolution of the debate. Now investigators think they know at least some of the molecules that act as morphogenetic determinants in early development. They have been able to show that, in a sense, cell determination begins even before an egg is fertilized. Studying sea urchins, biologist Paul Gross found that an unfertilized egg contains substances that function as morphogenetic determinants. They are located in the cytoplasm of the egg cell; i.e., in that part of the cell’s protoplasm that lies outside of the nucleus. In the unfertilized egg, the substances are inactive and are not distributed homogeneously. When the egg is fertilized, the substances become active and, presumably, govern the behavior of the genes they interact with. Since the substances are unevenly distributed in the egg, when the fertilized egg divides, the resulting cells are different from the start and so can be qualitatively different in their own gene activity. The substances that Gross studied are maternal messenger RNA’s—products of certain of the maternal genes. He and other biologists studying a wide variety of organisms have found that these particular RNA’s direct, in large part, the synthesis of histones, a class of proteins that bind to DNA. Once synthesized, the histones move into the cell nucleus, where section of DNA wrap around them to form a structure that resembles beads, or knots, on a string. The beads are DNA segments wrapped around the histones; the string is the intervening DNA. And it is the structure of these beaded DNA strings that guide the fate of the cells in which they are located.
9. The passage is most probably directed at which kind of audience? (A) State legislators deciding about funding levels for a state-funded biological laboratory (B) Scientists specializing in molecular genetics (C) Readers of an alumni newsletter published by the college that Paul Gross attended (D) Marine biologists studying the processes that give rise to new species (E) Undergraduate biology majors in a molecular biology course 我选了B 答案为E, 原因是：文中解释了大量的basic knowledge，所以不应该是给科学看到，而是给undergraduate讲述的一个世纪前的一段生物研究的发展。
10. It can be inferred from the passage that the morphogenetic determinants present in the early embryo are (A) located in the nucleus of the embryo cells (B) evenly distributed unless the embryo is not developing normally (C) inactive until the embryo cells become irreversibly committed to their final function (D) identical to those that were already present in the unfertilized egg (E) present in larger quantities than is necessary for the development of a single individual 依然选错：这是一道非常infer的题：biologists found that if they separated an invertebrate animal embryo into two parts at an early stage of its life, it would survive and develop as two normal embryos. 答案为E，原文讲embryo被biologists分成两个parts并且非开后的两个都作为了normal embryos生存和发展support了选项E：embryo比单一个体的发展所需要的quantities更大
11. The main topic of the passage is D (A) the early development of embryos of lower marine organisms (B) the main contribution of modern embryology to molecular biology (C) the role of molecular biology in disproving older theories of embryonic development (D) cell determination as an issue in the study of embryonic development (E) scientific dogma as a factor in the recent debate over the value of molecular biology 终于阴差阳错对了一道主旨题：全文都在讲上个世纪的biologists围绕试图解决这个问题（Which embryo cells are determined, just when do they become irreversibly committed to their fates, and what are the “morphogenetic determinants” that tell a cell what to become?）
12. According to the passage, when biologists believed that the cells in the early embryo were undetermined, they made which of the following mistakes? (A) They did not attempt to replicate the original experiment of separating an embryo into two parts. (B) They did not realize that there was a connection between the issue of cell determination and the outcome of the separation experiment. (C) They assumed that the results of experiments on embryos did not depend on the particular animal species used for such experiments. (D) They assumed that it was crucial to perform the separation experiment at an early stage in the embryo’s life. (E) They assumed that different ways of separating an embryo into two parts would be equivalent as far as the fate of the two parts was concerned. 继续选错：答案为E，很容易定位undermined在文中只出现了一次-This led them to believe that the cells in the early embryo are undetermined in the sense that each cell has the potential to develop in a variety of different ways...If it is cut in a plane different from the one used by the early investigators, it will not form two whole embryos. biologists发现事情并不简单，因为不同的切法会有不同的结果（所以原本认为不同的切法和normal embryo的生存发展一致）
13. It can be inferred from the passage that the initial production of histones after an egg is fertilized takes place A (A) in the cytoplasm (B) in the maternal genes (C) throughout the protoplasm (D) in the beaded portions of the DNA strings (E) in certain sections of the cell nucleus 又懵对了一道：文中histones只出现在了最后一段，but！！在最后一段完全找不到答案，后来结合了后面几道题：我才理解到最后一段的substances是对倒数第二段中G研究了unfertilized egg包含的物质及性能，然后又说when fertilized egg时，这些物质发生了改变（active and...和其他genes相互作用）最后一段解释了这些substances在fertilized egg中是发生了什么，答案对应原文（They are located in the cytoplasm of the egg cell; i.e., in that part of the cell’s protoplasm that lies outside of the nucleus.) 梳理一下，They are located in the cytoplasm of the egg cell; they是unfertilized eggs包含的物质，随后指出这些物质在fertilized eggs中变得不同，最后一段说了histones是....也就是这些substances
14. It can be inferred from the passage that which of the following is dependent on the fertilization of an egg? (A) Copying of maternal genes to produce maternal messenger RNA’s (B) Synthesis of proteins called histones (C) Division of a cell into its nucleus and the cytoplasm (D) Determination of the egg cell’s potential for division (E) Generation of all of a cell’s morphogenetic determinants 毫无意外；继续错 答案为B 示意同13题 unfertilized情况下inactive 无变化；fertilized后 Once synthesized, the histones move into the cell nucleus；所以histones在两种情况下呈现不同活动
15. According to the passage, the morphogenetic determinants present in the unfertilized egg cell are which of the following? (A) Proteins bound to the nucleus (B) Histones (C) Maternal messenger RNA’s (D) Cytoplasm (E) Nonbeaded intervening DNA 答案C G研究的这些物质 are maternal messenger RNA’s—products of certain of the maternal genes. 可能是后面的一系列基因活动是fertilized情况下所发生的，然而原始研究的为这个母基因，所以选C，或许用排除法 A. Once synthesized, the histones move into the cell nucleus, fertilized active才进入细胞核，unfertilized的物质是在细胞核之外（They are located in the cytoplasm of the egg cell; i.e., in that part of the cell’s protoplasm that lies outside of the nucleus）排除 B. histones：RNA指导H合成，同A，依然是fertilized eggs active的活动 D. Cytoplasm只是substance所在的一个place E. intervening DNA只是描述了一种fertilized active内部物质改变后的一种形态，并未提到unfertilized时是否或者为其他形态
16. The passage suggests that which of the following plays a role in determining whether an embryo separated into two parts will develop as two normal embryos? I. The stage in the embryo’s life at which the separation occurs II. The instrument with which the separations is accomplished III. The plane in which the cut is made that separates the embryo (A) I only (B) II only (C) I and II only (D) I and III only (E) I, II, and III I into two parts at an early stage of its life,文中第一句话 II 未提到 III If it is cut in a plane different from the one used by the early investigators, it will not form two whole embryos. 答案为D
17. Which of the following circumstances is most comparable to the impasse biologists encountered in trying to resolve the debate about cell determination (lines 6-8)? (A) The problems faced by a literary scholar who wishes to use original source materials that are written in an unfamiliar foreign language (B) The situation of a mathematician who in preparing a proof of a theorem for publication detects a reasoning error in the proof (C) The difficulties of a space engineer who has to design equipment to function in an environment in which it cannot first be tested (D) The predicament of a linguist trying to develop a theory of language acquisition when knowledge of the structure of language itself is rudimentary at best (E) The dilemma confronting a foundation when the funds available to it are sufficient to support one of two equally deserving scientific projects but not both 最后一题了，一边实习一边得空做题，这篇文章研究了一天 终于在这里要结束了 问和文章中biologists面临的困境相对的是以下哪个困境 D. 语言学家只懂理论基础知识的困境对应原文 But the debate could not be resolved because no one was able to ask the crucial questions in a form in which they could be pursued productively.
小女不才，纯靠自己的理解，有任何错误与偏差 请您指正，谢谢~
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发表于 2020-11-18 22:35:09
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