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  3. Nearly a century ago, biologists found that if they separated an invertebrat

Nearly a century ago, biologists found that if they separated an invertebrat

游客2024-01-13  5
问题     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 deter- minants 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 pro- toplasm 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 guides the fate of the cells in which they are located. [br] Which of the following circumstances is most comparable to the impasse biologists encountered in trying to resolve the debate about cell determination in lines 16-24?
选项 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.
答案 D
解析 哪一个选项类似于生物学家在试图解决关于细胞决定的争执时所遇到的困境?类比题，先总结原文。此问题无法解决，因为没有人能将此问题表述为一个可以继续进行有效研究的方式，即没有任何途径能将此研究进行下去，其原因是当时生物学家的认识有限。A．一学者想使用原始资料，但又不懂这些资料所使用的语言。没有指出这个学者用这些资料是要进行研究还是干别的什么。B．一数学家发现证明过程有错。显然不符。C．工程师面临问题是要设计适应某种环境，而这种环境无法先被检测。此困难不是由于他们本身缺陷引起，是客观环境条件所限。D．正确。一个试图研究语言习得理论(语言学一个分支)的语言学家，其本身关于语言结构的知识也就是初级水平。符合上面判断。一个问题无法解决、无从下手，是因为研究者本身的缺陷。E．只能资助两个科学计划中的一个，需进行选择。不合上文总结。
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