2. 外语
  3. Nearly a century ago, biologists found that if they separated an invertebrate animal embryo into two parts at an early stage of

Nearly a century ago, biologists found that if they separated an invertebrate animal embryo into two parts at an early stage of

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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.
The passage is most probably directed at which kind of audience?
选项 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.
答案E
解析 本文适合哪类读者?这是特殊的主题题型问法。本文是一篇讲生物学说发展的文章，最后讲明了从分子生物学的角度怎样解决一个早已提出的问题，但是本文并不涉及前沿新知，而只是很浅显地、略带科普性地讲了这个问题。A．国家立法者和C．校友会通讯读者与本文内容完全无关。B．分子遗传学方面的专家。前已论述，此文并不是学术文章，而是讲一个生物学中的基本问题，专家有可能是写作者，不会成为读者。D．研究发现新物种步骤的海洋生物学家，无关。E．本科生物学生学习分子生物学课程。正确，符合上面的分析。
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