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  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.
According to the passage, when biologists believed that the cells in the early embryo were undetermined, they made which of the following mistakes?
选项 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.
答案E
解析 如果生物学家相信，早期胚胎中细胞是不确定的，将会发生哪个错误?本文结论为：早期胚胎中细胞是确定的，如果此结论被否，则推出此结论的论据也应该不成立。∴E正确，他们将认为不同的切割方法产生的效果将是一样的。正是原文论据“切割平面不同，产生的效果不同”的取非后结果。A、B、C、D都和原文推理过程无关。
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