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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.
It can be inferred from the passage that the morphogenetic determinants present in the early embryo are
选项 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.
答案E
解析 从原文可以推出，早期胚胎中的“形态发生决定体”是：A．位于细胞核中。明显与原文不符，L38指出，位于细胞质中。B．除非胚胎发育不正常，应均匀分布。LA6—48明确说是不均匀的。C．在胚胎细胞不可逆转发挥其作用之前，不活跃。胚胎细胞发挥其作用，就是这些“形态发生决定体”活跃并发挥作用的结果，见原文第三、四段叙述。D．和未受精卵中已存在的完全一样。文中无。E．正确。有比一个体胚胎发育所需更多的量。本文一开始提到，在胚胎发育早期，切割胚胎为两部分将产生两个独立个体。如果此结论为非(一个胚胎中形态发生决定体的数目只够它自身发育)，此实验结果就不可能出现。
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