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 suggests that which of the following plays a role in determining whether an embryo separated into two parts and 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.
选项 A、I only
B、II only
C、I and II only
D、I and III only
E、I , II and III
答案D
解析 哪一个因素决定了将一个胚胎分成两部分、这两部分将发育为正常的个体?这个现象是文中一开始提出的例子。I．在胚胎发育的什么时候进行分割。原文L3一L4“at an early stage of its life”∴此题正确。Ⅱ．用什么工具进行分割。显然原文未提。Ⅲ．在哪个平面进行分割。原文L11—15对此进行过论述。∴D．I和Ⅲ。
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