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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 ceils in the early embryo are undetermined in the sense that each cell has the potential to develop in a variety of different ways. Late 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 irreversible 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 determinants in early development. They have been able to show that, in a sense, ceil 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 protoplasm that lies outside of the nucleus. In the unfertilized egg, the substances ate inactive and are not distributed homogeneously.  When the egg is fertilized, the substances become active and, presumable, 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 round them to form a structure that resembles beads, or knots, on a string.  The beads are DNA segments wrapped around the histone; 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.
   Choose the most appropriate from the four choices to complete the sentence or answer the question.
It can be inferred form the passage that the morphogenetic determinants present in the early embryo are ______.
选项 A、evenly distributed unless the embryo is not developing normally
B、inactive until the embryo cells become irreversibly committed to their final function
C、identical to those that were already present in the unfertilized egg
D、present in larger quantities than is necessary for the development of a single individual
答案D
解析
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