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那位NN帮忙解答，拜谢了！ og-5-28: 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 (5) 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 (10) 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 (15) 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 (20) opened up prospects for a resolution of the debate. Now investigators think they know at least some of the molecules that act as morphogenetic determinants in early development. They have been able o show that, in a sense, cell determination begins even before an egg (25) is fertilized. Studying sea urchins, biologist Paul Gross found that an unfertilized egg contains substances that func- tion as morphogenetic determinants. They are located in the cytoplasm of the egg cell; i.e., in that part of the (30) cell’s protoplasm 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 (35) 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 (40) 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 (45) histones move into the cell nucleus, where section of DNA wrap around them to form a structure that resem- bles 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 (50) DNA strings that guides the fate of the cells in which they are located. 28. It can be inferred from the passage that the initial production of histones after an egg is fertilized takes place (A) in the cytoplasm (B) in the maternal genes (C) throughout the protoplasm (D) in the beaded portions of the DNA strings（A） (E) in certain sections of the cell nucleus 28. A is the best answer. In the third paragraph, the author asserts that substances that function as morphogenetic determinants are located in the cytoplasm of the cell and become active after the cell is fertilized. In the fourth paragraph we learn that these substances are “maternal messenger RNA’s” and that they “direct, in large part, the synthesis of histones,” which, after being synthesized, “move into the cell nucleus” (lines 59-68). Thus, it can be inferred that after the egg is fertilized, the initial production of histones occurs in the cytoplasm.
文章说substanceare located in the cytoplasm of the egg cell; i.e., in that part of the cell’s protoplasm that lies outside of the nucleus， 然后又说The substances that Gross studied are maternal messenger RNA’s 接下来these particular RNA’s direct, in large part, the synthesis of histones,
再下来 Once synthesized, the histones move into the cell nucleus, 如果选A的话，小弟不太理解。我觉得substance应该是RNA，而RNA direct（指导？不知对不对？）histones的合成。 而文章说的是substance，也就是RNA存在于cytoplasm中，怎么就能得出histones存在于cytoplasm中呢？ 不知小弟的逻辑存在什么问题，请NN指正！谢谢啦！
接下来these particular RNA’s direct, in large part, the synthesis of histones,
再下来 Once synthesized, the histones move into the cell nucleus, 如果选A的话，小弟不太理解。我觉得substance应该是RNA，而RNA direct（指导？不知对不对？）histones的合成。 而文章说的是substance，也就是RNA存在于cytoplasm中，怎么就能得出histones存在于cytoplasm中呢？ 不知小弟的逻辑存在什么问题，请NN指正！谢谢啦！
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发表于 2007-7-29 23:35:00
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