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Passage 14 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. 请问绿色部分的是什么意思呢？我的理解是synthesized DNA有三种不同的structure. but what does intervening DNA mean? 而且，这三种structure的DNA只有一种，the beaded DNA controls the fate of the cells. 是这样的吗？ 那如果是这样的话，那也就是DNA决定了细胞的命运。可是文中第二段里面的 morphogenetic determinants （红色部分）应该指的是RNA啊，那也就是说RNA决定了cell啊，那这不是自相矛盾吗？ 高中学的生物也都忘光了，大家讨论下啊：） 谢谢！ | 
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发表于 2006-5-27 20:02:00
NA片断包裹着得组蛋白）; the string is the intervening DNA（排成一列是因为DNA得相互连接，所以看起来是一列，像一条绳子）. 