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这篇文章当时做的是丢盔弃甲泪流满面啊。希望上战场时不要遇到这么BT的。
请看以下我表示出来的几个句子，联合起来就可以推出你这个问题的答案，即在cytoplasm 中。
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 develops 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 embryos 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 to 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 function 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 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
50) DNA strings that guides the fate of the cells in which they are located. |
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发表于 2010-3-25 20:08:11
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