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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 determinants 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 (any of numerous echinoderms (class Echinoidea) that are usually enclosed in thin brittle globular tests covered with movable spines海胆), 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 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 guide the fate of the cells in which they are located.
2. It can be inferred from the passage that the morphogenetic determinants present in the early embryo are
(A) located in the nucleus of the embryo cells
(B) evenly distributed unless the embryo is not developing normally
(C) inactive until the embryo cells become irreversibly committed to their final function
(D) identical to those that were already present in the unfertilized egg（E）
(E) present in larger quantities than is necessary for the development of a single individual
看了ETS的解释是明白了，但是如果我做，我肯定会错，因为我绝对不会象ETS说的那样去思考。因为它说根据文中开头和结尾段提供的信息（用红色highlight的部分），答案是E。但我不会这样去定位。
请问NN们，如果你们遇到这样的题，如何处理？如何会原文去定位。
而且许多大N们曾不值一次的说到，为了节省时间，文章有些内容可以略读（比如举例、作者的进一步说明等），那哪些内容该略读，而往往很多时候，考题也许就出在那些略读的部分，这样一来反而容易出错。请问NN是如何处理这些问题的。顺便说明，整篇文章我是明白意思的，如果全部文字都读过的话，不是要花很长时间？但略读的话，肯定有些信息又会丢失。这不是一个矛盾吗。而且，如果记忆不好的话，回文章定位也会花时间。请问NN们是如何解决这些问题的？
多谢了。~~~ | 
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发表于 2005-3-5 01:25:00
