|
问题解决型 生物类
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.
第一段：无论如何分，胚胎会独立发展，不足:没有考虑到不同的平面，讲的是事件的背景
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.解决：MD
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 （n.[生]细胞质）of the egg cell; i.e., in that part of the cell’s protoplasm that lies outside of the nucleus（nucleus[nuclear的复数, 见nuclear]
n.核子）. 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.以SU为例，说明ＭＤ如何起作用，受精与未受精对比
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.说明MD如何起作用，maternal messenger RNA’就是ＭＤ:RàHàCNàbeaded string
2. It can be inferred from the passage that the morphogenetic determinants present in the early embryo are infer, 这题用ＰＯＥ好
(A) located in the nucleus of the embryo cells 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
(B) evenly distributed unless the embryo is not developing normally 反In the unfertilized egg, the substances are inactive and are not distributed homogeneously.
(C) inactive until the embryo cells become irreversibly（irreversible damage, change etc is so serious or so great that you cannot change something back to how it was before） committed to their final function 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.
(D) identical to those that were already present in the unfertilized egg（E）无
(E) present in larger quantities than is necessary for the development of a single individual
这题我一直没明白 读了OG的解释也没读懂 E为何对 另外OG为何说C是与原文相反的呢 我认为是对的
3. The main topic of the passage is 主题题
(A) the early development of embryos of lower marine organisms无
(B) the main contribution of modern embryology （embryologyn.胚胎学）to molecular biology无
(C) the role of molecular biology in disproving older theories of embryonic development
(D) cell determination as an issue in the study of embryonic development（D）
(E) scientific dogma as a factor in the recent debate over the value of molecular biology无
C为何不对
谢谢
[此贴子已经被作者于2004-6-16 6:58:47编辑过] | 
京公网安备11010202008513号 京ICP证101109号 京ICP备12012021号
发表于 2004-6-16 06:49:00
楼主
