|
Passage 5 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 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. 26. The main topic of the passage is (A) the early development of embryos of lower marine organisms (B) the main contribution of modern embryology to molecular biology (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 (E) scientific dogma as a factor in the recent debate over the value of molecular biology
D is the best answer. In identifying the main topic of the passage, you must consider the passage as a whole. In the first paragraph, the author provides a historical context for the debate described in the second paragraph, concerning when and how the determination of embryo cells takes place. The third and forth paragraphs provide a specific example of the “Recent discoveries in molecular biology” (lines 28-29) that may lead to the resolution of that debate.
做题的时候就在C和D之间挣扎，看了答案还是不知道为什么C是错的。大家给分析一下行么？ 26. The main topic of the passage is (A) the early development of embryos of lower marine organisms (B) the main contribution of modern embryology to molecular biology (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 (E) scientific dogma as a factor in the recent debate over the value of molecular biology
D is the best answer. In identifying the main topic of the passage, you must consider the passage as a whole. In the first paragraph, the author provides a historical context for the debate described in the second paragraph, concerning when and how the determination of embryo cells takes place. The third and forth paragraphs provide a specific example of the “Recent discoveries in molecular biology” (lines 28-29) that may lead to the resolution of that debate. 做题的时候就在C和D之间挣扎，看了答案还是不知道为什么C是错的。大家给分析一下行么？ | 
京公网安备11010202008513号 京ICP证101109号 京ICP备12012021号
发表于 2007-9-27 16:57:00
楼主
