SAT文章阅读模拟题之embryo cells

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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 o show that, in a sense, cell determination begins even before an egg 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 cells 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 RNAs --products of certain of the maternal genes. He and other biologists studying a wide variety of organisms have found that these particular RNAs 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 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 DNA strings that guides the fate of the cells in which they are located.

　　1. The passage is most probably directed at which kind of audience?

　　 State legislators deciding about funding levels for a state-funded biological laboratory

　　 Scientists specializing in molecular genetics

　　 Readers of an alumni newsletter published by the college that Paul Gross attended

　　 Marine biologists studying the processes that give rise to new species

　　 Undergraduate biology majors in a molecular biology course

　　2. It can be inferred from the passage that the morphogenetic determinants present in the early embryo are

　　 located in the nucleus of the embryo cells

　　 evenly distributed unless the embryo is not developing normally

　　 inactive until the embryo cells become irreversibly committed to their final function

　　 identical to those that were already present in the unfertilized egg

　　 present in larger quantities than is necessary for the development of a single individual

　　3. The main topic of the passage is

　　 the early development of embryos of lower marine organisms

　　 the main contribution of modern embryology to molecular biology

　　 the role of molecular biology in disproving older theories of embryonic development

　　 cell determination as an issue in the study of embryonic development

　　 scientific dogma as a factor in the recent debate over the value of molecular biology

　　4. According to the passage, when biologists believed that the cells in the early embryo were undetermined, they made which of the following mistakes?

　　 They did not attempt to replicate the original experiment of separating an embryo into two parts.

　　 They did not realize that there was a connection between the issue of cell determination and the outcome of the separation experiment.

　　 They assumed that the results of experiments on embryos did not depend on the particular animal species used for such experiments.

　　 They assumed that it was crucial to perform the separation experiment at an early stage in the embryos life.

　　 They assumed that different ways of separating an embryo into two parts would be equivalent as far as the fate of the two parts was concerned.

　　5. It can be inferred from the passage that the initial production of histones after an egg is fertilized takes place

　　 in the cytoplasm

　　 in the maternal genes

　　 throughout the protoplasm

　　 in the beaded portions of the DNA strings

　　 in certain sections of the cell nucleus

　　6. It can be inferred from the passage that which of the following is dependent on the fertilization of an egg?

　　 Copying of maternal genes to produce maternal messenger RNAs

　　 Sythesis of proteins called histones

　　 Division of a cell into its nucleus and the cytoplasm

　　 Determination of the egg cells potential for division

　　 Generation of all of a cells morphogenetic determinants

　　7. According to the passage, the morphogenetic determinants present in the unfertilized egg cell are which of the following?

　　 Proteins bound to the nucleus

　　 Histones

　　 Maternal messenger RNAs

　　 Cytoplasm

　　 Nonbeaded intervening DNA

　　8. The passage suggests that which of the following plays a role in determining whether an embryo separated into two parts will two parts will develop as two normal embryos?

　　Ⅰ.The stage in the embryos life at which the separation occurs

　　Ⅱ. The instrument with which the separations is accomplished

　　Ⅲ. The plane in which the cut is made that separates the embryo

　　 Ⅰonly

　　 Ⅱ only

　　 Ⅰ and Ⅱ.only

　　 Ⅰ and Ⅲ.only

　　 Ⅰ,Ⅱ, and Ⅲ

　　9. Which of the following circumstances is most comparable to the impasse biologists encountered in trying to resolve the debate about cell determination ?

　　 The problems faced by a literary scholar who wishes to use original source materials that are written in an unfamiliar foreign language

　　 The situation of a mathematician who in preparing a proof of a theorem for publication detects a reasoning error in the proof

　　 The difficulties of a space engineer who has to design equipment to function in an environment in which it cannot first be tested

　　 The predicament of a linguist trying to develop a theory of language acquisition when knowledge of the structure of language itself is rudimentary at best

　　 The dilemma confronting a foundation when the funds available to it are sufficient to support one of two equally deserving scientific projects but not both

　　Correct Answers：EEDEABCDD