GMAT综合阅读精解之十二

2022-06-08 19:43:46

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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

  (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 func-

  tion 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 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

  (50) 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?

  (A) State legislators deciding about funding levels for a

  state-funded biological laboratory

  (B) Scientists specializing in molecular genetics

  (C) Readers of an alumni newsletter published by the

  college that Paul Gross attended

  (D) Marine biologists studying the processes that give

  rise to new species

  (E) 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

  (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) present in larger quantities than is necessary for the

  development of a single individual

  3. 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

  4. According to the passage, when biologists believed that

  the cells in the early embryo were undetermined, they

  made which of the following mistakes?

  (A) They did not attempt to replicate the original

  experiment of separating an embryo into two parts.

  (B) They did not realize that there was a connection

  between the issue of cell determination and the

  outcome of the separation experiment.

  (C) They assumed that the results of experiments on

  embryos did not depend on the particular animal

  species used for such experiments.

  (D) They assumed that it was crucial to perform the

  separation experiment at an early stage in the

  embryo’s life.

  (E) 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

  (A) in the cytoplasm

  (B) in the maternal genes

  (C) throughout the protoplasm

  (D) in the beaded portions of the DNA strings

  (E) 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?

  (A) Copying of maternal genes to produce maternal

  messenger RNA’s

  (B) Sythesis of proteins called histones

  (C) Division of a cell into its nucleus and the cytoplasm

  (D) Determination of the egg cell’s potential for division

  (E) Generation of all of a cell’s morphogenetic

  determinants

  7. According to the passage, the morphogenetic

  determinants present in the unfertilized egg cell are

  which of the following?

  (A) Proteins bound to the nucleus

  (B) Histones

  (C) Maternal messenger RNA’s

  (D) Cytoplasm

  (E) 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 embryo’s 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

  (A) Ⅰonly

  (B) Ⅱ only

  (C) Ⅰ and Ⅱ.only

  (D) Ⅰ and Ⅲ.only

  (E) 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

  (lines 12-18)?

  (A) The problems faced by a literary scholar who wishes

  to use original source materials that are written in

  an unfamiliar foreign language

  (B) The situation of a mathematician who in preparing a

  proof of a theorem for publication detects a

  reasoning error in the proof

  (C) The difficulties of a space engineer who has to

  design equipment to function in an environment in

  which it cannot first be tested

  (D) 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

  (E) 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

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