2. 考研
  3. Nearly a century ago, biologists found that if they separated an invertebrate animal embryo into two parts at an early stage of

Nearly a century ago, biologists found that if they separated an invertebrate animal embryo into two parts at an early stage of

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问题    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 irreversible 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, 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, presumable, 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 sections 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 guides the fate of the cells in which they are located.
The passage is most probably directed at which kind of audience?
选项 A、State legislators deciding about funding levels for a state-funded biological laboratory.
B、Readers of an alumni newsletter published by the college that Paul Gross attended
C、Marine biologists studying the processes that give rise to new species.
D、Undergraduate biology majors in molecular biology cours
答案C
解析 本文介绍了形态发生决定子(morphogenetic determinants)的存在和功能，以及它是如何细胞发展以至生物形态形成，形态发生决定子在细胞内的不同分布决定了细胞的发展命运，这就使通过改变形态决定子分布而研究新物种成为可能，关于这个问题的研究是基于海胆类生物的，所以最有可能的读者是研究新物种形成的海洋生物学家。
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