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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? Butthe 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 ofthe egg cell; i.e., in that part of the cell’s protoplasm that liesoutside 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 messengerRNA’s—products of certain of the maternal genes. He and other biologists studying a wide variety of organisms have found thatthese particular RNA’s direct, in large part, the synthesis ofhistones, 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.
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
8. The passage suggests that which of the following plays a role in determining whether an embryo separated into two parts will develop as two normal embryos?
[size=10.666666984558105px]I[size=10.666666984558105px] The stage in the embryo’s life at which the separation occurs
[size=10.666666984558105px]II The instrument with which the separations is accomplished
III. The plane in which the cut is made that separates the
embryo
(A) I only
(B) II only
(C) I and II only
(D) I and III only
(E) I, II, and III
不明白以上的两道题，答案分别为E. D。
第一道题没有找到原文的定位在哪，很没头绪；第二道题中的[size=10.666666984558105px]I不知道从何而来。
困扰多时，求解答！
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发表于 2018-8-24 14:43:50
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