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查了一下以往的帖子，没有对这道题的详细解答，故再放上来请教大家~~！！
Passage 14 (14/63)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 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[生]细胞质 of the egg cell; i.e., in that part of the cell’s protoplasm[生]原生质
that lies outside of the nucleus. In the unfertilized egg, the substances are inactive and are not distributed homogeneously(Of the same or similar nature or kind).
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(参差不齐的; 不规则的) 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 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
答案选E，我选的是c，思路跟cd上以前有人错的一样，但是可惜这道题没有人回答啊啊啊啊啊啊啊
C.inactive until the embryo cells 是第三段的叙述，而irreversibly committed to their final function是第二段的叙述，这种结构通常是错误的。但是原文"When the egg is fertilized, the substances become active and, presumably, govern the behavior of the genes they interact with"是不是可以理解为第二段叙述的irreversibly committed to their final function，
我从正确答案上推，是不是因为以前debate的时候发现because no one was able to ask the crucial questions in a form in which they could be pursued productively. 然后现在科学家又能够know at least some of the molecules that act as morphogenetic determinants in early development.所以通过productiviely这个词推出这个cd量很多？？？？？
望NN帮忙解答。
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发表于 2011-8-21 21:28:31
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