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PASSAGE 14
* 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 developas 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 embryois 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 questionsin 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 foundthat 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 maternalgenes. He and other biologists studying a wide varietyof organisms have found that these particular RNA's direct, in large part, the synthesis of histones, a classof 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 DNAsegments 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 ofaudience?
(A) State legislators deciding about funding levels for astate-funded biological laboratory
(B) Scientists specializing in molecular genetics
(C) Readers of an alumni newsletter published by thecollege that Paul Gross attended
(D) Marine biologists studying the processes that giverise to new species
(E) Undergraduate biology majors in a molecularbiology course
答案是E.我选的B.不知道怎么判断是E的..
2. It can be inferred from the passage that the morphogenetic determinants present in theearly embryo are
(A) located in the nucleus of the embryo cells
(B) evenly distributed unless the embryo is notdeveloping normally
(C) inactive until the embryo cells become irreversiblycommitted to their final function
(D) identical to those that were already present in theunfertilized egg
(E) present in larger quantities than is necessary for thedevelopment of a single individual
为什么会是E呢？偶选的是D。不懂E是怎么定位的
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发表于 2005-10-3 23:30:00
