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 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, 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 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.
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
答案是E，我选得是B。
我的思路是：最后一段指出全文讨论的物质是一种基因信息素，那么文章的受众不应该是那些学基因学的人吗？
请问大家正确的思路是什么，谢啦
5.Itcan 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
这题选是选对了，可是真心没谱啊！
Wangsiwei大牛的回答是
原文第三段－Once synthesized, the histones move into the cell nucleus, 前一句－these particular RNA’s direct the synthesis of H,而第二段种说了这个RNA就是located in the cytoplasm of the egg cell说明开始的发生地是C...., 从另一个角度讲，说了是move into the cell nucleus, 说明了开始是在nucleus的外面，第二段说了那个C...就是在nucleus的外面。
但是direct也不一定表明是在一起的呀，还是我钻牛角尖了，谢谢啦作者: 醉清风额 时间: 2013-1-14 02:43
1.The passage is most probably directed at which kind of audience?
5.Itcan be inferred from the passage that the initial production of histones after an egg is fertilized takes place作者: 醉清风额 时间: 2013-1-14 02:57 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 asmorphogenetic 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 substancesthat function as morphogenetic determinants. They are located in the cytoplasm of the eggcell; 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, 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 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.
morphogenetic determinants = substances =messengerRNA’s （信使RNA ） 而信使RNA很大程度上决定histones的合成 由文章可知morphogenetic determinants = substances in the cytoplasm 因此 histones的合成同样in the cytoplasm作者: aaron22 时间: 2013-1-14 20:21
谢谢，原来定位是substance，恍然大悟啊
文章还是理解不到位啊作者: laurachen310 时间: 2021-10-6 00:41
5.Itcan 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
第五题还是不太懂，文章问的是after an egg is fertilized，但是定位这里说的不是unfertilized吗？“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”