首页
登录
职称英语
Nearly a century ago, biologists found that if they separated an invertebrat
Nearly a century ago, biologists found that if they separated an invertebrat
游客
2025-02-26
43
管理
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 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 deter- minants 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 pro- toplasm 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 guides the fate of the cells in which they are located. [br] It can be inferred from the passage that which of the following is dependent on the fertilization of an egg?
选项
A、Copying of maternal genes to produce maternal messenger RNA’s.
B、Synthesis of proteins called histones.
C、Division of a cell into its nucleus and the cytoplasm.
D、Determination of the egg cell’ s potential for division.
E、Generation of all of a cell’s morpho-genetic determinants.
答案
B
解析
以下哪一个有赖于卵子的受精?A.母系基因的复制从而产生母系信使RNA。RNA是“形态发生决定体”中一种,L35~37指出,它们在受精前已存在。B.组蛋白的合成。正确。原文L55—59指出组蛋白合成受RNA控制(direct),而L43—46指出受精后,“形态发生决定体”才活跃并决定(govern)某些基因的行为。所以,此行为有赖于受精。C.细胞分裂,分化为细胞核和细胞质。文中未提。D.卵细胞分裂趋势的确定。无。E.一个细胞内所有“形态发生决定体”的产生。无。
转载请注明原文地址:http://tihaiku.com/zcyy/3973764.html
相关试题推荐
Attheendofthenineteenthcentury,arisinginterestinNativeAmericanc
Nearlyacenturyago,biologistsfoundthatiftheyseparatedaninvertebrat
Nearlyacenturyago,biologistsfoundthatiftheyseparatedaninvertebrat
Nearlyacenturyago,biologistsfoundthatiftheyseparatedaninvertebrat
Nearlyacenturyago,biologistsfoundthatiftheyseparatedaninvertebrat
Nearlyacenturyago,biologistsfoundthatiftheyseparatedaninvertebrat
Nearlyacenturyago,biologistsfoundthatiftheyseparatedaninvertebrat
Milankovitchproposedintheearlytwentiethcenturythattheiceageswere
Milankovitchproposedintheearlytwentiethcenturythattheiceageswere
Milankovitchproposedintheearlytwentiethcenturythattheiceageswere
随机试题
HowmanypeoplediedoneregionaloneaftercycloneIvan?[originaltext]In20
Thechangeinthatvillagewasmiraculou
引起急性血源性骨髓炎最常见的致病菌是A:溶血性金黄色葡萄球菌 B:嗜血属流感杆
各种运输方式内外部的各个方面的构成和联系,就是( )。 A.运输系统
对于新员工来讲,所谓挑战性工作主要是()。A.独立完成某一具体工作任务 B.主
向上级机关汇报工作用()。A.请求 B.报告 C.通告 D.通报
男,40岁。24小时前颊部外伤,全层组织缺损约3.5cm*4cm。清创后适合的缝
根据《环境影响评价技术导则—地面水环境》,关于水质取样垂线布设的说法,错误的是(
下列可作为施工作业指导书主要编制依据的有()。A.国家、行业和总公司有关设计、施
根据《计价规范》规定,下列有关工程量清单计价下的投标报价的编制原则,正确的有()
最新回复
(
0
)