首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
考研
Nearly a century ago, biologists found that if they separated an invertebrate animal embryo into two parts at an early stage of
Nearly a century ago, biologists found that if they separated an invertebrate animal embryo into two parts at an early stage of
admin
2012-01-05
35
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 irreversible 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, presumable, 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 sections 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.
It can be inferred form the passage that the morphogenetic determinants present in the early embryo are ______.
选项
A、evenly distributed unless the embryo is not developing normally
B、inactive until the embryo cells become irreversibly committed to their final function
C、identical to those that were already present in the unfertilized egg
D、present in larger quantities thanis necessary for the development of a single individual
答案
D
解析
第三段第三句指出,正常状态下,形态发生决定子的分布就是不均匀的,故A项错误。第三段第四句指出,它是在卵子受精后被激活,B项错误。第三段第一句提到,Paul Gross发现未受精的鸡蛋含有一些起到形态发生决定子功能的物质,但它们并不能起到作用,要在卵子受精后被激活才成为形态发生决定子,C项错误。最后一段第二句指出,这些物质要大量存在才能指导组织蛋白的合成,故D项正确。
转载请注明原文地址:https://kaotiyun.com/show/uNua777K
本试题收录于:
翻译硕士(翻译硕士英语)题库专业硕士分类
0
翻译硕士(翻译硕士英语)
专业硕士
相关试题推荐
ANationofHypochondriacsThemainimpressiongrowingoutoftwelveyearsonthefacultyofamedicalschoolisthattheNo.1
ConvertingtheMasses:StarbucksinChinaItsoundslikeMissionImpossible:sellcoffeetoChina’steadrinkers.Starbucks’so
Itisnecessarythathe______theassignmentwithoutdelay.
MillionsofelderlyGermansreceivedanoticefromtheHealth&SocialSecurityMinistryearlierthismonththatstruckadamagi
Theeighteenth-centuryBritishthinkerEdmundBurkemadethefollowingobservationaboutthebenefitsofopposition:"Hethatwr
Ihadno______aboutspeakingthelanguagewhenIwasinGreece;itwasdrivingontheothersideoftheroadwhichbothersme.
Somebelievethatintheageofidentikitcomputergames,massentertainmentandconformityonthesupermarketshelves,trulyin
AtHarvardUniversity’smostrecentCommencementCeremony,femalePresidentDrewFausthadanimportantreminderforstaffands
Atseveno’clockeachmorningabellsoundsintheredbrickbuildingsonthesteepbankoftheHudsonRiveratOssining,NewYo
Youwon’tdieofunhappiness.Contrarytothelong-heldandwidespreadbeliefthatthemorestressedandoutofcontrolyouare
随机试题
诺贝尔希望人们把他作为热爱和平的人来记住他。
职业教育在学制的地位有()个,是()的、()的和()的。
骨折固定分为
如果乙公司对外负债100余万元无力偿还,而该债务是在甲公司决策、指示下以乙公司的名义进行贸易造成的,甲公司对此债务的责任应如何判定?()。如果甲公司董事长万某以王某不懂经营为由,欲解除王某所担任的丙公司董事长职务,而丙公司的董事会成员均表示反对
【2011专业案例真题上午卷】某企业的35kV变电所,35kV配电装置选用移开式交流金属封闭开关柜,室内单层布置,10kV配电装置采用移开式交流金属封闭开关柜,室内单层布置,变压器布置在室外,平面布置示意图如下图所示,请回答下列问题,并列出解答过程:
某厂为了提高质量检验的效率和效果,合理地配置检验资源,对本厂的产品质量检验系统、质量检验等各方面进行了更为规范的规划和分类。从事质量检验工作首先要明确质量检验的作用,严格把关使不合格品_______。
小李开了一个多小时会议,会议开始时看了手表,会议结束又看了手表,发现时针与分针恰好互换了位置,问这个会议大约开了1小时多少分?
以特定目的作为犯罪构成必要要件的犯罪是()。
GigabitEthernet的传输速率比传统的10MbpsEthernet快100倍,但是它们仍然保留着和传统Ethernet相同的______。
ReadthearticlebelowabouttheBritishlongerworkinghours.Foreachquestion23-28,choosethecorrectanswer.Markonele
最新回复
(
0
)