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
外语
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
2014-09-18
71
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.
The passage suggests that which of the following plays a role in determining whether an embryo separated into two parts and two parts will develop as two normal embryos? I . The stage in the embryo’ s life at which the separation occurs. II. The instrument with which the separations is accomplished. III. The plane in which the cut is made that separates the embryo.
选项
A、I only
B、II only
C、I and II only
D、I and III only
E、I , II and III
答案
D
解析
哪一个因素决定了将一个胚胎分成两部分、这两部分将发育为正常的个体?这个现象是文中一开始提出的例子。I.在胚胎发育的什么时候进行分割。原文L3一L4“at an early stage of its life”∴此题正确。Ⅱ.用什么工具进行分割。显然原文未提。Ⅲ.在哪个平面进行分割。原文L11—15对此进行过论述。∴D.I和Ⅲ。
转载请注明原文地址:https://kaotiyun.com/show/N0tO777K
本试题收录于:
GMAT VERBAL题库GMAT分类
0
GMAT VERBAL
GMAT
相关试题推荐
SpeakerA:Hello.CanIbeofservicetoyou?SpeakerB:I’mjustwindowshopping.SpeakerA:Youarealwayswelcome.【D8】_____
Lookingbackonmychildhood,Iamconvincedthatnaturalistsarebornandnotmade.Althoughwewerebroughtupinthesameway
Newspapersarenotnearlyaspopulartodayastleywereinthepast.Therearenotverymanypeoplewhoseriouslyreadanewspa
SpeakerA:CouldIgetaridewithyoutotheconcerttonight?SpeakerB:Ican’tgo,______.Ithinkshe’sleavingaround7:30
IfyoulookcloselyatsomeoftheearlycopiesoftheDeclarationofIndependence,beyondtheflourishedsignatureofJohnHanc
IfyoulookcloselyatsomeoftheearlycopiesoftheDeclarationofIndependence,beyondtheflourishedsignatureofJohnHanc
Themulti-billion-dollarWesternpopmusicindustryisunderfire.ItisbeingblamedbytheUnitedNationsforthedramaticris
Themulti-billion-dollarWesternpopmusicindustryisunderfire.ItisbeingblamedbytheUnitedNationsforthedramaticris
Atthebeginningoflastcentury,medicalscientistsmadeaninterestingdiscovery:wearebuiltnotjustoffleshandbloodbut
Weallknowthatitispossibleforordinarypeopletomaketheirhomesontheequator(道),althoughoftentheymayfeeluncomfo
随机试题
黄老师教了八年的书,一天,刚上课她就很兴奋地宣布:“我想告诉你们,咱们班出了个诗人,小于写了一首很美的诗,我想读给大家听。”黄老师朗读了这首诗,它的确很美,然而,黄老师注意到小于的脸红了,看上去非常不安,班上有些同学在窃窃私语。后来,黄老师问小于是否愿意再
治疗痰湿咳嗽,宜选用的药物是()治疗痰热咳嗽,宜选用的药物是()
对乙酰氨基酚没有抗炎抗风湿作用是由于( )。
设计单位在施工图设计过程中.对不涉及重大设计原则问题的设计修改意见存在分歧,由()决定。
《中华人民共和国对外贸易法》正式实施的时间是()。
维果斯基的“最近发展区”意指()。
①海围着山,山围着我。沙田山居,峰回路转,我的朝朝暮暮,②日起日落,月望月朔,全在此中度过,我成了山人。问余何事栖碧山,笑而不答,山已经代我答了。其实山并未回答,是鸟代山答了,是虫,是松风代山答了。③山是禅机深藏的高僧,轻易不开口的。人在楼上倚栏杆,山列坐
Whycouldn’tToddandJunefindoutwhosentthemthetickets?WhatdidToddandJunedowiththetickets?
Whichofthefollowingitalicizedpartsindicatesarelationshipoflinkingverbandpredicative(系表结构)?
Whatdoesthewomanthinkofhercourse?
最新回复
(
0
)