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
23
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 ceils in the early embryo are undetermined in the sense that each cell has the potential to develop in a variety of different ways. Late 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 determinants in early development. They have been able to show that, in a sense, ceil 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 ate 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 round them to form a structure that resembles beads, or knots, on a string. The beads are DNA segments wrapped around the histone; 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.
Choose the most appropriate from the four choices to complete the sentence or answer the question.
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 than is necessary for the development of a single individual
答案
D
解析
转载请注明原文地址:https://kaotiyun.com/show/IAua777K
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
ReportssuggestHSBCcouldseektocutanadditional8,000~10,000jobsfromitsheadcountof238,000(aspokespersondeclinedto
请将以下作品填入对应的作者右边的空格内TheCelebratedJumpingFrogofCalaverasCounty;
请将以下作品填入对应的作者右边的空格内AnimalFarm;
A______isagrowthoffeathers,furorskinalongthetopoftheheadsofsomeanimals,especiallybirds.
Thecompactdictionariespublishedinrecentyearsarenotasunwieldyassomeoftheoldereditions.
Windstormshaverecentlyestablishedarecordwhichmeteorologistshopewillnotbeequaledformanyyears______.
IfaMeccaexistsfortoday’syouth,itmusttaketheformofarockconcert.Suchacolossaleventdrawspeoplefromsurprisin
Whyhasmiseducationbecomeasocietalnorm?(ThisquestionisbasedonPassage5)
Youwon’tdieofunhappiness.Contrarytothelong-heldandwidespreadbeliefthatthemorestressedandoutofcontrolyouare
随机试题
简述多样化分销模式的原因。
关于电离室控时自动曝光控制,叙述错误的是
具有联缀四肢百骸、主司关节运动作用的是全身皮肤分为十二个部分,分属于十二经脉,称为
患者因外伤被路人送到医院抢救,被送到医院时因大量失血已经昏迷,此时急需输血治疗但其家人还未赶到医院,对患者输血时采取的以下措施,符合临床输血技术规范的是()
市场竞争突出地表现为价格竞争,产品价格不断下跌,这反映的是()的特征。
《婚姻法》第15条规定:“父母对子女有抚养教育的义务,子女对父母有赡养扶助的义务。”这一规定是法的创制的哪种形式?()
服务企业的()是服务人员向顾客提供服务、使顾客满意的过程。
下列各项中,应征收增值税的是()。
如何应对网络谣言?(北京大学,2009年;大连理工大学,2016年)
Asaphysicianwhotravelsquitealot,Ispendalotoftimeonplaneslisteningforthatdreaded"Isthereadoctorunboard?"
最新回复
(
0
)