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
It was once assumed that all living things could be divided into two fundamental and exhaustive categories. Multicellular plants
It was once assumed that all living things could be divided into two fundamental and exhaustive categories. Multicellular plants
admin
2014-09-18
47
问题
It was once assumed that all living things could be divided into two fundamental and exhaustive categories. Multicellular plants and animals, as well as many unicellular organisms, are eukary-otic—their large, complex cells have a well-formed nucleus and many organelles. On the other hand, the true bacteria are prokaryotic cell, which are simple and lack a nucleus. The distinction between eukaryotes and bacteria, initially defined in terms of subcellular structures visible with a microscope, was ultimately carried to the molecular level. Here prokaryotic and eukaryotic cells have many features in common. For instance, they translate genetic information into proteins according to the same type of genetic coding. But even where the molecular processes are the same, the details in the two forms are different and characteristic of the respective forms. For example, the amino acid sequences of various enzymes tend to be typically prokaryotic or eukaryotic. The differences between the groups and the similarities within each group made it seem certain to most biologists that the tree of life had only two stems. Moreover, arguments pointing out the extent of both structural and functional differences between eukaryotes and true bacteria convinced many biologists that the precursors of the eukaryotes must have diverged from the common ancestor before the bacteria arose.
Although much of this picture has been sustained by more recent research, it seems fundamentally wrong in one re- spect. Among the bacteria, there are organisms that are significantly different both from the cells of eukaryotes and from the true bacteria, and it now appears that there are three stems in the tree of life. New techniques for determining the molecular sequence of the RNA of organisms have produced evolutionary information about the degree to which organisms are related, the time since they diverged from a common ancestor, and the reconstruction of ancestral versions of genes. These techniques have strongly suggested that although the true bacteria indeed form a large co- herent group, certain other bacteria, the archaebacteria, which are also prokaryotes and which resemble true bacteria, represent a distinct evolutionary branch that far antedates the com- mon ancestor of all true bacteria.
The passage is primarily concerned with
选项
A、detailing the evidence that has led most biologists to replace the tri-chotomous picture of living organisms with a dichotomous one.
B、outlining the factors that have contributed to the current hypothesis concerning the number of basic categories of living organisms.
C、evaluating experiments that have resulted in proof that the prokaryotes are more ancient than had been expected.
D、summarizing the differences in structure and function found among true bacteria, archaebacteria, and eukaryotes.
E、formulating a hypothesis about the mechanisms of evolution that resulted in the ancestors of the prokaryotes.
答案
B
解析
主题题型:A.说反了,现在生物学家应更趋向三分法。B.正确。给出几个关于当今对基本生物类型划分数目的看法的因素。符合原文叙述,即关于三分法的一些描述。C.评价实验。D.总结差异。E.表达一假说(关于进化机制)。均不是原文主要内容。
转载请注明原文地址:https://jikaoti.com/ti/R1zYFFFM
本试题收录于:
GMAT VERBAL题库GMAT分类
0
GMAT VERBAL
GMAT
相关试题推荐
Musiccomesinmanyforms:mostcountrieshaveastyleoftheirown.【C1】______theturnofthecenturywhenjazzwasborn,Ameri
______couldnotarousethestudents’interest.
TheannualcampaigntomakeSingapore’sthreemillionpeoplemorepoliteendedyesterdayandwasimmediatelyfollowedbyanother
Concerningmoneyoranythingelse,conflictsbetweenhusbandandwifeusuallyreflectapowerstruggle.Conflictsbetweenparent
IfyoulookcloselyatsomeoftheearlycopiesoftheDeclarationofIndependence,beyondtheflourishedsignatureofJohnHanc
OnNovember19,1863,AbrahamLincolnwenttoGettysburginPennsylvaniatospeakattheNationalSoldiersCemetery.TheCivilW
Visitorstothezoousuallypitytheanimalsowingtotheirparticularemotionalassociations(联想).Whichanimalsshouldbeindee
Visitorstothezoousuallypitytheanimalsowingtotheirparticularemotionalassociations(联想).Whichanimalsshouldbeindee
Thepassageasawholecanbestbecharacterizedaswhichofthefollowing?TheauthorimpliesthatthetitleofGlatthaar’sbo
随机试题
下述哪一种是非特异性梅毒血清试验
纽曼保健系统模式主要包括的三个部分不包括
某男,52岁,哮喘已六年,反复缠绵不愈,喉中痰鸣,胸闷胁痛,爪甲青紫,面色晦黯,有时夹有口苦口干,便干腹胀,舌紫暗或有瘀斑,脉沉涩。应为
A.脾阳虚证B.寒湿困脾证C.湿热蕴脾证D.肝胆湿热证E.肾气不固证
直接接触防护应选用()。
受甲公司委托,乙锅炉压力容器检测检验站委派具有检验资格的张某,到甲公司对一200立方米的球形液氧储罐进行检测检验。该球罐是由丙公司制造、丁施工公司安装的。依据《特种设备安全法》的规定,下列关于张某检测和执业的说法,正确的是()。
建设项目安全设施设计有下列()情形的,不予批准,并不得开工建设。
根据皮亚杰的理论,图式从低级到高级发展是通过同化与顺应的形式进行的。()
党的十一届三中全会的主要功绩是()。
Between1883and1837,thepublishersofa"pennypress"provedthatalow-pricedpaper,editedtointerestordinarypeople,cou
最新回复
(
0
)