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Passage 41
   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 unicellu-lar organisms,are eukaryotic-their large,complex cells(5) have a well-formed nucles and many organelles. On the other hand,the true bacteria are prokaryotic cell,which are simple and lack a nucleus2. The distinction between eukaryotes and bacteria,initially defined in terms of subcellular structures visible with a microscope, was ulti-(10) mately carried to the molecular3 level. Here prokaryotic and  eukaryotic cells have many features in common. For instance,they translate genetic4 information into proteins according to the same type of genetic coding. But even where the molecular processes are the same,the details in(15)the two forms are different and characteristic of the respec-tive forms. For example, the amino acid sequences of vari-ous enzymes5 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(20) that the tree of life had only two stems. Moreover,argu-ments pointing out the extent of both structural6 and func-tional differences between eukaryotes and true bacteriaconvinced many biologists that the precursors7 of the eukaryotes must have diverged8 from the common(25)ancestor before the bacteria arose.
   Although much of this picture has been sustained bymore recent research,it seems fundamentally wrong in onerespect. Among the bacteria,there are organisms that aresignificantly different both from the cells of eukaryotes and(30) from the true bacteria,and it now appears that there are three stems in the tree of life. New techniques for deter-mining the molecular sequence of the RNA of organismshave produced evolutionary9 information about the degreeto which organisms are related,the time since they diverged(35) from a common ancestor,and the reconstruction10 of ances-  tral versions of genes11. These techniques have strongly suggested that although the true bacteria indeed form a     large coherent group,certain other bacteria, the archaebac-teria, which are also prokaryotes and which resemble true(40) bacteria, represent a distinct evolutionary branch that far antedates12 the common ancestor of all true bacteria.

  1. The passage is primarily concerned with
   (A) detailing the evidence that has led most biologists to replace the trichotomous 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) formulating13 a hypothesis about the mechanisms14 of evolution that resulted in the ancestors of the prokaryotes

  2. According to the passage, investigations15 of eukaryotic and prokaryotic cells at the molecular level supported the conclusion that
   (A) most eukaryotic organisms are unicellular
   (B) complex cells have well-formed nuclei16
   (C) prokaryotes and cukaryotes form two fundamental categories
   (D) subcellular structures are visible with a microscope
   (E) prokaryotic and eukaryotic cells have similar enzymes

  3. According to the passage, which of the following statements about the two-category hypothesis is likely to be true?
   (A) It is promising17 because it explains the presence of true bacteria-like organisms such as organelles in eukaryotic cells.
   (B) It is promising because it explains why eukaryotic cells,unlike prokaryotic cells,tend to form multicellular organisms.
   (C) It is flawed because it fails to account for the great variety among eukaryotic organisms.
   (D) It is flawed because it fails to account for the similarity between prokaryotes and eukaryotes.
   (E) It is flawed because it fails to recognize an important distinction among prokaryotes.

  4. It can be inferred from the passage that which of the following have recently been compared in order to clarify the fundamental classifications of living things?
   (A) The genetic coding in true bacteria and that in other prokaryotes
   (B) The organelle structures of archaebacteria, true bacteria, and eukaryotes
   (C) The cellular1 structures of multicellular organisms and unicellular organisms
   (D) The molecular sequences in eukaryotic RNA, true bacterial18 RNA, and archaebacterial RNA
   (E) The amino acid sequences in enzymes of various eukaryotic species and those of enzymes in archaebecterial species

  5. If the “new techniques” mentioned in line 31 were applied19 in studies of biological classifications other than bacteria, which of the following is most likely?
   (A) Some of those classifications will have to be reevaluated.
   (B) Many species of bacteria will be reclassified
   (C) It will be determined20 that there are four main categories of living things rather than three.
   (D) It will be found that true bacteria are much older than eukaryotes.
   (E) It will be found that there is a common ancestor of the eukaryotes, archaebacteria, and true bacteria.

  6. According to the passage, researchers working under the two-category hypothesis were correct in thinking that
   (A) prokaryotes form a coherent group
   (B) the common ancestor of all living things had complex properties
   (C) eukaryotes are fundamentally different from true bacteria
   (D) true bacteria are just as complex as eukaryotes
   (E) ancestral versions of eukaryotic genes functioned differently from their modern counterparts.

  7. All of the following statements are supported by the passage EXCEPT:
   (A) True bacteria form a distinct evolutionary group.
   (B) Archaebacteria are prokaryotes that resemble true bacteria.
   (C) True bacteria and eukaryotes employ similar types of genetic coding.
   (D) True bacteria and eukaryotes are distinguishable at the subcellular level.
   (E) Amino acid sequences of enzymes are uniform for eukaryotic and prokaryotic organisms.

  8. The author‘s attitude toward the view that living things are divided into three categories is best described as one of
   (A) tentative acceptance
   (B) mild skepticism
   (C) limited denial
   (D) studious oriticism
   (E) whole hearted endorsement


1 cellular aU1yo     
  • She has a cellular telephone in her car.她的汽车里有一部无线通讯电话机。
  • Many people use cellular materials as sensitive elements in hygrometers.很多人用蜂窝状的材料作为测量温度的传感元件。
2 nucleus avSyg     
  • These young people formed the nucleus of the club.这些年轻人成了俱乐部的核心。
  • These councils would form the nucleus of a future regime.这些委员会将成为一个未来政权的核心。
3 molecular mE9xh     
  • The research will provide direct insight into molecular mechanisms.这项研究将使人能够直接地了解分子的机理。
  • For the pressure to become zero, molecular bombardment must cease.当压强趋近于零时,分子的碰撞就停止了。
4 genetic PgIxp     
  • It's very difficult to treat genetic diseases.遗传性疾病治疗起来很困难。
  • Each daughter cell can receive a full complement of the genetic information.每个子细胞可以收到遗传信息的一个完全补偿物。
5 enzymes 7881ad8ce9c83424f7874e70266ed2d8     
n. 酶,酵素
  • It was said that washing powders containing enzymes remove stains more efficiently. 据说加酶洗衣粉除污更有效。
  • Among the enzymes which are particularly effective are pepsin, papain. 在酶当中特别有效的是胃朊酶、木瓜酶。
6 structural itXw5     
  • The storm caused no structural damage.风暴没有造成建筑结构方面的破坏。
  • The North American continent is made up of three great structural entities.北美大陆是由三个构造单元组成的。
7 precursors 5e19fce64ab14f5a4b5c8687640c2593     
n.先驱( precursor的名词复数 );先行者;先兆;初期形式
  • Phenyl (or polyphenyl) substituted epoxides serve as excellent precursors to phenyl (or diphenyl) carbenes. 某些苯代(或多苯)环氧乙烷是制取带苯环(或二苯)碳烯的极好原料。 来自辞典例句
  • Note the presence of megakaryocytes, erythroid islands, and granulocytic precursors. 可见巨核细胞,红细胞岛和粒细胞前体细胞。 来自互联网
8 diverged db5a93fff259ad3ff2017a64912fa156     
分开( diverge的过去式和过去分词 ); 偏离; 分歧; 分道扬镳
  • Who knows when we'll meet again? 不知几时咱们能再见面!
  • At what time do you get up? 你几时起床?
9 evolutionary Ctqz7m     
  • Life has its own evolutionary process.生命有其自身的进化过程。
  • These are fascinating questions to be resolved by the evolutionary studies of plants.这些十分吸引人的问题将在研究植物进化过程中得以解决。
10 reconstruction 3U6xb     
  • The country faces a huge task of national reconstruction following the war.战后,该国面临着重建家园的艰巨任务。
  • In the period of reconstruction,technique decides everything.在重建时期,技术决定一切。
11 genes 01914f8eac35d7e14afa065217edd8c0     
n.基因( gene的名词复数 )
  • You have good genes from your parents, so you should live a long time. 你从父母那儿获得优良的基因,所以能够活得很长。 来自《简明英汉词典》
  • Differences will help to reveal the functions of the genes. 它们间的差异将会帮助我们揭开基因多种功能。 来自英汉非文学 - 生命科学 - 生物技术的世纪
12 antedates c38d55a1436b985fa21ef0979713a131     
v.(在历史上)比…为早( antedate的第三人称单数 );先于;早于;(在信、支票等上)填写比实际日期早的日期
  • This event antedates the discovery of America by several centuries. 这件事比发现美洲早几个世纪。 来自辞典例句
  • This old carriage antedates the invention of the car. 这辆旧马车是在汽车发明前就存在的。 来自互联网
13 formulating 40080ab94db46e5c26ccf0e5aa91868a     
v.构想出( formulate的现在分词 );规划;确切地阐述;用公式表示
  • At present, the Chinese government is formulating nationwide regulations on the control of such chemicals. 目前,中国政府正在制定全国性的易制毒化学品管理条例。 来自汉英非文学 - 白皮书
  • Because of this, the U.S. has taken further steps in formulating the \"Magellan\" programme. 为此,美国又进一步制定了“麦哲伦”计划。 来自百科语句
14 mechanisms d0db71d70348ef1c49f05f59097917b8     
n.机械( mechanism的名词复数 );机械装置;[生物学] 机制;机械作用
  • The research will provide direct insight into molecular mechanisms. 这项研究将使人能够直接地了解分子的机理。 来自《简明英汉词典》
  • He explained how the two mechanisms worked. 他解释这两台机械装置是如何工作的。 来自《简明英汉词典》
15 investigations 02de25420938593f7db7bd4052010b32     
(正式的)调查( investigation的名词复数 ); 侦查; 科学研究; 学术研究
  • His investigations were intensive and thorough but revealed nothing. 他进行了深入彻底的调查,但没有发现什么。
  • He often sent them out to make investigations. 他常常派他们出去作调查。
16 nuclei tHCxF     
  • To free electrons, something has to make them whirl fast enough to break away from their nuclei. 为了释放电子,必须使电子高速旋转而足以摆脱原子核的束缚。
  • Energy is released by the fission of atomic nuclei. 能量是由原子核分裂释放出来的。
17 promising BkQzsk     
  • The results of the experiments are very promising.实验的结果充满了希望。
  • We're trying to bring along one or two promising young swimmers.我们正设法培养出一两名有前途的年轻游泳选手。
18 bacterial dy5z8q     
  • Bacterial reproduction is accelerated in weightless space. 在失重的空间,细菌繁殖加快了。
  • Brain lesions can be caused by bacterial infections. 大脑损伤可能由细菌感染引起。
19 applied Tz2zXA     
  • She plans to take a course in applied linguistics.她打算学习应用语言学课程。
  • This cream is best applied to the face at night.这种乳霜最好晚上擦脸用。
20 determined duszmP     
  • I have determined on going to Tibet after graduation.我已决定毕业后去西藏。
  • He determined to view the rooms behind the office.他决定查看一下办公室后面的房间。