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[机经题库答案] 托福阅读生物类真题The Identification of the Genetic Material原文+题目汇总

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发表于 2017-5-15 15:06 |显示全部楼层
类别:生物类
真题150711CN-P2
Title:The Identification of the Genetic Material
The history of biology is filled with incidents in which research  on one specific topic has  contributed richly to another, apparently unrelated  area. Such a case is the work of Frederick  Griffith, an English physician whose attempts to prevent the disease  pneumonia led to the  identification of the material in cells that contains genetic informationthe  information that determines an  organism's characteristic structure. In the 1920s, Griffith was studying the bacterium Streptococcus pneumoniae,  or pneumococcus, one of the organisms that  cause pneumonia in humans. He was trying to develop a vaccine against  this devastating illness. He was working with two strains of the bacteria pneumococcus. A bacterial strain is a population  of cells descended from a single parent cell; strains differ in one or more inherited characteristics. Griffith's  strains were designated S and R because, when grown in the laboratory, one produced shiny, smooth (S) colonies or groups of bacteria, and the other produced colonies that look rough (R).  
When the S strain was injected into mice, the mice became  diseased. When the R strain was  injected, the mice did not become diseased. Bacteria of the S strain are  virulent (able to cause disease)  because they are surrounded by a protective jelly-like coating that prevents the mouse's immune defense mechanisms  from destroying the bacteria before they can  multiply. The R strain lacks this coating.  
With the hope of developing a  vaccine against pneumonia, Griffith injected some mice with heat-killed S pneumococci. These heat-killed bacteria did  not produce infection. Griffith  assumed the mice would produce antibodies to the bacteria that would allow  them to fight the virulent form if  they were exposed to it. However, when Griffith inoculated other mice with a mixture of living R bacteria and  heat-killed S bacteria, to his astonishment,  the mice became ill with  pneumonia. When he examined blood from these mice, he found it full of living bacteriamany with  characteristics of the virulent S strain. Griffith concluded that, in the presence of the dead S pneumococci,  some of the living R pneumococci had been  transformed into virulent S-strain organisms.  
Did this transformation of the bacteria depend on something the  mouse did to the bacteria No. It  was shown that simply putting living R and heat-killed S bacteria together in  a test tube yielded the same  transformation. Next it was discovered that a cell-free extract of heat-killed S cells also transformed  R cells. (A cell-free extract contains all the contents of cells, but no intact cells.) This result demonstrated that some  substance called at the time a  chemical transforming principle from the extract of S pneumococci could cause a heritable change (a change that could  be passed on to future generations) in the  affected R cells. From these observations, some scientists  concluded that this transforming material  carried heritable information, and thus was the genetic material that scientists  had been searching for.   
The identification of the transforming material was a crucial  step in the history of biology,  accomplished over a period of several years by Oswald Avery and his colleagues at what is now Rockefeller University. They treated samples of the  transforming extract in a variety of  ways to destroy different types of substancesproteins, nucleic acids,  carbohydrates, and lipidsand tested  the treated samples to see if they had retained transforming activity. The answer was always the same: If the  DNA (deoxyribonucleic acid) in the extract  was destroyed, transforming activity was lost; everything else could  be eliminated without removing  the transforming ability of the extract. As a final step, Avery, with Colin  MacLeod and Maclyn McCarty, isolated virtually pure DNA from a sample of  pneumococcal transforming extract and showed that it caused bacterial  transformation.  
In retrospect, the  work of Avery, MacLeod, and  McCarty, published in 1944, was a milestone  in establishing that DNA is the genetic material. However, at the time, it  had little impact on scientists'  view about the physical basis of inheritance. The genetic material had to  encode all the information needed  to specify an organism, and the chemical complexity and diversity of proteins were known to be  impressive. So during the first half of the twentieth century, the hereditary material was generally  assumed to be a protein. Nucleic acids, by contrast, were known to have only a few components and seemed too  simple to carry such complex  information.  
  
1. The  word apparently in the passage is closest  in meaning to  
A.       seemingly  
B.         surprisingly  
C.         relatively  
D.       previously  
  
2. According  to paragraph 1, Griffith experimented with strains of the pneumococcus bacteria because he wanted to discover which  of the following  
A.       A  strain of bacteria that could be used to develop a vaccine  
B.         How bacterial strains developed under  laboratory conditions  
C.         Why the strains of bacteria differed in appearance  
D.       Which  bacterial strains were most infectious in humans  
  
3. Why  does the author provide the information that The R strain lacks this coating  
A.       To provide an example of variations within  strains of pneumococcus bacteria  
B.         To explain  why the R strain is not able to cause disease  
C.         To suggest  that the R strain has other ways to defend itself from immune defense mechanisms  
D.       To explain why mice became diseased when  injected with the R strain  
  
4. The  word astonishment in the passage is  closest in meaning to  
A.       alarm
B.         surprise  
C.         disappointment  
D.       interest
    
5. According  to paragraph 3, why did Griffith conclude from his experiment injecting both  R and S strains pneumococci into  mice that some of the R strain bacteria transformed into disease-causing  S strain pneumococci  
A.       All  the living bacteria he found in the blood of the injected mice were S strain bacteria.  
B.         He already knew from earlier experiments that  R strain pneumococci sometimes transform  into S strain pneumococci.  
C.         He could tell from examining the bacteria  under a microscope that some individual  pneumococci cells had characteristics of both the S and R strains.  
D.       He  observed living cells in the mice's blood with S strain characteristics, but  the only living cells injected were  R strain pneumococci.  
  
6. According  to paragraph 4, why was Griffith's experiment repeated in a test tube  
A.       To provide additional support for the  transformation of R-strain into S-strain pneumococci  
B.         To establish  whether or not the transformation of R cells was caused by something the mouse's body did  
C.         To determine  why the S-strain pneumococci somehow survived if they were in the presence of   the R-strain  
D.       To test the results of adding a cell-free  extract to the mixture  
  
7. Which of the sentences below best expresses the  essential information in the highlighted  sentence in the passage. Incorrect  choices change the meaning in important ways or leave out essential information.  
A.        This result showed that the chemical  transforming principle in S pneumococci was passed on to future generations of S pneumococci.  
B.         After exposure to the cell-free extract from  the S pneumococci, R pneumococci strain cells  acquired the ability to transform themselves into S pneumococci.  
C.         The transformation of R cells by a cell-free  extract of S pneumococci demonstrated the  existence of a chemical transforming principle that brought about heritable change.  
D.       This  transformation showed that the characteristics that the S pneumococci possess are superior to the characteristics of  R pneumococci.  
  
8. According  to paragraph 5, why did Oswald Avery and  his colleagues treat the transforming  extract in a variety of destructive ways  
A.        They hoped to destroy the virulent part of  the transforming extract.  
B.         They wanted to identify the substance  responsible for the transforming activity.  
C.         They wanted to identify which methods would  destroy particular substances in the  transforming extract.  
D.       They  needed to determine which treatments were most successful in destroying DNA.  
  
9. The  word virtually in the passage is closest  in meaning to  
A.       perfectly  
B.         nearly  
C.         partially  
D.       relatively  
  
10. The  phrase In retrospect in the passage is  closest in meaning to  
A.       By  general agreement  
B.         In reality  
C.         Looking back  
D.       Practically speaking  
  
11. According  to paragraph 6, why did scientists continue to believe that the hereditary material was a protein  
A.       Scientists  thought that the research of Avery and  his colleagues provided insufficient  information about the nature of DNA.  
B.         Scientists believed that only proteins were  complex enough to carry genetic information.  
C.         Scientists thought Avery and his colleagues had little understanding of the  physical basis of inheritance.  
D.       Scientists  ignored important milestones that indicated the chemical complexity of DNA.  
  
12. Which  of the following can be inferred from the passage about the transformed R-strain pneumococci  
A.       They  had acquired the genetic information for producing a protective coating.  
B.         They were unable to cause transformation in  other strains of pneumococci.  
C.         In the presence of heat-killed R-strain  bacteria, they lost their virulence.  
D.       They  did not multiply as quickly as nontransformed cells did.  
  
13. Look at the four squares     that indicate  where the following sentence could be added to the passage.  
Why  
Where would the sentence best fit?  
Click on a square to add the sentence to the passage.  
(题目不完整)  

  
14. Drag  your choices to the spaces where they belong. To review the passage, click on View Text. Answer Choices  
A.       From  the 1920s through 1944, researchers used pneumococcus bacteria to discover the properties of DNA because the  bacteria was relatively simple, having only two strains.  
B.         Frederick Griffith discovered that a  nonvirulent strain of bacteria could be transformed into a virulent strain by being exposed to  dead cells from the virulent strain.  
C.         By selectively destroying various substances  in the cells of pneumococci bacteria, Oswald  Avery and his colleagues identified  DNA as the substance that caused bacterial  transformation.  
D.       Oswald  Avery injected the combination of  heat-killed, virulent cells and nonvirulent  cells into mice because he hoped this would lead to a vaccine for pneumonia.  
E.         Avery and  his colleagues were able to isolate Griffith's transforming principle by injecting mice with the extract that  contained the transforming principle.  
F.         Scientists did not initially recognize the  importance of the discovery that DNA could  cause genetic transformation because the hereditary material was  assumed to be a protein.

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发表于 2017-6-26 19:48 |显示全部楼层
请问有参考答案吗?
该会员没有填写今日想说内容.
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发表于 2017-7-13 17:36 |显示全部楼层
第2题文中没有提到 D中most infectious 这件事情,并且实验是在mice身上做的,不是human,应该是A吧;第七题A选项明显有问题吧,不是was passed on to future generations of S pneumococci,而是 R pneumococci吧,C比较合适

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Skyegloz  沒錯 而且第七題重點在於 S cause Heritage change in R 選項中只有C提到heritage / genetic changes.  详情 回复 发表于 2017-12-23 17:10
yyyao  can't agree more! 我也觉得这两道题有问题,毕竟真题的答案不是标答吧。  详情 回复 发表于 2017-9-5 10:02
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audreyhepburn 发表于 2017-7-13 17:36
第2题文中没有提到 D中most infectious 这件事情,并且实验是在mice身上做的,不是human,应该是A吧;第七 ...

can't agree more! 我也觉得这两道题有问题,毕竟真题的答案不是标答吧。
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