托福阅读真题第194篇The Identification of the Genetic Material
The Identification of the Genetic Material
Paragraph 1：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 information, the 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).
1. According to paragraph 1, Griffith experimented with strains of the pneumococcus bacteria because he wanted to discover which of the following？
O A strain of bacteria that could be used to develop a vaccine
O How bacterial strains developed under laboratory conditions
O Why the strains of bacteria differed in appearance
O Which bacterial strains were most infectious in humans
Paragraph 2：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.
2. Why does the author provide the information that The R strain lacks this coating？
O To provide an example of variations within strains of pneumococcus bacteria
O To explain why the R strain is not able to cause disease
O To suggest that the R strain has other ways to defend itself from immune defense mechanisms
O To explain why mice became diseased when injected with the R strain
Paragraph 3：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 bacteria, many 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.
3. 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？
O All the living bacteria he found in the blood of the injected mice were S strain bacteria.
O He already knew from earlier experiments that R strain pneumococci sometimes transform into S strain pneumococci.
O He could tell from examining the bacteria under a microscope that some individual pneumococci cells had characteristics of both the S and R strains.
O He observed living cells in the mice's blood with S strain characteristics, but the only living cells injected were R strain pneumococci.
Paragraph 4：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.
4. According to paragraph 4, why was Griffith's experiment repeated in a test tube？
O To provide additional support for the transformation of R-strain into S-strain pneumococci
O To establish whether or not the transformation of R cells was caused by something the mouse's body did
O To determine why the S-strain pneumococci somehow survived if they were in the presence of the R-strain
O To test the results of adding a cell-free extract to the mixture
5. 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.
O This result showed that the chemical transforming principle in S pneumococci was passed on to future generations of S pneumococci.
O After exposure to the cell-free extract from the S pneumococci, R pneumococci strain cells acquired the ability to transform themselves into S pneumococci.
O 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.
O This transformation showed that the characteristics that the S pneumococci possess are superior to the characteristics of R pneumococci.
Paragraph 5：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 substances —— proteins, nucleic acids, carbohydrates, and lipids —— and 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.
6. According to paragraph 5, why did Oswald Avery and his colleagues treat the transforming extract in a variety of destructive ways？
O They hoped to destroy the virulent part of the transforming extract.
O They wanted to identify the substance responsible for the transforming activity.
O They wanted to identify which methods would destroy particular substances in the transforming extract.
O They needed to determine which treatments were most successful in destroying DNA.
Paragraph 6：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.
7. The phrase In retrospect in the passage is closest in meaning to
O By general agreement
O In reality
O Looking back
O Practically speaking
8. According to paragraph 6, why did scientists continue to believe that the hereditary material was a protein？
O Scientists thought that the research of Avery and his colleagues provided insufficient information about the nature of DNA.
O Scientists believed that only proteins were complex enough to carry genetic information.
O Scientists thought Avery and his colleagues had little understanding of the physical basis of inheritance.
O Scientists ignored important milestones that indicated the chemical complexity of DNA.
9. Which of the following can be inferred from the passage about the transformed R-strain pneumococci？
O They had acquired the genetic information for producing a protective coating.
O They were unable to cause transformation in other strains of pneumococci.
O In the presence of heat-killed R-strain bacteria, they lost their virulence.
O They did not multiply as quickly as nontransformed cells did.
10. Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selected THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points. Drag your choices to the spaces where they belong. To review the passage, click on View Text.
Answer Choices
O 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.
O 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.
O 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.
O 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.
O Avery and his colleagues were able to isolate Griffith's transforming principle by injecting mice with the extract that contained the transforming principle.
O 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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