大学英语六级117及答案解析.doc

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1、大学英语六级 117 及答案解析(总分：448.05，做题时间：132 分钟)一、Part I Writing (3(总题数：1，分数：30.00)1.For this part, you are allowed 30 minutes to write a composition on the topic: Cell Phones in China. You should write no fewer than 150 words and you should base your composition on the table below. Remember to write clearly

2、. （分数：30.00）_二、Part II Reading C(总题数：1，分数：71.00)THE PARTY GENE Steven Benner jokingly calls himself a dilettante (业余艺术爱好者). A biochemist at the University of Florida in Gainesville, Benner learns a wide range of disciplines, from bioinformatics to astrobiology. His aim is to gain insight into the ba

3、sic chemical rules that govern how life works both here and, ultimately, on other planets. But although science drew his gaze to the skies, it was alcohol that brought him back down to Earth. Or, to be more exact, the enzymes that can both make and consume it. Alcohol dehydrogenase (脱氢酶) is best kno

4、wn as the enzyme (酶) that breaks down alcohol in the body, and as such it has been studied exhaustively. But Benner and other researchers in the field have now turned to its evolution, and their work is providing fresh insight into the puzzle of why some creatures, such as yeast, came to make alcoho

5、l and why so many others, including ourselves, can tolerate it. Alcohol dehydrogenase ADH for short is a blanket term applied to a large and diverse group of enzymes. In many creatures, including ourselves, they help to convert alcohols, such as ethanol, into compounds that other enzymes can break d

6、own and extract energy from. But in a number of microorganisms, they can help the reverse reaction, making alcohols as part of the process of extracting energy from sugars. The stars of these alcohol-producers are the yeasts. Not only do Saccharomyces species of yeast churn out oodles of ethanol, th

7、ey can also tolerate far higher concentrations of it than other microorganisms. Brewers yeast owes this ability to two alcohol dehydrogenases: ADH1, which makes ethanol, and ADH2, which breaks it down for use as an energy source. Yeast not only brews its own moonshine, it consumes it too “to the las

8、t drop“, as Benner says. At first sight, this makes no sense. Making ethanol from sugar and then consuming it is energetically far more wasteful than simply consuming the sugar. Researchers have long pondered why yeast goes to all that trouble. Although it might be nice to think that there is a crea

9、ture out there whose raison detre is to party, evolution doesnt work that way. Make or break Benner and his team came across the explanation when hunting for the origins of ADH in yeast. Benner is interested in combining the study of genes and proteins with geology and palaeontology to gain insight

10、into the history of life on Earth and present-day protein function. “Every biomolecule is better understood if we know its history as well as its structure,“ he says. The ADH genes in yeast make an intriguing subject for this approach. When yeast gained its ability to make alcohol, it must have done

11、 so as a result of a selection pressure in its environment and, what is more, this would have had a knock-on effect on other creatures. So working out when and how the ADH enzymes came to be could open a small window onto what ecosystems were like back then. ADH genes and the proteins they make are

12、well studied and have been isolated from many different species of yeast, so Benner s team had plenty of useful material to work with. The goal was to reconstruct the original gene that was duplicated to give rise to ADH1 and ADH2, and to ask what its function was did it make alcohol, or did it brea

13、k it down? From a database of the sequences of related ADH genes in various yeasts combined with additional ADH genes specially sequenced for this study Benner and his colleagues assembled an evolutionary tree of yeast ADH. This showed where the ancestral gene would have fitted in and helped the res

14、earchers work out its most likely aminoacid (氨) sequence. Inferring the past from the present isnt perfect, so they ended up with 12 slightly different candidate genes. Fruitful collaboration The group then reconstructed all 12 genes and tested them in yeast to see how the enzymes they produced comp

15、ared with todays ADH enzymes. The supposed ancestor turned out to be most similar to modern-day ADH1, the one that helps yeast make alcohol. The same evolutionary tree helped the team to estimate when the ancestor gave rise to the two present ADH genes. This information offers some insight into what

16、 drove the strategy. Was it humans breeding yeasts and selecting them to accumulate alcohol? Or did the event take place long before that? The group found that duplication of the ancestral gene took place between 80 million and 60 million years ago, which means that humans could not have had anythin

17、g to do with it. Rather, Benner thinks it was down to flowering plants. “The hypothesis is that it occurred near the time Earth first provided yeast with fleshy fruits,“ he says. With their temptingly large amounts of sugar, the fruit called for a clever strategy. “Yeast realized there was a lifesty

18、le opportunity, which involved making large amounts of alcohol as a way of defending the resources against competing organisms,“ Benner explains. In other words, yeast came up with a way of “pickling“ the fruit by producing alcohol, which would have made the fruit toxic to its competitors. This had

19、a knock-on effect on its wider ecosystem: as well as killing off its competitors, yeast had created a niche in fermenting fruit for any organism that could devise a way to cope with the alcohol. It was around this time that the fruitflies emerged. Feeding on yeast and fruit juices in rotting fruit t

20、hat can easily contain alcohol concentrations of 4% or more (about the same as beer), the fruitfly (Drosophila) and its larvae (幼虫) found themselves in need of a mechanism for breaking down alcohol. Drosophila came up with its own form of ADH, structurally unrelated to that of mammals and yeast. In

21、fruitflies, ADH plays a role in alcohol tolerance but also in energy metabolism, allowing the fly to use alcohol indeed many different alcohols as energy sources. Different species of Drosophila live on different fruits, which in turn produce different combinations of alcohols when they ferment. Giv

22、en that the biology of ADH is well understood, and that fruitflies are ideal for doing genetics studies, scientists have turned to studying the enzyme to understand how natural selection shapes it to prefer different alcohols in different species. Such studies provide an elegant link between a creat

23、ures ecology and the molecular changes that allowed it to exploit its niche. Luciano Matzkin, an evolutionary biologist at the University of Arizona in Tucson, recently looked at ADH in two species of Drosophila that feed on different plants. He compared the different versions of the ADH gene in eac

24、h fly, and identified key changes to the enzymes structures that could have helped the flies adapt to different alcohols. Although alcohol tolerance is clearly an important trait for fruitflies, it is not the only function ADH seems to have in Drosophila. “It has played various roles during the evol

25、ution of the fruitfly,“ Matzkin points out “It pops up in many different places.“ One of these is related to how well flies can resist a hot environment. Different populations of flies living at different latitudes have different versions of the Adh gene. And these patterns can shift rapidly in resp

26、onse to climate change, giving scientists a ringside seat for watching evolution at work, as well as a way of seeing the effects of global warming on ecosystems. Rapid response Together with others, Ary Hoffmann, evolutionary geneticist at La Trobe University near Melbourne, Australia, found that a

27、particular version of the Adh gene, called AdhS, in Australia has spread south by some 400 kilometres in only 20 years. This version of the gene is associated with heat resistance. “Twenty years is rapid in evolutionary terms,“ Hoffmann points out. The speed of change suggests that different version

28、s of Adh can make a big difference to a fruitflys survival. ADH, it seems, is a versatile enzyme that has evolved in different times and settings. In fact, ADH activity is carried out by three families of enzymes that seem to have arisen independently. The families are spread among most major life f

29、orms from bacteria to plants, yeast and animals. It seems as though the structure of ADH, which allows it to bind to alcohol as well as to several other chemicals, made it a useful enzyme under different circumstances. The original purpose of the ADH now found in humans probably wasnt breaking down

30、alcohol: the fact that the enzyme can do this simply came in handy later on. So, what was its original function? At the moment, nobody knows. But some are hazarding a guess. Ricard Albalat, an evolutionary geneticist at the University of Barcelona in Spain, believes it was used to break down other p

31、otentially harmful chemicals, such as formalde-hyde. “Formaldehyde can react with DNA and cause mutations,“ notes Jan-Olov H?g, a medical biochemist at the Karolinska Institute in Stockholm, Sweden. “The ability to break it down is a crucial function of ADH.“ But whatever their true origins, there i

32、s clearly a lot more to these multitalented enzymes than just allowing us to get drunk. As researchers delve further into their history, these molecules are shedding light on the big questions of evolutionary biology. （分数：71.00）(1).ADH is a cover term for a large and diverse group of enzymes.（分数：7.1

33、0）A.YB.NC.NG(2).Saccharomyces can stand far higher concentrations of ethanol than other microorganisms.（分数：7.10）A.YB.NC.NG(3).ADHland ADH2 are same brewers yeasts responsible for breaking down ethanol.（分数：7.10）A.YB.NC.NG(4).According to Benner, biomolecules structure and history are helpful for its

34、understanding.（分数：7.10）A.YB.NC.NG(5).Benner and his colleagues assembly of an evolutionary tree of yeast ADH showed _and helped the researchers_.（分数：7.10）_(6).The supposed ancestor turned out to_, the one that helps yeast make alcohol.（分数：7.10）_(7).Benner and his team hypothesized that duplication o

35、f the ancestral gene occurred near the time_.（分数：7.10）_(8).Drosophilas form of ADH is structurally_.（分数：7.10）_(9).Patterns of Adh gene of flies living at different latitudes can help scientists to see_.（分数：7.10）_(10).Different versions of Adh can_ to a fruitflys survival.（分数：7.10）_三、Listening Compre

36、hens(总题数：1，分数：15.00)A.The man should try to be more understanding.B.The mans wife should be more understanding.C.The mans negative attitude may be derived from his childhood.D.The pessimism of mans wife may be the result of her past experiences.A.The man is a football fan.B.The man needs the womans

37、help.C.The man didnt watch TV last night.D.The man often has power failure at home.A.12:15.B.1:00.C.l:10.D.12:30A.Because he reads too many books.B.To learn how to separate the important from the unimportant.C.To be well-informed.D.Because he always wastes his time.A.She wants to live in the suburbs

38、.B.She is offended by her naughty children.C.She disagrees with father.D.She turns a deaf ear to her husband s words.A.He is good at drawing pictures.B.He likes paintings very much.C.He likes visiting the art museum very much.D.He thinks the art museum is a very quiet place.A.Buy something in the Bi

39、ll supermarket.B.Get money to pay his bill.C.Check to see whether he can pay in cash.D.Get a check for the woman.A.He dislikes museums and galleries.B.He does hot care about the hot weather.C.Going to the beach is the best choice.D.He doesnt want to go to Washington.四、Section A(总题数：2，分数：10.00)A.What

40、 the mans plans are for tonight.B.Why the man does not want to play tennis.C.Why they do not have time to play tennis after class today.D.What time they can meet in the library.A.He drank too much coffee last night.B.He has the flu.C.He is nervous about his anthropology project.D.He missed dinner la

41、st night.A.Let him win a tennis game.B.Help him finish his anthropology project.C.Give him some medicine for his stomach.D.Lend him her anthropology book.A.Shed like to take her book back.B.Shed like to borrow a book.C.Shed like to know whether she could order a book that has come in.D.Shed like to

42、know whether a book that shed ordered has come in.A.A psychology book.B.A physical book.C.An architecture book.D.Not mentioned yet.A.A library.B.A publishing house.C.A museum.D.A bookstore.五、Section B(总题数：3，分数：30.00)A.Because it changes color in a regular 24-hour rhythm.B.Because it changes color in

43、 response to the suns rays.C.Because it constantly changes color.D.Because it changes color every hour.A.To tell the crab what time it is.B.To protect the crab from the sunlight and enemies.C.To keep the crab warm.D.To be of no real use.A.They change color every 50 minutes.B.They change color more q

44、uickly.C.They change color more slowly.D.They change color on the same schedule.A.The Rhythmic Cycle of the Sun and the TideB.Discoveries in BiologyC.A Scientific StudyD.A Living ClockA.They should not be too strict with the children.B.They should limit their demands on some children.C.They should d

45、emand more of their children.D.They should demand more of the bright children.A.To do comprehensive exercises.B.To read simple sentences.C.To copy out from the textbooks.D.To do all of the above.A.She assigned people to do this report.B.She made investigations in the 700 schools.C.She supported the

46、report.D.She wrote this report.A.The largest sea creatures ever known.B.Fish living in shallow waters.C.Plant-eating land animals.D.Meat-eating land animals.A.Waters off a level coast.B.The forest areas near the sea.C.The areas near the mouths of rivers.D.Both A and C.A.To escape from their enemies.

47、B.To catch more food.C.To use their developing flippers.D.To swim.A.From Land Animal to King of the SeaB.The Food of the WhaleC.The Whales Ancient AncestorD.How Whales Catch Food六、Section C(总题数：1，分数：10.00)For many women choosing whether to work or not to work outside their home is a luxury: They must work to survive. Others face a hard decision. Perhaps the easiest choice has to do with (36) 1. One husband said, “M