大学英语六级67及答案解析.doc

1、大学英语六级 67 及答案解析(总分：448.04，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 short essay entitled Computer and Online Games. You should write at least 150 words following the outline given below. 1大学生接触电脑游戏的概况 2电脑游戏对大学生的利与弊 3我对这一问题的看法 （分数：30.00）_二、P

2、art 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 basic chemical rul

3、es 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 known as the enzyme

4、 (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 alcohol and why so man

5、y 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 down and extract

6、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, they can also tole

7、rate 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 last drop“, as Benn

8、er 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 creature out there w

9、hose 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 into the history

10、 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 so as a result

11、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 well studied and

12、 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 break it down? From

13、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 researchers work ou

14、t 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 compared with todays

15、 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 drove the strat

16、egy. 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 anything to do with it.

17、 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 lifestyle opportunity,

18、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 a knock-on effec

19、t 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 that can easily c

20、ontain 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 fruitflies, ADH

21、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. Given that the biol

22、ogy 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 creatures ecology and

23、 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 each fly, and ident

24、ified 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 evolution of the fru

25、itfly,“ 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 response to climate

26、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 particular versi

27、on 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 versions of Adh can mak

28、e 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 forms from bacter

29、ia 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 alcohol: the fac

30、t 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 potentially harmf

31、ul 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 is clearly a lot

32、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：7.10）A.YB.NC.NG(2).

33、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 understanding.（分

34、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 of the ancestral

35、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 Comprehens(总题数：1，分数：15

36、00)A.He is not to blame.B.He was responsible for the accident.C.He will take all the responsibility.D.He will be more careful next time.A.Nick cant go on studying because he has to work in a steel plant.B.Nick has earned enough money for his senior year.C.Nick prefers working in a steel plant to st

37、udying.D.Nick doesnt have enough money so hell work during his senior year.A.A new record.B.An old record.C.A popular record.D.A record of classical music.A.There wont be enough seats for everybody.B.The speaker wont show up.C.The seminar wont be open to the public.D.The tickets may have been sold o

38、ut.A.She is still under training for her new job.B.She is not sure if her boss would allow her to go.C.She is having some trouble now.D.She doesnt want to go on holiday with the man.A.Because she feels very hot in the room.B.Because she wants to avoid meeting people.C.Because she wants to smoke a ci

39、garette outside.D.Because she doesnt like the smell of smoke inside.A.He doesnt write well enough.B.He is not a professional writer.C.He hasnt got any professional experience.D.He didnt perform well in the interview.A.He has too many dreams.B.He likes to sleep.C.He doesnt put his ideas into practice

40、D.He doesnt have many ideas.四、Section A(总题数：2，分数：10.00)A.He drives too fast.B.He plays his guitar too loudly.C.His radio wakes her children up.D.His friends are too noisy.A.At midnight.B.In the morning.C.When the car is turned off.D.When the neighbors son leaves for work.A.Nancy should call the nei

41、ghbors to complain.B.Nancy should introduce her children to the neighbors.C.Nancy should ask the neighbor s son to babyish.D.Nancy should bring the neighbors a gift.A.She will stay home.B.She will go the concert with Abruzzi.C.She will invite Abruzzi to a concert.D.She has not decided yet.A.She does

42、nt want to hurt Abruzzi by refusing again.B.She is afraid that she might lose Abruzzi.C.She wants to have a change after a busy week.D.She cant think of a reason not to accept Abruzzis invitation again.A.She has been studying late into night all week.B.She has made plans to work late all week.C.She

43、has not been free to go out with Abruzzi all week.D.She has been busy tidying up her apartment all week.A.Abruzzi is very patient and determined.B.Lindas roommate does most of the cleaning in their apartment.C.Linda doesnt really care for Abruzzi.D.Linda likes concert better than shows.五、Section B(总

44、题数：3，分数：30.00)A.Students from America.B.Students from England.C.Students from Australia.D.Students from Japan.A.Those who know how to program computers.B.Those who get special aid from their teachers.C.Those who are very hardworking.D.Those who have well-educated parents.A.Japanese students study mu

45、ch harder than Columbian students.B.Columbian students score higher than Japanese students in maths.C.Columbian students are more optimistic about their maths skills.D.Japanese students have better conditions for study.A.Physics.B.Mathematics.C.Environmental science.D.Life science.A.About 45 million

46、B.About 5.4 million.C.About 50 million.D.About 4.5 million.A.The actors and actresses are not paid for their performance.B.The actors and actresses only perform in their own communities.C.They exist only in small communities.D.They only put on shows that are educational.A.It provides them with the

47、opportunity to watch performance for free.B.It provides them with the opportunity to make friends.C.It gives them the chance to do something creative.D.It gives them a chance to enjoy modem art.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 mouth