1、大学英语六级 157 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 composition on the topic Long Holidays. You should write at least 150 words according to the outline given below in Chinese: 1.长假给大学生带来的好处 2.长假可能给大学生带来问题 3.我应当怎术过好长假 （分数：3
2、0.00）_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 basic che
3、mical 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 known as t
4、he 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 alcohol and w
5、hy 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 down and
6、 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, they can
7、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 last drop“
8、, 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 creature ou
9、t 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 into th
10、e 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 so as
11、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 well st
12、udied 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 break it do
13、wn? 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 researcher
14、s 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 compared wi
15、th 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 drove
16、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 anything to do
17、 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 lifestyle oppo
18、rtunity, 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
19、-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 that can
20、 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 fruitfl
21、ies, 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. Given that
22、 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 creatures ec
23、ology 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 each fly,
24、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 evolution o
25、f 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 response to
26、 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 particu
27、lar 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 versions of Ad
28、h 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 forms fr
29、om 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 alcohol
30、: 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 potentia
31、lly 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 is clear
32、ly 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：7.10）A.YB.
33、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 underst
34、anding.（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 of the a
35、ncestral 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
36、数：1，分数：15.00)A.She was approving.B.She was apologetic.C.She was annoyed.D.She was careless.A.A visitor has borrowed it.B.She had given it to her guide.C.A friend took it to the West.D.Bill gave it back to his friend.A.She agrees with the man.B.She hasnt seen Kevin recently.C.She thinks Kevin will be
37、 late.D.She doesnt know what the man means.A.Anthony fixed it.B.The plumber fixed it.C.It was left leaking.D.It was removed.A.In a clothing store.B.In a garden.C.At a cleaners.D.At a printing shop.A.Being too hot.B.Being delayed.C.Catching the beach ball.D.Getting up early.A.It looks better than the
38、 other one.B.The man needs more wall space for it.C.It should be put on another wall.D.It makes the wall a little more attractive.A.The experiment will take a long time.B.The students should count their time in the lab.C.Dr. Walters teaches biology in the afternoon.D.Biology lab is open one hour eve
39、ry afternoon.四、Section A(总题数：2，分数：10.00)A.Inviting him to dinner.B.Inviting him to a party.C.Asking him for money.D.Asking him to talk to his girlfriend.A.They have a big fight.B.They have a quarreling.C.They are breaking up.D.They will get married.A.Talk to his girlfriend.B.Recover with his girlfri
40、end.C.Invite her to the party.D.He didnt be certain.A.A new fuel for buses.B.The causes of air pollution.C.A way to improve fuel efficiency in buses.D.Careers in environmental engineering.A.Her car is being repaired.B.She wants to help reduce pollution.C.Parking is difficult in the city.D.The cost o
41、f fuel has increased.A.A fuel that burns cleanly.B.An oil additive that helps cool engines.C.A material from which filters are made.D.An insulating material sprayed on engine parts.A.The high temperatures required for its use.B.The high cost of materials used in its production.C.The lack of trained
42、environmental engineers.D.The opposition of automobile manufacturers.五、Section B(总题数：3，分数：30.00)A.Experience in negotiating.B.A high level of intelligence.C.The time they spend on preparation.D.The amount of pay they receive.A.Study the case carefully beforehand.B.Stick to a set target.C.Appear frie
43、ndly to the other party.D.Try to be flexible about their terms.A.Make sure there is no misunderstanding.B.Try to persuade by giving various reasons.C.Repeat the same reasons.D.Listen carefully and patiently to the other party.A.Working too long.B.Accumulated stress.C.Bad sleeping habits.D.Anti-socia
44、l lifestyles.A.A third.B.Less than three-fifths.C.Almost half.D.More than three-fifths.A.It is not as bad as it used to be.B.Americans do not want to give up any more sleep.C.Americans guarantee to change the sleeping habits.D.Americans want to know just how bad it is to their health.A.Bringing in a
45、 guest expert on the show.B.Discussing how to change the situation.C.Getting feedback from listeners who call the show.D.Discussing whether this is a trend in all developing countries.A.He is a commercial diver.B.He is an independent photographer.C.He is a camera manufacturer.D.Both A and B.A.Michae
46、l has been diving for nine years.B.Michael dives on holidays with his parents.C.Michael loves diving ever since he first tried it.D.Michael has never taken any diving courses.A.Taking pictures under water.B.Connecting pipelines.C.Planting sea weeds.D.Placing explosives under the water.A.Because he w
47、as never afraid of anything.B.Because he was protected by a special medium.C.Because he had enough experience.D.Because it was his job.六、Section C(总题数：1，分数：10.00)Two United Nations agencies have (36) 1for more money to supply food to (37) 2camps in Africa. They say they have had to cut food aid to hundreds of thousands of people. Most are in West Africa and the Great Lakes area. The two agencies say the cuts have created (38) 3among tho
