Many species of animals which once lived on the earth are no longer _________.A. in practiceB. in existenceC. in commonD. in turn
题目
A. in practice
B. in existence
C. in common
D. in turn
相似考题
更多“Many species of animals which once lived on the earth are no longer _________. A. in practiceB. in existenceC. in commonD. in turn”相关问题
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第1题：
Which of the following is the result of the volcanic eruption?
A．People stop traveling in Europe
B．Airlines suffer from the loss of planes
C．It becomes dangerous for animals to eat outside
D．Parmers have lost many of their animals
正确答案：C
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第2题：
Modern zoos are very different from zoos that were built fifty years ago.At that time, zoos were places (1) people could go to see animals from many parts of the world.They animals lived in cages that were made (2) Concrete with iron bars, cages that were easy to keep clean.
Unfortunately for the animals, the cages were small and impossible to hide in.the zoos environment was anything (3) natural.Al though the zoo keepers took good care of the animals and fed them well, many of the animals did not thrive: they behaved in strange ways, and they often became ill
In modern zoos, people can see animals in more natural habitats.The animals are(4)more freedom in large areas so that they can live more comfortably as they would in nature Even the appearance of zoos has changed, trees and grass grow in the cages, and streams of water flow (5) the areas that animals live in.(完型填空)
A.Of
B.But
C.Where
D.Over
E.Given
参考答案：CABED
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第3题：
The loss of biological diversity has become most spectacularly noticeable in extinction or decline of populations of large and well-known animals.Many animal species have become extinct since the 1700’s, among them the California grizzly bear, the dodo, and the passenger pigeon.Hundreds of other animal species are threatened.In North America, endangered species include the black-footed ferret, the California condor, the desert tortoise, and the whooping crane.
Most species in danger of extinction anywhere in the world are suffering from the intrusion (侵入) of human beings.The Asian elephant, for example, has become an endangered species due to the expansion of the human population throughout its range in southern Asia.In Africa, the African elephants are being killed off for their tusks.Monkeys and other primates throughout the world are threatened by hunting, capture for medical use, and the destruction of their habitats.And the rhinoceroses, lions, and other large mammals of the African savanna (grassy plains with scattered trees) compete for land and life with some of the most rapidly growing human populations in the world.
The loss of biological diversity is most severe, however, in the tropical rain forests.The forests of the tropics are particularly vulnerable to disturbance because the soils have a low capacity for retaining nutrients.Most of the forests’ nutrient elements are held in the tissues of plants.When loggers and farmers destroy existing plant cover, the nutrients are washed from the land into streams, and the land itself becomes less able to support life.Destruction of these forests destroys the habitat of hundreds, sometimes thousands, of species from such creatures as the howler monkey and indigo macaw, to less visible species of plants, insects, and microbes, many of which are not yet known to science.
1.The author points out that species extinction ().
A.is most noticeable in North America
B.is mainly taking place among large animals
C.quickened with industrialization of the society
D.never took place before the 1700’s
2.Which of the following is NOT mentioned with regard to human intrusion?
A.Elephants are killed off for their tusks
B.Some primates are hunted for medical use
C.Growing human population results in the competition for land and life among animals
D.Men live increasingly on animals as their source of food.
3.The chief cause of the gradual disappearance of biological diversity is ().
A.human cruelty
B.human interference
C.the increase of human population
D.the disappearance of the forests
4.In tropical forests，().
A.animals live a hard life due to competition
B.a lot of species die out for lack of food
C.rains constitute the major threat to species
D.a greater number of animals are yet to be known
5.It can be inferred that species extinction ().
A.proceeds of a faster rate than noticeable
B.is the worst among well-known animals
C.is the most severe in Africa
D.is chiefly due to competition among species
参考答案：CDBBA
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第4题：
Many students start each term with an award check, but by the time books are bought, food is paid for, and a bit of social life ＿＿, it looks rather emaciated.A. lives
B. lived
C. was lived
D. has lived答案：B解析：考查省略。句中出现了三个并列结构：books are bought, food is paid for, and a bit of social lif。e一般情况下，谓语动词是不可省略的，如果内容与其前所说的内容是并列关系，谓语动词为be时，则可以省略be动词。按照are brought, is paid和并列结构来看，这里应该填一般现在时的被动语态，排除C选项，live a life这里应该用被动，排除A,D，所以正确答案为B。句意为“很多学生在学期伊始有支票奖励，但在买了书和食物及社交花费之后，钱就变少了”。 -
第5题：
共用题干
第二篇
The Book of Life
So far,scientists have named about 1.8 million living species,and that's just a fraction of what proba-
bly exists on Earth.With so many plants,animals,and other creatures covering the planet,it can be tough
to figure out what type of spider is moving up your leg or what kind of bird is flying by.
A soon-to-be-launched Web site might help. An international team of researchers has announced the
creation of a Web-based Encyclopedia of Life(EoL).The project aims to catalog every species on Earth in a
single,easy-to-use reference guide.
To get the encyclopedia started,the creators will use information from scientific databases that already
exist.And eventually,in special sections of the site,nonscientists with specialized knowledge will get to join
in.Bird-watchers,for example,will be able to input which birds they've seen and where.The technology
for this kind of tool has only recently become available.
As the EoL develops, you might find it useful for school projects.The site will feature special pages for
kids who are studying ecosystems（生态系统）in their neighborhoods. To make sure the encyclopedia is
accurate,scientists will review much of the information added to it. People who visit the site will be able to
choose to leave out pages that haven't been reviewed.
Another convenient feature of the EoL is that you'll be able to pick the level of detail you see to match
your interests,age,and current knowledge.If you wanted to learn about bears for a science class report,for
example,you could use the"novice"setting to get basic information about the animals.On the"expert"
setting,on the other hand,you could get much more detailed information about the history,literature and
exploration of bears.
It now takes years for scientists to collect all the data they need to describe and analyze species.The
creators of the Encyclopedia of Life hope that their new tool will speed that process."Spider" and"bird"mentioned in Paragraph 1 are examples to illustrate_______________.
A:animals are people's friends
B:there are numerous living species on Earth
C:it is easy to see spiders and birds
D:there are many types of spiders and birds答案：B解析：由文章内容可知，文章开头运用“蜘蛛”和“鸟”的例子主要说明地球上生物的种类太 多了。
根据第二段的最后一句话可知正确答案。
由第三段的第一句话可知答案。
novice在文中的意思为“新手，初学者”，选项中的beginner符合题意。
由文中第一句话可知“科学家已经命名1.8百万种物种了”，而不是“仅仅一小部分”, 其他选项在文中都能找到依据。 第三篇 文章主要介绍了为什么野生的大象比圈养的大象要长寿。 -
第6题：
共用题干
第三篇
Longer Lives for Wild Elephants
Most people think of zoos as safe places for animals,where struggles such as having difficulty finding
food and avoiding predators（猛兽）don't exist. Without such problems,animals in zoos should live to a ripe
（成熟的）old age.
But that may not be true for the largest land animals on Earth.Scientists have known that elephants in
zoos often suffer from poor health.Sometimes,they even become unable to have babies.
To learn more about how captivity（圈养）affects elephants, a team of international scientists compared
the life spans of female elephants born in zoos with female elephants living outdoors in their native lands.
Zoos keep detailed records of all the animals in their care,documenting factors such as birth dates,illnes-
ses,weight and death.These records made it possible for the researchers to analyze 40 years of data on 800
African and Asian elephants in zoos across Europe.The scientists compared the life spans of the zoo-born fe-
male elephants with the life spans of thousands of wild female elephants in Africa and Asian elephants that
work in logging camps（伐木场）, over approximately the same time period.
The team found that female African elephants born in zoos lived an average of 16.9 years. Their wild
counterparts who died of natural causes lived an average of 56 years一more than three times as long. Female
Asian elephants followed a similar pattern. In zoos,they lived 18.9 years,while those in the logging camps
lived 41.7 years.
Scientists don't know yet why wild elephants seem to get on so much better than their zoo-raised coun-
terparts.Georgia Mason,a biologist at the University of Guelph in Canada who led the study,thinks stress
and obesity（肥胖症）may be to blame. Zoo elephants don't get the same kind of exercise they would in the
wild,and most are very fat. Social lives of elephants are also much different in zoos than in the wild,where
they live in large herds and family groups.
The study raises some questions about acquiring more elephants to keep in zoos.While some threatened
and endangered species living in zoos reproduce（生殖）successfully and maintain healthy populations,that
doesn't appear to be the case with elephants.It can be inferred from the last paragraph that______________.
A:zoo-born elephants should be looked after more carefully
B:zoos should keep more animals except elephants
C:it may not be wise to keep elephants in zoos
D:elephants are no longer an endangered species答案：C解析：由文章第二段第二句话可知答案。
由文章第三段可知，是动物园记录他们所照顾的大象的详细信息，而不是科学家们。
由第四段两组数据对比可知，在动物园出生的雌象比野生的寿命短很多，即死得更早。
由第五段内容可知，圈养的大象之所以短命是因为它们不是成群大家庭生活的。
最后一段暗示大象和其他适合圈养的动物不一样、它需要野外生活，因此圈养大象可 能是不明智的。第5部分：补全短文 -
第7题：
共用题干
第二篇
The Book of Life
So far, scientists have named about 1.8 million living species（物种）,and that's just a small part of
what probably exists on Earth.With so many plants,animals,and other creatures covering the planet,it can
be tough to figure out what type of spider is moving up your leg or what kind of bird is flying by.
A soon-to-be-launched Web site mighit help. An international team of researchers has announced the
creation of a Web-based Encyclopedia（百科全书）of Life(EoL).The project aims to catalog every species
on Earth in a single,easy-to-use referenice guide.
To get the encyclopedia started,the creators will use information from scientific databases（数据库）that
already exist. And eventually , in special sections of the site , nonscientists with specialized（专门的）
knowledge will get to join in Bird-watchers,for example,will be able to input which birds they'ye seen and
where.The technology for this kind of tool has only recently become available.
As the EoL develops,you might find it useful for school projects.The site will feature special pages for
kids who are studying ecosystems（生态系统）in their neighborhoods. To make sure the encyclopedia is
accurate,scientists will review much of the information added to it.People who visit the site will be able to
choose to leave out pages that haven't been reviewed.
Another convenient feature of the EoL is that you'11 be able to pick the level of detail you see to match
your interests,age,and current knowledge.If you wanted to learn about bears for a science class report,for
example,you could use the"novice"setting to get basic information about the animals.On the"expert"
setting,on the other hand,you could get much more detailed information about the history,literature,and
exploration of bears.
It now takes years for scientists to collect all the data they need to describe and analyze species.The
creators of the Encyclopedia of Life hope that their new tool will speed that process.The goal of the creation of the EoL is to________.
A:collect enough data to analyze the living species on Earth
B:enlarge an existing Web site of the living species on Earth
C:work out an easy-to-use catalog of every living species on Earth
D:add new information to the existing databases of the living species on Earth答案：C解析：由文章内容可知，文章开头运用“蜘蛛”和“鸟”的例子主要说明地球上生物的种类太 多了。
根据第二段的最后一句话可知正确答案。
由第三段的第一句话可知答案。
novice在文中的意思为“新手，初学者”，选项中的beginner符合题意。
由文中第一句话可知“科学家已经命名1.8百万种物种了”，而不是“仅仅一小部分”, 其他选项在文中都能找到依据。第二篇 文章主要讲一些科学家，设计者和艺木家为帮助一个没有电的地区设计了一种“便携灯”及 这种灯的工作原理。 -
第8题：
Let's not wait any longer, he might not（）at all.
- A、turn over
- B、turn up
- C、turn on
- D、turn down
正确答案:B -
第9题：
单选题According to the experts, the reversal of the Earth’s magnetic field would _____.Adestroy almost all the creatures on the Earth
Bcause some species extinctions on the Earth
Cnot be as disastrous as the previous one
Dcause no big trouble for man and animals
正确答案： A解析：
第三段最后一句：但是专家说这种影响不会造成大的灾难，尽管世界末日之称以及一些不明确的证据把物种灭绝与上一次地球磁场转移相提并论。 -
第10题：
单选题The relationship of jati to varna is most comparable to which of the following relationships?AThe relationship of individual playing cards to a 52-card deck
BThe relationship of a group of related species of animals to the genus to which the species belong
CThe relationship of a compact disk to the songs contained on that disk
DThe relationship of a transmission to the car in which the transmission has been installed
EThe relationship of an updated version of software to the original version of that software
正确答案： C解析：
推断题。根据文章第二、三段的叙述，可以推测jati和varna的关系是有相关性的群体的不同划分，故本题应选B项。 -
第11题：
单选题During the transition of the Earth’s magnetic field _____.Athe compass will become useless
Bman and animals will be confused as to directions
Cthe magnetic strength of the Earth will disappear
Dthe magnetic strength of the Earth will be stronger
正确答案： A解析：
第三段第四句开始：磁场的转换给人类和动物带来许多问题。宇航员和卫星会遇到很多困难，依靠磁场指示方向迁徙的鸟类、鱼类和其他动物会在迁徙中失去方向。 -
第12题：
单选题Which of the italicized parts functions as an attribute（定语）?AThis is where Luxun once lived.
BI don’t know where Luxun once lived.
CWe haven’t yet settled the question where we are going to spend our summer vacation.
DShe is going to live in Macao, where she has some close friends.
正确答案： C解析：
考查斜体部分在句中所做的句子成分。A选项中的斜体部分在句中做表语；B选项中的斜体部分在句中做宾语；C选项中的斜体部分在句中做同位语；D选项中的斜体部分在句中做定语；故选D。 -
第13题：
This is the _____ house in which Mr.Zhou once lived.A.certain
B.sure
C.real
D.very
答案：D -
第14题：
We now know that about ____ of all the kinds of seaweed are animals.
A: one four
B: once four
C: once fourth
D: one fourth
参考答案：D
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第15题：
翻译句子Several years passed on Earth, which were only a few days in heaven. Niu Lang and Zhi Nv lived happily together and had two children before the Queen of Heaven discovered Zhi Nv's absence.
参考答案：天上数日，地上数年。牛郎织女幸福地生活，而且还有了两个孩子。后来，王母娘娘发现了织女不在天庭。
-
第16题：
Many kinds of animals have vanished from the earth.A:developed
B:disappeared
C:linked
D:renewed答案：B解析：vanish“消灭，消失”。本句意思是：很多种动物从地球上消失了。disappear也是“消失”的意思。 -
第17题：
共用题干
第三篇
Longer Lives for Wild Elephants
Most people think of zoos as safe places for animals,where struggles such as having difficulty finding
food and avoiding predators（猛兽）don't exist. Without such problems,animals in zoos should live to a ripe
（成熟的）old age.
But that may not be true for the largest land animals on Earth.Scientists have known that elephants in
zoos often suffer from poor health.Sometimes,they even become unable to have babies.
To learn more about how captivity（圈养）affects elephants, a team of international scientists compared
the life spans of female elephants born in zoos with female elephants living outdoors in their native lands.
Zoos keep detailed records of all the animals in their care,documenting factors such as birth dates,illnes-
ses,weight and death.These records made it possible for the researchers to analyze 40 years of data on 800
African and Asian elephants in zoos across Europe.The scientists compared the life spans of the zoo-born fe-
male elephants with the life spans of thousands of wild female elephants in Africa and Asian elephants that
work in logging camps（伐木场）, over approximately the same time period.
The team found that female African elephants born in zoos lived an average of 16.9 years. Their wild
counterparts who died of natural causes lived an average of 56 years一more than three times as long. Female
Asian elephants followed a similar pattern. In zoos,they lived 18.9 years,while those in the logging camps
lived 41.7 years.
Scientists don't know yet why wild elephants seem to get on so much better than their zoo-raised coun-
terparts.Georgia Mason,a biologist at the University of Guelph in Canada who led the study,thinks stress
and obesity（肥胖症）may be to blame. Zoo elephants don't get the same kind of exercise they would in the
wild,and most are very fat. Social lives of elephants are also much different in zoos than in the wild,where
they live in large herds and family groups.
The study raises some questions about acquiring more elephants to keep in zoos.While some threatened
and endangered species living in zoos reproduce（生殖）successfully and maintain healthy populations,that
doesn't appear to be the case with elephants.Unlike other animals in zoos,zoo-raised elephants
A:live a long life
B:give birth to many babies
C:develop poor health
D:have difficulty getting food答案：C解析：由文章第二段第二句话可知答案。
由文章第三段可知，是动物园记录他们所照顾的大象的详细信息，而不是科学家们。
由第四段两组数据对比可知，在动物园出生的雌象比野生的寿命短很多，即死得更早。
由第五段内容可知，圈养的大象之所以短命是因为它们不是成群大家庭生活的。
最后一段暗示大象和其他适合圈养的动物不一样、它需要野外生活，因此圈养大象可 能是不明智的。第5部分：补全短文 -
第18题：
共用题干
第二篇
Human and the Earth
In terms of the evolution of life on Earth,human beings have just arrived.Despite their shori time on Earth,however,people have brought about enormous changes to the surface of the planet一changes far out of proportion to the interval of time they have occupied it.
People have more control over their surroundings than any other species on Earth.With the combination of intelligence and manual skill(allowing us to make and use tools),people have found ways to use plant and animal resources,mineral ores,fuels,and many other materials and resources from the Earth.
As the number of people on Earth increases,it becomes increasingly difficult for the population to survive on the resources of the land.The amount of land is limited.Although agricultural production can be increased by use of machinery such as tractors and the addition of fertilizer,the land ultimately can produce only so much food and no more.
As the human population grows,people consume more.Clearly,some locations on Earth already have too many people;in many of these areas,future increases will surely bring about more poverty and suffering. Yet people in rich nations use proportionally far more of the available resources than people in poorer nations.As they consume these resources without restraint,they also waste large amounts of them.
People are only a very small fraction of all the living things on the planet.Yet their numbers create a drain on resources that can't be renewed.For example,the amount of water on Earth is limited,this water is cleaned through natural processes.However,the natural processes for filtering water can clean only a certain amount of water by removing the pollution.Likewise,a limited amount of petroleum can be found under the Earth's surface.Petroleum is a valuable resource.Should people use it up to manufacture petrol for automobiles?As people work to control the planet and make life comfortable,are they using up resources that are needed for their survival?Why have people more control over their surroundings than any other species on Earth?
A:Because of their intelligence and manual skill.
B:Because they can use plant and animal resources,mineral ores and fuels.
C:Because they are cleverer than any other animals on Earth.
D:Both A and B.答案：D解析：文章第一段第一句话提到“就地球上生命的进化而言，人类是姗姗来迟者”，因此比较而言，人类的历史并不长。选项A是正确的。
从文章第二段第二句话“With the combination of intelligence and manual skill...”我们知道选项A和B都是正确的，因此选择D项。
文章第三段提到，土地的总量是有限的。虽然使用拖拉机等机械和施用化肥可以提高农业产量，但是土地最终所能产出的粮食只有那么多，不会再增加。因此只有选项C是正确的。
文章第四段第三句说富裕国家的人们所使用的能源在比例上远远大于贫穷国家的人们。选项C中提到的“发达国家的人们”最符合题意。
根据文章最后一段关于资源的描述我们可以发现选项A、B、C都是正确的，只有选项D的表述错误，因此选择D项。 -
第19题：
共用题干
第二篇
The Book of Life
So far, scientists have named about 1.8 million living species（物种）,and that's just a small part of
what probably exists on Earth.With so many plants,animals,and other creatures covering the planet,it can
be tough to figure out what type of spider is moving up your leg or what kind of bird is flying by.
A soon-to-be-launched Web site mighit help. An international team of researchers has announced the
creation of a Web-based Encyclopedia（百科全书）of Life(EoL).The project aims to catalog every species
on Earth in a single,easy-to-use referenice guide.
To get the encyclopedia started,the creators will use information from scientific databases（数据库）that
already exist. And eventually , in special sections of the site , nonscientists with specialized（专门的）
knowledge will get to join in Bird-watchers,for example,will be able to input which birds they'ye seen and
where.The technology for this kind of tool has only recently become available.
As the EoL develops,you might find it useful for school projects.The site will feature special pages for
kids who are studying ecosystems（生态系统）in their neighborhoods. To make sure the encyclopedia is
accurate,scientists will review much of the information added to it.People who visit the site will be able to
choose to leave out pages that haven't been reviewed.
Another convenient feature of the EoL is that you'11 be able to pick the level of detail you see to match
your interests,age,and current knowledge.If you wanted to learn about bears for a science class report,for
example,you could use the"novice"setting to get basic information about the animals.On the"expert"
setting,on the other hand,you could get much more detailed information about the history,literature,and
exploration of bears.
It now takes years for scientists to collect all the data they need to describe and analyze species.The
creators of the Encyclopedia of Life hope that their new tool will speed that process."Spider" and "bird" mentioned in Paragraph 1 are examples to illustrate________.
A:animals are people’s friends
B:there are numerous living species on Earth
C:it is easy to see spiders and birds
D:there are many types of spiders and birds答案：B解析：由文章内容可知，文章开头运用“蜘蛛”和“鸟”的例子主要说明地球上生物的种类太 多了。
根据第二段的最后一句话可知正确答案。
由第三段的第一句话可知答案。
novice在文中的意思为“新手，初学者”，选项中的beginner符合题意。
由文中第一句话可知“科学家已经命名1.8百万种物种了”，而不是“仅仅一小部分”, 其他选项在文中都能找到依据。第二篇 文章主要讲一些科学家，设计者和艺木家为帮助一个没有电的地区设计了一种“便携灯”及 这种灯的工作原理。 -
第20题：
单选题Why do some people get sick after drinking the water from the river?ABecause the river is polluted
BBecause the river makes the earth more beautiful
CBecause the river can only offer water to animals
DBecause there are too many animals and plants in the river
正确答案： C解析：
题目问的是“为什么一些人在喝了河水之后生病了”，由The river的描述可知，树很难过，因为人类向河水中排放废物，这些废物杀死了许多喝我的河水的动物和植物。一些人喝了这种水也生病了，因此答案为A项，意思是“因为河流受到了污染”。 -
第21题：
单选题Some people argue that it would do more harm than good for plants and animals to develop through artificial selection. Which statement best supports this argument?ARoses no longer smell like roses.
BPurebred dogs are disappearing.
CHumans are harmful agents of plant and animal evolution.
DMany domesticated plants and animals can no longer survive in the wild.
正确答案： C解析：
题干中指出一些人认为人工选择植物和动物对它们发展造成的危害要大于益处，问文中那个陈述可以支撑这一论点。文章第二段指出人们通过人工选择来使植物或动物进行繁殖，而很多栽培植物和驯化动物在野外已无法生存，因此符合题意。其他选项在文中均未提及。 -
第22题：
单选题The problem with Elkhorn and stag horn corals lies in that _____.Athey were not much-liked by many people
Bthey were not well-known enough to be an icon
Cthey were not considered animals by many people
Dthey were not added to the list of Endangered Species
正确答案： B解析：
第二段第三句说麋角珊瑚（elkhorn coral）和小鹿角珊瑚（staghorn coral）成功地被列入了濒危物种名单，但作为标志性物种还有欠缺，因为很多人并没有意识到珊瑚是活的动物。 -
第23题：
问答题Practice 2 Transgenic plants and animals result from genetic engineering experiments in which genetic material is moved from one organism to another, so that the latter will exhibit a characteristic. Business corporations, scientists, and farmers hope that transgenic techniques will allow more cost-effective and precise plants and animals with desirable characteristics that are not available using up to date breeding technology. Transgenic techniques allow genetic material to be transferred between completely unrelated organisms. In order for a transgenic technique to work, the genetic engineer must first construct a transgene, which is the gene to be introduced plus a control sequence. When making a transgene, scientists usually substitute the original promoter sequence with one that will be active in the correct tissues of the recipient plant or animal. The creation of transgenic animals is one of the most dramatic advances derived from recombinant DNA technology. A transgenic animal results from insertion of a foreign gene into an embryo. The foreign gene becomes a permanent part of the host animals’ genetic material. As the embryo develops, the foreign gene may be present in many cells of the body, including the germ cells of the testis or the ovary. If the transgenic animal is fertile, the inserted foreign gene (transgene) will be inherited by future progeny. Thus, a transgenic animal, once created, can persist into future generations. Transgenic animals are different from animals in which foreign cells or foreign organs have been engrafted. The progeny of engrafted animals do not inherit the experimental change. The progeny of transgenic animals do. The techniques for creating a transgenic animal include the following: 1) picking a foreign gene, 2) placing the foreign gene in a suitable form called a “construct” which guides the insertion of the foreign gene into the animal genome and encourages its expression, and 3) injecting the construct into a single fertilized egg or at the very early embryo stage of the host animal. Much genetic engineering goes into the choice of a foreign gene and building a construct. The construct must have promoters to turn on foreign gene expression at its new site within the host animal genome. By choosing a particular promoter and splicing it in front of the foreign gene, we can encourage expression of our transgene within a specific tissue. One of the most important applications of transgenic animals is the development of new animal models of human disease. Transgenic animals can serve as models for many malignant tumors. Mice have been the most frequent hosts for transgenic modification, other domestic animals have also been used. One idea has been to create transgenic cows which secrete important pharmaceutical substances in their milk. Other attempts are being made to express human interferon in the milk of sheep. A transgenic crop plant contains a gene or genes which have been artificially inserted instead of the plant acquiring them through pollination. The inserted gene sequence (known as the transgene) may come from another unrelated plant, or from a completely different species: transgenic BT corn, for example, which produces its own insecticide, containing a gene from a bacterium. Plants containing transgenes are often called genetically modified or GM crops although in reality all crops have been genetically modified from their original wild state by domestication, selection and controlled breeding over long periods of time.正确答案： 参考译文
转基因动植物系遗传工程实验的产物,在这个实验中,一个生物体的遗传物质被移入另一个生物体内,因而后者就能显示出新的特征。商界、科学家和农民都希望转基因技术能够培育出更具价格优势、而且是最近的培育技术无法培育的恰遂人愿的动植物。转基因技术能使遗传物质在两种完全不相干的生物体之间进行转移。
为了启用转基因技术,基因工程师必须首先培育一个转基因,转基因即含有受控序列的用于移植的基因。在培育转基因时,科学家通常用一个将在受体动物或植物的恰当组织中活动的启动子序列替代原有的启动子序列。
转基因动物的问世是重组DNA技术最激动人心的进展之一。把异体基因植入胚胎后即培育出转基因动物。这样异体基因就永远地成了宿主动物基因物质的一部分。随着胚胎的发育,异体基因有可能进入身体的许多细胞中,包括睾丸或卵巢的生殖细胞。如果转基因动物具有繁殖能力,植入的异体基因(转基因)就会遗传给下一代。因此,转基因动物一经培育成功,就能世代相传。转基因动物与植入异体细胞或异体器官的动物不一样。植入异体细胞或异体器官的动物发生的实验性变异不会遗传给下一代,而转基因动物的后代会继承转基因引起的变异。
转基因动物的培育主要采用以下的技术手段:1)选取一个异体基因,2)把异体基因置于某个合适的“结构体”(译者注:专业术语称plasmid,汉译“质粒”)中,这个结构体能把异体基因导入动物的基因组并能激活基因显现,3)把结构体注入宿主动物的一个单独的受精卵内或宿主动物的发育刚起步的胚胎中。基因工程的重头戏在于选择异体基因和培育结构体。这个结构体必须要有启动子,并靠启动子激活异体基因在宿主动物基因组的新部位上的基因显现。通过选择某个特别的启动子并把它粘接在异体基因表面,就能在某个特别的组织内激发转基因显现。
转基因动物最为重要的用途之一是开发培育出罹患人类疾病的新型动物。转基因动物可以用来培育罹患多种人类的恶性肿瘤的“模特儿”,老鼠是最常见的用于基因改造的宿主动物,当然还有别的家畜。人们设想培育一种转基因奶牛,能分泌出含有珍贵药物成分的牛奶,此外,人们正在尝试使羊奶中含有人类干扰素。
转基因植物内有一个或多个基因由人工导入,而并非通过天然授粉获得。导入的基因序列(即转基因)可能源于另一种与之毫不相干的植物,甚至会来自于完全不同的物种。例如,转基因Bt玉米就含有某种细菌的基因,能自身产生杀虫物质。体内含有转基因的植物常常称为遗传修饰过的作物,或GM作物,尽管事实上所有的农作物都是由最初的野生状态经过漫长岁月的驯化、优选和调控性育种使其基因得以改良。解析： 暂无解析