人教版英语课本原文(必修1~选修9)
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发布时间:2010-03-04 10:37:00
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CLONING: WHERE IS IT LEADING US?
Cloning has always been with us and is here to stay. It is a way of making an exact copy of another animal or plant. It happens in plants when gardeners take cuttings from growing plants to make new ones. It also happens in animals when twins identical in sex and appearance are produced from the same original egg. The fact is that these are both examples of natural clones.
Cloning has two major uses. Firstly, gardeners use it all the time to produce commercial quantities of plants. Secondly, it is valuable for research on new plant species and for medical research on animals. Cloning plants is straightforward while cloning animals is very complicated. It is a difficult task to undertake. Many attempts to clone mammals failed. But at last the determination and patience of the scientists paid off in 1996 with a breakthrough - the cloning of Dolly the sheep. The procedure works like this:
On the one hand, the whole scientific world followed the progress of the first successful clone, Dolly the sheep. The fact that she seemed to develop normally was very encouraging. Then came the disturbing news that Dolly had become seriously ill. Cloning scientists were cast down to find that Dolly's illnesses were more appropriate to a much older animal. Altogether Dolly lived six and a half years, half the length of the life of the original sheep. Sadly the same arbitrary fate affected other species, such as cloned mice. The questions that concerned all scientists were: "Would this be a major difficulty for all cloned animals? Would it happen forever? Could it be solved if corrections were made in their research procedure?"
On the other hand, Dolly's appearance raised a storm of objections and had a great impact on the media and public imagination. It became controversial. It suddenly opened everybody's eyes to the possibility of using cloning to cure serious illnesses and even to produce human beings.
Although at present human egg cells and embryos needed for cloning research are difficult to obtain, newspapers wrote of evil leaders hoping to clone themselves to attain their ambitions. Religious leaders also raised moral questions. Governments became nervous and more conservative. Some began to reform their legal systems and forbade research into human cloning, but other countries like China and the UK, continued to accumulate evidence of the abundant medical aid that cloning could provide. However, scientists still wonder whether cloning will help or harm us and where it is leading us.
THE RETURN OF THE DINOSAURS?
The possibility of cloning fierce and extinct wild animals has always excited film makers. And they are not the only ones! The popularity of films such as Jurassic Park, in which a scientist clones several kinds of extinct dinosaurs, proves how the idea struck a mixture of fear and excitement into people's hearts. But in fact we are a long way from being able to clone extinct animals. Scientists are still experimenting with cloning mammals. This is because the cloning of mammals is still a new science and its story only began seriously in the 1950s as this list shows:
1950s cloning of frogs 1996 first clone of a mammal: Dolly the sheep
1970s research using the embryos of mice 2000 cow gave birth to a bison
1979 work on embryos of sheep and mice 2001 China's first cloned twin calves
1981 first experimental clones of mice 2002 first cloned cats
1983 first experimental clones of cows 2005 first cloned dog
…
From time to time people suggest that extinct animals like dinosaurs, can possibly be brought back to life through cloning. Unfortunately, with what we know now, this is either impossible or unsuitable. There are many reasons.
◎ The initial requirement is that you need perfect DNA (which gives information for how cells
are to grow).
◎ All efforts of cloning an animal will be in vain if there is not enough diversity in the group to overcome illnesses. Diversity in a group means
having animals with their genes arranged in different ways. The advantage is that if there is a new illness some of these animals may die,
but others will survive and pass on the ability to resist that disease to the next generation. The great drawback to cloning a group of
animals is that they would all have the same arrangement of genes and so might die of the same illness. Then none of them would be left to
continue the species.
◎ It would be unfair to clone any extinct animals if they were to live in a zoo. A suitable habitat would be needed for them to lead a natural life.
Based on what we know now, you cannot clone animals that have been extinct longer than 10,000 years. Actually, dinosaurs disappeared 65,000,000 years ago. So the chance of dinosaurs ever returning to the earth is merely a dream.
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