普通高中课程标准实验教科书 英语选修8 Unit 2 Cloning-Using La（在线收听）

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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