British scientists have done something that.many people thought.was impossible:

    British scientists have done something that，many people thought，was impossible: they created the first clone of an adult mammal（哺乳动物）．The clone，named Dolly，is a seven-month-old sheep，grown from a cell of an adult female sheep.

    What is a clone? A clone is a copy—a genetic copy. Genes are the instructions for life. They tell a cell what to do and how to do it. Genes make each individual plant or animal different and unique. But a clone has the same genes as the original organism(生物体）．

    Cloning is not new. Plants have been cloned for centuries. New plants are often grown from cutting from another plant. Also，plants like pineapples，strawberries and carnations(康乃馨）are sometimes cloned. One benefit of cloning is that we can make many copies of the best and healthiest plants.

    It has been much more difficult to clone animals. Frogs have been cloned，but they died as tadpoles，never living long enough to become adult frogs，which made the scientists despondent. Mice，sheep and cattle have been cloned from embryos. But Dolly is the first healthy clone of an adult mammal. How did they do it?

    （1）An unfertilized egg was taken from a sheep.

    （2）The egg nucleus(细胞核）was removed.

    （3)A cell was taken from the udder(乳房）of a pregnant(怀孕的）sheep. It was“starved”，so that it stopped growing.

    （4）The two cells were combined into one，and then implanted in a third sheep，where it grew normally.

    （5）The clone was born. It was genetically identical to the pregnant sheep.

    This sounds much easier than it actually was. To end up with one clone，the scientists started with 277 udder cells. And they needed in a way to make the egg cell accept a new nucleus. They did that by starving the cell.

    It may soon be possible to clone many types of animals. Will we also be able to clone human organs，or even whole human beings? We have only begun to think about the morality of cloning technology. Maybe the question to ask is not whether we can clone humans，but instead，should we clone humans?

59. What can replace the underlined word“despondent” in the fourth paragraph?

A. calmed down          B. set down

C. cut down             D. cast down

60. Dolly is a sheep________because she is grown from a cell of an adult female sheep.

A. that has no father         B. that has no mother

C. that has no parents        D. that has no tail

61. According to the passage we can know that__________．

A. only plants can be cloned

B. only strawberries and carnations can sometimes be cloned

C. animals and plants have quite a long history of cloning

D. genes can tell a cell what to do and how to do it during cloning

62. From the fourth paragraph we can know that__________．

A. a tadpole is the early stage of a grown-up frog

B. a tadpole is a frog without a tail

C. a tadpole is a kind of animal

D. a tadpole is a clone of an adult mammal

试题答案

59. 【解析】选D。词义理解题。由第四段前两句可推知科学家们很不高兴。cast down“使沮丧；使不愉快”。

60. 【解析】选C。推理判断题。从文中可以看出，多莉是从一只成年羊的细胞中克隆出来的，所以它没有父母，而是那只羊的一个翻版。

61. 【解析】选D。细节理解题。D项“基因在克隆过程中可以告诉细胞做什么及如何去做”，文章第二段正是说的这个意思。第三段指植物可以克隆，第四段指动物也可以克隆，所以A项不正确；从文中第三段可知菠萝也是可以克隆的，所以B项不正确；第三段中指植物已有几个世纪的克隆历史，而没有提到动物的克隆历史，所以C项与题意不符。

62.【解析】选A。推理判断题。文章第四段第二句说：人们曾经克隆过青蛙，但在蝌蚪阶段就死了，并没有进入成年青蛙阶段，所以我们可以判断，蝌蚪是青蛙的早期形态。