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  3. Dolly was once an awfully lonely sheep. When the famous cloned (of an exact copy of a plant or animal made by taking a cell from

Dolly was once an awfully lonely sheep. When the famous cloned (of an exact copy of a plant or animal made by taking a cell from

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问题     Dolly was once an awfully lonely sheep. When the famous cloned (of an exact copy of a plant or animal made by taking a cell from it and developing it artificially) animal made headlinesin 1997, she was the only mammal ever to be manufactured from the cell of an adult donor. Since then, the clone ranks have swelled, with mice and cattle also making their way out of the labs. Last week cloning technology took another step forward when an international biotechnology company announced that it had created a litter of five genetically identical piglets (young pigs), and that it had a pretty good idea of how they could one day be used as organ donors for ailing humans.
    The idea of turning pigs into tissue factories has been around for at least 30 years. Pigs breed easily and mature quickly, and their organs are roughly the same size.as those of humans, meaning operations can be performed with a relative snap-out, snap-in simplicity. The problem is, once the donor organ is stitched in place, the body rebels, rejecting it even more violently than it would a human transplant. " A pig heart transplanted in a person would turn black within minutes, " says David Ayares, a research director with PPL Therapeutics, the biotech firm that helped clone Dolly and also produced the piglets.
    What causes pig organs to be rejected so quickly is a sugar molecule on the surface of pig cells that identifies the tissue as unmistakably nonhuman. When the immune system spots this marker, it calls out its defenses. PPL scientists recently succeeded in finding the gene responsible for the sugar and knocking it out of the nucleus of a pig cell. Their next step would be to extract that nucleus, insert it into a pig ovum, and then into the womb of a host pig. The sugar free piglet that was eventually born could then be cloned over and over as a source of safe transplant organs. The idea is to arrive at the ideal animal and repeatedly copy it exactly as it is. The cloned piglets PPL introduced to the world last week were created in just this way, though for this first experiment in pig replication the scientists left the sugar genes intact.
    Despite this recent success, PPL is not likely to be setting up its organ shop anytime soon. Knocking out the key sugar gene solves only the problem of short-term rejection. Much more has to be done before any solution to long-term rejection can be found. Nonetheless, Ayares is optimistic, insisting that pig organs could be available in as little as five years. For the present, even a little new transplant material is a big improvement over what’s available, and for gravely ill patients awaiting a donor, that’s no small thing.
PPL scientists will try to solve their problem by
选项 A、removing the sugar molecule on the surface of pig cells.
B、distinguishing human tissues form those of pigs.
C、using the sugar-free nucleus in cloning a pig.
D、taking out the sugar genes from the cell of a pig.
答案C
解析 根据第三段第三句“PPL scientists recently succeeded in finding the gene responsible for the sugar and knocking it out of the nucleus of a pig cell．”可知，与这些糖分子相关的基因存在于猪的细胞核，而不是细胞表面，故A选项错误；B选项的观点文中没有涉及；根据第三段第三句、第四句“PPL scientists recently…the gene responsible for the sugar and knocking it out of the nucleus of a pig cell．Their next step…into the womb of a host pig．”可知，PPL的科学家想用不含糖的细胞核克隆猪，以解决他们的问题，故正确答案为C选项；D选项只是解决该问题过程当中的一个步骤，不能完全解决问题，故不选。
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