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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

admin2009-03-21  104
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.
What is true about Dolly according to the text?
选项 A、She was a lonely sheep in the first place.
B、She was manufactured out of the lab.
C、She was cloned from the cell of a mature sheep.
D、She was replaced by cloned piglets in terms of importance.
答案C
解析 根据第一段首句“Dolly was once all awfully lonely sheep．”可知，多利曾经是一只孤单的绵羊，A选项中的“in the first place”意为“起初，首先”，与文意不符；根据该段第二句“…she was the only mammal ever to be manufactured from the cell of an adult donor”，可知，多利是唯一一只由原体提取的细胞培养而成的哺乳动物，并不是完全依靠实验室制造出来的，故B选项错误；adult donor与mature sleep意思相同，故C选项为正确答案；此处的adult donor即指代c中所说mature sheep；根据文章最后一句话“For the present，even a little new transplant material is a big improvement over what’s available…”可知，即使是一个细微的新的移植物在已有的技术基础上都将是一个重大的进步，全文并没有将克隆猪与克隆羊的重要性进行比较，故D选项错误。
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