问答题

Practice 3　　Dolly was no ordinary lamb. She was cloned from a single mammary cell of an adult ewe, overturning long- held scientific dogma that had declared such a thing biologically impossible. Her birth set off a race in laboratories around the world to duplicate the breakthrough and raised the specter of human cloning.　　A decade later, scientists are starting to come to grips with just how different Dolly was. Dozens of animals have been cloned since that first little lamb and it’s becoming increasingly clear that they are all, in one way or another, defective.　　It’s tempting to think of clones as perfect carbon copies of the original—down to every hair and quirk of temperament. It turns out, though, that there are various degrees of genetic replication. Not only are clones separated from the original template by time—-in Dolly’s case, six years—but they are also the product of an unnatural molecular mechanism that turns out not to be very good at making identical copies.　　But scientists see a role for cloning in treating human diseases—and perhaps someday conquering some of man’s most intractable conditions. It may be another 10 years or more before the approach yields anything safe and reliable enough to be used in real patients, and there is no guarantee that it will ever be successful. But nobody thought Dolly was possible until she made history that warm July night 10 years ago.