Section (C)
Twins Six Years Apart
Scientists in Scotland recently announced that, for the first time, they have cloned an exact copy of an adult mammal. The cloned lamb, named Dolly, has the exact same genes as the adult sheep from which she was cloned. In other words, the two are identical twins; only Dolly is six years younger. The goal of the Scottish scientist Ian Wilmut is to develop a way to raise identical sheep that produce medicines for humans.
A week after Wilmut's announcement, other scientists revealed that they had used a different technique to clone monkeys, which are much more closely related to humans. These accomplishments immediately set off a worldwide debate: Should scientists be allowed to clone animals? Is cloning morally wrong and dangerous — or is it a valuable research tool?
All attempts at cloning were largely unsuccessful until 1984. That's when a scientist in Denmark separated cells from a sheep's embryo (胚胎). An embryo is an early stage of development in which cells are busy dividing and "transforming" into specialized cells like skin, eye, or muscle cells.
Unlike a skin cell, an embryo is on its way to becoming a complete living thing: The Danish scientist combined an embryo cell with an egg cell from another sheep. He put the combined cell — then a newly growing embryo — into a grown female sheep. To much surprise, the embryo grew into a baby lamb.
Since then, other scientists have used embryos to clone cattle, pigs, goats, rabbits - and, now, even monkeys. So what makes Wilmut's sheep unique? Instead of using early-stage embryo cells, Wilmut used cells from the udder (乳腺) of an adult sheep. In theory, that's like using one of your skin cells to clone a new you!
Wilmut knew that each cell of the body contains a full set of genetic instructions — instructions to grow a complete individual. (The only exceptions are egg and sperm cells, each of which contains half the genes to grow a new individual.) Once cells have specialized, on their way to becoming skin or eye or udder cells, most of the genetic instructions to make a full being are turned off. Until now, scientists believed that specialized cells could not be used to form a complete living thing.
Wilmut proved them wrong. He found a way to take an udder cell and make it grow into a new cloned lamb. An amazing fact: Dolly has no biological father. Wilmut's success didn't come easily. He has been studying this problem for more than two decades.
Last year, he used embryos to successfully clone two sheep. Then he went ahead to clone an adult sheep. But, of 277 udder cells he fused with egg cells, only 30 began to develop into embryos. He implanted 29 of those into female sheep. Only one adult gave birth to a lamb.
Other scientists have jumped in to repeat Wilmut's experiment with other animals, including cows. And that's what has scientists, animal-rights advocates, politicians — even President Clinton — up in arms. How far, they wonder, will cloning go?
Wilmut maintains that cloning animals has tremendous potential for helping people. Cloned sheep, he says, could be used as living drug factories. Scientists could "engineer" sheep that produce drugs in their milk. And by altering the proteins on the surfaces of animal organs to make them more like human organs, scientists believe they may be able to create a plentiful source of organ donors for people.
Why not clone humans as organ donors? Theoretically, Wilmut says, there is no reason his techniques couldn't someday be used to clone people. Think about the possibilities: a whole basketball team of Michael Jordans, a scientific panel of Albert Einsteins, a movie starring and co-starring Brad Pitts.
On a more serious note, some experts argue that couples who have difficulty having a baby could make copies of themselves. And parents whose child has a fatal disease like cancer might be able to clone the child, creating a twin who could be a bone-marrow (骨髓) donor.
But even Ian Wilmut draws the line at cloning humans. "All of us would find that offensive," he says. Several countries, including Britain, Denmark, Germany, and Australia, have made all scientific work on cloning humans illegal. The U.S. has no such law, but President Clinton has set up a panel of scientists and philosophers to study the issue. In the meantime, Clinton has imposed a ban on using federal money to clone humans.
Humans are more than the sum of their genes, argues a philosopher at one research institute. Though they look exactly the same, clones are not necessarily exact copies. The younger twin might grow up with different influences — say, unusual friends or special teachers. A cloned Albert Einstein might fail his physics class. A cloned pop star might sing terribly.
Say you were cloned. Would your twin live a shorter life because he or she started out with DNA that was already 10, 20, or 30 years old? Scientists aren't sure. And how could you prevent someone from taking a sample of your hair and making a clone of you? Again, no solutions.
Some people who oppose cloning also object to the use of animals as research tools. "Next, they'll be cloning foxes to make more fur (毛皮) coats," says the president of an animal rights group.
What do you think? Should scientists be allowed to clone animals? How about humans?
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相差六歲的雙胞胎
蘇格蘭科學(xué)家最近宣稱,他們第一次克隆了一只成年哺乳動物。 克隆出的羊羔名為多莉,她具有和其被克隆的成年綿羊完全一樣的基因。 換言之,二者是一對同卵雙胞胎,只是多莉小六歲。 蘇格蘭科學(xué)家伊恩·威爾莫特的目的是要研制出一種方法,來培育能為人類提供藥物的同卵綿羊。
在威爾莫特發(fā)布消息一周后,其他科學(xué)家透露說,他們使用了另一種技術(shù)來克隆猴子。這種動物與人類的關(guān)系更為密切。 這些成果立即引發(fā)了一場世界范圍的辯論: 應(yīng)該允許科學(xué)家克隆動物嗎? 克隆是違背或危害道德的——還是一種有價值的研究手段?
1984年之前,所有的關(guān)于克隆的努力多半未獲成功。 但1984年,一位丹麥科學(xué)家從一只綿羊的胚胎中分離出了細(xì)胞。 胚胎是發(fā)育的最初階段,在這個階段,細(xì)胞不停地分裂并"轉(zhuǎn)化"成特化的細(xì)胞,如皮膚細(xì)胞、眼睛細(xì)胞、或肌肉細(xì)胞等。
與單個皮膚細(xì)胞不同,一個胚胎將逐漸形成一個完整的生命體: 這位丹麥科學(xué)家將一個胚胎細(xì)胞和一個取自另一只綿羊的卵細(xì)胞結(jié)合起來。 他把結(jié)合后的細(xì)胞——此時已是一個重新生長的細(xì)胞——植入一只成熟母羊的體內(nèi)。 極為奇特的是,這只胚胎長成了一個小羊羔。 從那時起,科學(xué)家們就用胚胎來克隆牛、豬、山羊、兔子等——而現(xiàn)在,居然還克隆了猴子。
那威爾莫特的綿羊有什么獨特之處呢? 威爾莫特使用的不是初期胚胎細(xì)胞,而是從一只成年綿羊的乳腺提取的細(xì)胞。 理論上說,這就像是用你的一個皮膚細(xì)胞來克隆一個新的你。
威爾莫特知道,身體的每一個細(xì)胞都含有一整套基因指令——長成一個完整個體的指令。 (惟一的例外是卵子和精子細(xì)胞,它們的每個細(xì)胞分別含有長成新個體的一半基因。) 一旦細(xì)胞特化,進入成為皮膚細(xì)胞、眼睛細(xì)胞或乳腺細(xì)胞的過程,大部分長成完整個體的基因指令就被關(guān)閉。 在此之前,科學(xué)家相信特化細(xì)胞不能被用來形成完整的生命體。
威爾莫特證明他們是錯的。 他找到了提取乳腺細(xì)胞并把它培育成一只新的克隆羊的方法。 一個有趣的事實是:多莉沒有生物學(xué)意義上的父親。
威爾莫特的成功得來不易。 他對這個問題研究了20多年。 去年,他用胚胎成功地克隆了兩只綿羊。 后來他又進一步克隆了一只成年綿羊。 但是,在他將卵細(xì)胞與之結(jié)合的277個乳腺細(xì)胞中,只有30個開始長成胚胎。 他將其中29個植入母羊體內(nèi),只有一只成年羊分娩了羊羔。
其他科學(xué)家立即投入工作,在其他動物(包括母牛)身上重復(fù)威爾莫特的實驗。 而這正是讓一些科學(xué)家、動物權(quán)利倡議者和政治家——包括克林頓總統(tǒng)——竭力反對的事情。 他們困惑不解:克隆究竟要走多遠(yuǎn)?
威爾莫特堅持認(rèn)為,克隆動物具有幫助人類的巨大潛力。 他說,克隆出來的綿羊可以用作活的藥物工廠。 科學(xué)家可以"設(shè)計"乳汁中含有藥物的綿羊。 而且科學(xué)家們相信,通過改變動物器官表面的蛋白質(zhì),它們更像人類器官,就能夠找到一個為人類提供器官的豐富來源。
那為什么不克隆人類來作為器官捐獻(xiàn)者呢? 威爾莫特說,理論上,有理由相信他的技術(shù)某一天能被用來克隆人。 請考慮這些可能性:一支全由邁克爾·喬丹組成的籃球隊,一組全由阿爾伯特·愛因斯坦組成的科學(xué)家,或者一部主配角全是布拉德·皮特的電影。
有些專家用更為嚴(yán)肅的語氣爭辯說,那些生育孩子有困難的夫婦可以復(fù)制他們自身。 而那些孩子患有諸如癌癥等不治之癥的父母也能克隆一個孩子,創(chuàng)造出一個能提供骨髓的雙胞胎。
但是,就連伊恩·威爾莫特也在克隆人類問題上劃了一條界線。 他說:"我們大家都覺得那個想法令人反感。" 有些國家,包括英國、丹麥、德國和澳大利亞,都宣布克隆人類的研究工作是非法的。 美國雖然還沒有這樣的法律,但克林頓總統(tǒng)已經(jīng)建立了一個由科學(xué)家和哲學(xué)家共同組成的小組來研究這個問題。 同時,克林頓還下令禁止用聯(lián)邦基金來克隆人類。
某研究機構(gòu)的一位哲學(xué)家爭論說,一個人不僅僅是其基因的總和。 盡管克隆的人看上去一模一樣,但他們并非必定是一模一樣的翻版。 年少的雙胞胎成長時會受到不同的影響——比如說,不一樣的朋友和特別的老師等。 克隆出的阿爾伯特·愛因斯坦可能物理課不及格。 克隆出的流行歌星也許唱得很糟糕。
就說你被克隆了吧。 你的孿生兄弟或姐妹出生時DNA就已經(jīng)10、20或30歲了,難道就因為這樣他或她的壽命會更短嗎? 科學(xué)家也不能肯定。 而且,你怎樣防止別人從你的頭發(fā)里取得樣本來克隆一個你呢? 同樣是沒有答案的。
有些反對克隆的人,同樣也反對用動物做實驗工具。 一家動物權(quán)利組織的主席說,"接下來,他們就會克隆狐貍來做更多的狐皮大衣了。"
你怎么看? 應(yīng)該允許科學(xué)家克隆動物嗎? 還有人呢?