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Double Takes


Frogs, mice, sheep, and cows are among the animals that scientists have successfully cloned. (The computer-generated artwork on the previous screen, however, shows a purely imaginary view of that process.)

A clone is an exact physical copy of one “parent,” created using sophisticated scientific techniques. If you had a clone, it would be an exact physical copy of you, from your hair color to any inherited disease. Scientists have not cloned humans—yet.

But scientists are continuing research on cloning animals. Their motives, most people would agree, are good: new medicines, improved food production, and a future for endangered species. Some people disagree, however, about whether cloning itself is good—for any reason.

All living things are made of cells. Each cell contains a nucleus, a kind of personal treasure chest. Inside this treasure chest are “necklaces” of deoxyribonucleic acid (DNA), with genes—millions of bits of biological information—strung in a certain order along the strands of DNA. The exact arrangement of genes determines how someone looks and grows. The arrangement is unique to each person. DNA is what makes you—you.


A researcher examines actual DNA. Found in all cells, DNA determines everything from hair color to foot size.

Photograph by Volker Steger / Science Photo Library / Photo Researchers, Inc.

Strands of DNA sparkle in this computer-aided painting.

Photograph by Tony Stone Images

All people and most animals inherit DNA from each of two parents. A cloned creature, however, receives DNA from only one parent.

Animal cloning experiments began in the 1960s. Frogs were the first subjects. By 1987 scientists had begun cloning cows and other mammals. These experiments involved making clones from the animals’ embryo (EM-bree-oh) cells. These are the early, developing cells of unborn young. But, since there’s no way to predict how unborn young will turn out, scientists sought a way to clone from adult cells instead of from embryo cells. That way farmers, for example, could have their adult prize pigs or cows duplicated. Specially treated sheep, goats, and cows that produce milk rich in medicines for treating human diseases could be cloned into herds.

Cloning from adult cells is more difficult than cloning from embryo cells. That difficulty helps explain the excitement in 1997, when Ian Wilmut, a biologist at the Roslin Institute in Edinburgh, Scotland, cloned a lamb named Dolly from an adult cell. How did he do it?



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