Meet the Mammal that Made History

Dolly the sheep was the first successfully cloned mammal in history. Dolly was different from other cloning attempts because she was created from a specialized adult cell, not from an unspecialized stem cell.

To create Dolly, Ian Wilmut of the Roslin Institute in Edinburgh, Scotland, and his colleagues used two cells from two different sheep.

1. A donor cell was taken from the udder of a six-year-old sheep in the final stage of pregnancy.
2. A host cell, which needed to be an egg cell, was taken from the ovaries of another sheep.
3. The scientists needed the nucleus from the udder (donor) cell so they removed it.
4. The scientists then removed the nucleus from the egg (host) cell because they needed the egg cell, but not its nucleus.
5. Finally, the scientists took the nucleus from the donor cell and fused it, using an electrical current, with the egg cell.

The key to Dolly's success was to make the nucleus of the donor cell "quiet" so that it stopped behaving like an udder cell and instead reprogrammed itself to become an embryo.

The resulting embryo, which became Dolly, carried all of the chromosomes from the donor udder cell and none of the nuclear chromosomes from the host egg cell. Dolly ends up being an exact genetic copy, a clone, of her donor-cell "mother." Wilmut and his colleagues published their findings in a scientific paper in the 27 February, 1997, issue of Nature.

Did the cloning process affect Dolly's health? Researchers announced early in 2002 that Dolly developed arthritis in her left hind leg. While arthritis isn't unheard of in sheep, Dolly's condition surfaced at an unusually early age, prompting the question: is the arthritis a genetic defect from the cloning process? More research is required to answer that and many other questions about the general health and aging process of clones.