This Is How Dolly the Sheep Was Cloned

To create Dolly, the [Roslin Institute] team concentrated on arresting the cell cycle—the series of choreographed steps all cells go through in the process of dividing. In Dolly‘s case, the cells the scientists wanted to clone came from the udder of a pregnant sheep. To stop them from dividing, researchers starved the cells of nutrients for a week. In response, the cells fell into a slumbering state that resembled deep hibernation.

At this point, [embryologist Ian] Wilmut and his colleagues switched to a mainstream cloning technique known as nuclear transfer. First they removed the nucleus of an unfertilized egg, or oocyte, while leaving the surrounding cytoplasm intact. Then they placed the egg next to the nucleus of a quiescent donor cell and applied gentle pulses of electricity. These pulses prompted the egg to accept the new nucleus—and all the DNA it contained—as though it were its own. They also triggered a burst of biochemical activity, jump-starting the process of cell division. A week later, the embryo that had already started growing into Dolly was implanted in the uterus of a surrogate ewe.

An inkling that this approach might work, says Wilmut, came from the success his team experienced in producing live lambs from embryonic clones. “Could we do it again with an adult cell?” wondered Wilmut, a reserved, self-deprecating man who likes gardening, hiking in the highlands and drinking good single-malt Scotch (but who was practical enough to file for a patent before he went public).

It was a high-risk project, and in the beginning Wilmut proceeded with great secrecy, limiting his core team to four scientists. His caution proved to be justified; the scientists failed far more often than they succeeded. Out of 277 tries, the researchers eventually produced only 29 embryos that survived longer than six days. Of these, all died before birth except Dolly, whose historic entry into the world was witnessed by a handful of researchers and a veterinarian.