THE AGE OF CLONING

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Scientists who have focused their cloning efforts on more forgiving embryonic tissue have met with greater success. A simple approach, called embryo twinning (literally splitting embryos in half), is commonly practiced in the cattle industry. Coaxing surrogate cells to accept foreign DNA is a bit trickier. In 1952 researchers in Pennsylvania successfully cloned a live frog from an embryonic cell. Three decades later, researchers were learning to do the same with such mammals as sheep and calves. "What's new," observes University of Wisconsin animal scientist Neal First, "is not cloning mammals. It's cloning mammals from cells that are not embryonic."

Embryo cells are infinitely easier to work with because they are, in the jargon of cell biologists, largely "undifferentiated." That is, they have not yet undergone the progressive changes that turn cells into skin, muscles, hair, brain and so on. An undifferentiated cell can give rise to all the other cells in the body, say scientists, because it is capable of activating any gene on any chromosome. But as development progresses, differentiation alters the way DNA--the double-stranded molecule that makes up genes--folds up inside the nucleus of a cell. Along with other structural changes, folding helps make vast stretches of DNA inaccessible, ensuring that genes in adult cells do not turn on at the wrong time or in the wrong tissue.

The disadvantage of embryonic cloning is that you don't know what you are getting. With adult-cell cloning, you can wait to see how well an individual turns out before deciding whether to clone it. Cloning also has the potential to make genetic engineering more efficient. Once you produce an animal with a desired trait--a pig with a human immune system, perhaps--you could make as many copies as you want.

In recent years, some scientists have speculated that the changes wrought by differentiation might be irreversible, in which case cloning an adult mammal would be biologically impossible. The birth of Dolly not only proves them wrong but also suggests that the difficulty scientists have had cloning adult cells may have less to do with biology than with technique.

To create Dolly, the Roslin 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, 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.