Udderly amazing. Scientists have for the first time used DNA from an adult mammal-specifically, genetic material from cells in the mammary glands of a 6-year-old ewe-to create a genetic duplicate. This clone, a healthy lamb named Dolly, was born last July, Ian Wilmut of the Roslin Institute in Edinburgh, Scotland, and his colleagues announce in the Feb. 27 Nature.
The spectacular feat builds upon cloning research dating back to the early 1980s. At that time, scientists developed a procedure called nuclear transfer that enables them to replace the DNA-containing nucleus of an egg cell with a nucleus from another cell. Researchers soon found that the altered egg could develop into a clone of the animal that provided the nucleus-but only if the nucleus came from a cell of a barely developed embryo. Cloning attempts using nuclei from adult animals invariably failed.
Last year, Wilmut and his coworkers described a modified nuclear transfer method that allowed them to clone sheep from older embryonic cells (SN: 3/9/96, p. 148). By maintaining the intended donor cells in a nutrient-deprived medium, the scientists forced the cells out of their normal growth cycle and into a quiescent stage called G0. For reasons still under study, nuclei from these cells are more readily accepted by eggs.
With the birth of Dolly, Wilmut's group has now proved that at least some adult cells prepared in the same manner can generate a viable clone when their nuclei are transferred to eggs.
Many biologists had concluded that this was impossible, speculating that the DNA inside the nuclei of adult cells undergoes irreversible changes as the cells mature into the specialized roles they perform-secreting milk, for example. …