In many ways, the cells of multicellular organisms -- daffodils or maple trees, frogs, sheep, or humans -- are genetic clones because they all come from the same fertilized egg cell. During embryonic development, all of the cells of these organisms are produced by mitosis (cell division) from the fertilized oocyte. Each time mitosis occurs, a cell copies its DNA and then splits in half.
The two daughter cells that result carry the same number and kind of chromosomes and genes, which are made of the same DNA as the oocyte. So in terms of their genetic content, all the cells of a multicellular organism -- except the egg cells and sperm cells -- are clones. But even though the cells of a multicellular organism contain identical genetic material, they do not look or act the same. An adult multicellular plant or animal is not simply a massive ball of identical cells.
This is because a complex process occurs during development that causes the cells of an embryo to assume different shapes and functions. During the process called differentiation, cells begin to acquire unique molecular and structural characteristics that allow them to perform specific functions. In mammals, for instance, red blood cells carry oxygen; skeletal muscle cells contract and move the body; and nerve cells conduct electrical impulses that signal other cells to respond. One of the greatest challenges in developmental biology is to understand how a single fertilized egg becomes a complex, differentiated multicellular organism.
What accounts for differentiation during embryonic development? How can cells that carry the same DNA, genes, and chromosomes look and function so differently? The simplest explanation is that, as an embryo grows, the genes in different cells become turned on or off in different patterns or combinations. One combination of active and inactive genes results in a skeletal muscle cell. A different combination of active and inactive genes results in a nerve cell. Researchers have learned much about the molecular processes that turn genes on and off during development and adulthood, although many questions remain unanswered.
Dolly, the cloned sheep, has changed the way that scientists think about a related question: Are the genes in an adult, differentiated cell permanently turned "on" or "off?" Put another way, is it possible to reprogram a differentiated, adult cell so that as it divides, it is capable of doing what a fertilized egg cell can do -- producing an entirely new organism? Dolly's existence suggests that differentiated cells from adult mammals can be reprogrammed to behave as if they were embryonic cells.
Researchers in Scotland created Dolly by fusing an adult, differentiated cell obtained from a sheep's udder with a fertilized egg cell from which they had removed the nucleus. 
The scientists, led by Wilmut of the Roslin Institute in Edinburgh, based their techniques on methods developed in the 1960s by Gurdon, Briggs, and King. Those researchers experimented with the African clawed frog, Xenopus laevis, and won a Nobel prize for their research. They used the nucleus from a tadpole's intestinal cell to generate an adult frog, but they were never able to use the nucleus from an adult cell to create an adult frog.
Wilmut and his colleagues have spent many years developing their experimental techniques, which are still far from perfect. But they have succeeded in doing what has never before been accomplished -- cloning an apparently healthy adult mammal from an adult differentiated cell.
