cloningof human embryos by dividing such fertilized eggs at a very early stage of development, but this technique actually produces a twin rather than a clone. In a true mammalian clone (as in Gurdon's frog clone) the nucleus from a body cell of an animal is inserted into an egg, which then develops into an individual that is genetically identical to the original animal.
Later experiments in cloning resulted in the development of a sheep from a cell of an adult ewe (in Scotland, in 1996), and since then rodents, cattle, swine, and other animals have also been cloned from adult animals. Despite these trumpeted successes, producing cloned mammals is enormously difficult, with most attempts ending in failure cloning succeeds 4% or less of the time in the species that have been successfully cloned. In addition, some studies have indicated that cloned animals are less healthy than normally reproduced animals.
In 2001 researchers in Massachusetts announced that they were trying to clone humans in an attempt to extract stem cells . The National Academy of Sciences, while supporting (2001) such so-called therapeutic or research cloning, has opposed (2002) the cloning of humans for reproductive purposes, deeming it unsafe, but many ethicists, religious and political leaders, and others have called for banning human cloning for any purpose. South Korean scientists announced in 2004 that they had cloned 30 human embryos, but an investigation in 2005 determined that the data had been fabricated. In 2013 scientists at Oregon Health and Science Univ. reported that they had created embryonic stem cells using genetic material from human skin cells and donated eggs the technique used to create the embryo, however, would not result in a viable human clone.
See G. Kolata, Clone (1997).
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