clone

clone group of organisms, all of which are descended from a single individual through asexual reproduction, as in a pure cell culture of bacteria. Except for changes in the hereditary material that come about by mutation , all members of a clone are genetically identical. Laboratory experiments in in vitro fertilization of human eggs led in 1993 to the "cloning" of 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 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.

Bibliography: See G. Kolata, Clone (1997).

clone
1. A group of cells, an organism, or a population of organisms arising from a single ancestral cell. All members of a particular clone are genetically identical. In nature clones are produced by asexual reproduction, for example by the formation of bulbs and tubers in plants or by parthenogenesis in certain animals. New techniques of cell manipulation and tissue culture have enabled the cloning of many plants and some animals. A wide range of commercially important plant species, including potatoes, tulips, and certain forest trees, are now cloned by micropropagation, resulting in more uniform crops. Cloning in animals is more complex, but has been accomplished successfully in sheep and cattle. The first mammal to be cloned experimentally from the body cell of an adult was a sheep (‘Dolly’) born in 1997 after over 200 previous failed attempts. The nucleus containing DNA was extracted from an udder cell (which had been deprived of nutrients) and inserted into an ‘empty’ egg cell (from which the nucleus had been removed) using the technique of nuclear transfer. This reconstituted egg cell was then stimulated to divide by an electric shock and implanted into the uterus of a surrogate mother ewe, who subsequently gave birth to a clone of the original sheep. This breakthrough offered the prospect of producing exact replicas of animals with certain genetically engineered traits, for example to manufacture drugs in their milk or provide organs for human transplantation.

2. (gene clone) An exact replica of a gene. See gene cloning.

clone Set of organisms obtained from a single original parent either by asexual reproduction or by artificial selection. Clones are genetically identical and may arise naturally from parthenogenesis in animals. Cloning is often used in plant propagation (including tissue culture) to produce new plants from parents with desirable qualities such as high yield. In 1997 scientists in Scotland produced a sheep embryo from a single cell of an adult sheep using nuclear-transfer technology (transfer of a cell nucleus). In 2003 Italian scientists announced they had cloned a horse, and the next year 30 human embryos were cloned in South Korea. See also genetic engineering