Cloning is the process of creating an identical copy of an original. A clone in the biological sense, therefore, is a multi-cellular organism that is genetically identical to another living organism. Sometimes this can refer to "natural" clones made when an organism reproduces asexually, but in common parlance the clone is an identical copy by some conscious design. The word was coined by the British geneticist J. B. S. Haldane in 1963, and is derived from the Greek word for "twig", klōn.
In biology, cloning is used in two contexts: cloning a gene, or cloning an organism. Cloning a gene means to extract a gene from one organism (for example by PCR) and insert it into a second organism (usually via a vector), where it can be used and studied. Cloning a gene sometimes can refer to success in identifying a gene associated with some phenotype. For example, when biologists say that the gene for disease X has been cloned, they mean that the gene's location and DNA sequence has been identified, although the ability to specifically copy the physical DNA is a side-effect of its identification.
Cloning an organism means to create a new organism with the same genetic information as an existing one. In a modern context, this can involve somatic cell nuclear transfer in which the nucleus is removed from an egg cell and replaced with a nucleus extracted from a cell of the organism to be cloned (currently, both the egg cell and its transplanted nucleus must be from the same species). As the nucleus contains (almost) all of the genetic information of a lifeform, the "host" egg cell will develop into an organism genetically identical to the nucleus "donor".
However, the term clone is used in horticulture to mean all descendants of a single plant, produced by vegetative reproduction. Many horticultural varieties of plants are in effect clones, having been derived from a single individual, multiplied by some process other than sexual reproduction. As an example, some European varieties of grapes represent clones that have been propagated for over two millennia.
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Clones
The modern cloning techniques involving nuclear transfers have been successfully performed on several species: (in chronological order)
However, the success rate has been very low: Dolly was born after 276 failed attempts; 70 calves have been created from 9,000 attempts and one third of them died young; Prometea took 328 attempts. With certain species such as dogs and rats no successful clones have been created at all. Many people believe that attempts to perform human cloning would be unethical, but some scientists have publicly announced their intention to do so. Some believe the Chinese may have already done so.
A surprising development to do with aging resulted from finds that Dolly was apparently born old; she developed arthritis at age six. Aging of this type is thought to be due to telomeres, regions at the tips of chromosomes which prevent genetic threads fraying every time a cell divides. Over time telomeres get worn down until cell-division is no longer possible - this is thought to be a cause of aging. However, when researchers cloned cows they appeared to be younger than they should be. Analysis of the cow's telomeres showed they had not only been 'reset' to birth-length, but they were actually longer - suggesting these clones would live longer life spans than normal cows (but many have died young after excessive growth). Researchers think that this could eventually be developed to reverse aging in humans.
Human cloning is a subject of great controversy regarding its ethical and practical consequences. A number of groups have made claims that they are working on or have already produced human clones. None of these claims has been independently confirmed. For more on these issues, see the article human cloning.Human cloning
