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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".

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 to 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. This can be done by 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".

This technique has been successfully performed on several species, such as frogs, mice, sheep, and cattle (the most famous example being the sheep "Dolly"). However the success rate is 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. With certain species such as dogs, rats and horses 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

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.

Computer Cloning and Computer Program Cloning

When IBM came out with the IBM PC in 1981, other companies such as Compaq decided to put out a clone of the PC as a legal reimplemenatation from the PC's documentation or reverse engineering. As most of the components except the PC's BIOS were publicly available, all Compaq had to do was reverse engineer the BIOS. The result was a machine that had more bang for the buck than the archetypes that the machine resembled. The term "PC clone" fell out of use in the 1990s; the class of machines it now describes are simply called PCs or Intel machines.

Software can also be cloned by reverse engineering or legal reimplementation from documentation or other sources. Software such as MS-DOS's edlin line editor and the Unix operating system have been cloned. The reasons for cloning may include getting around draconian licensing fees or as a curiosity; e.g. because the programmer can.

The Jargon File has this definition for clone:

1. An exact duplicate: "Our product is a clone of their product." Implies a legal reimplementation from documentation or by reverse-engineering. Also connotes lower price. 2. A shoddy, spurious copy: "Their product is a clone of our product." 3. A blatant ripoff, most likely violating copyright, patent, or trade secret protections: "Your product is a clone of my product." This use implies legal action is pending. 4. [obs] `PC clone:' a PC-BUS/ISA or EISA-compatible 80x86-based microcomputer (this use is sometimes spelled `klone' or `PClone'). These invariably have much more bang for the buck than the IBM archetypes they resemble. This term fell out of use in the 1990s; the class of machines it describes are now simply `PCs' or `Intel machines'. 5. [obs.] In the construction `Unix clone': An OS designed to deliver a Unix-lookalike environment without Unix license fees, or with additional `mission-critical' features such as support for real-time programming. Linux and the free BSDs killed off this product category and the term with it. 6. v. To make an exact copy of something. "Let me clone that" might mean "I want to borrow that paper so I can make a photocopy" or "Let me get a copy of that file before you mung it".

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