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 either when an organism Censored page or when two genetically identical individuals are produced by accident (as with identical twins), but in common parlance the clone is an identical copy by some conscious design. Also see clone (genetics).

The term clone is derived from κλων, the Greek word for "twig". In horticulture, the spelling clon was used until the twentieth century; the final e came into use to indicate the vowel is a "long o" instead of a "short o". Since the term entered the popular lexicon in a more general context, the spelling clone has been used exclusively.

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". Mitochondrial DNA, which is not transferred by this process, is generally ignored as its effects on organisms are thought to be relatively minor.

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 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. This is a genuine example of cloning in the broader biological sense, as it creates genetically identical organisms by biological means, but this particular kind of cloning has not come under ethical scrutiny and is generally treated as an entirely different kind of operation.

Therapeutic cloning is the procedure for creating stem cells genetically compatible with the patient.

Contents

1 Species cloned 2 Health aspects 3 Human cloning 4 Cloning extinct species 5 Commercial cloning 6 External links

Species cloned

The modern cloning techniques involving nuclear transfers have been successfully performed on several species: (in chronological order)

• frogs: (1962) Unsuccessful
• carp: (1963) ** Successfully cloned **
• sheep: (1996) Dolly
• rhesus monkey: Tetra (female, January 2000)
• pig: 5 Scottish PPL piglets (March 2000), Xena (female, August 2000)
• gaur: Noah (male, January 2001)
• cattle: Alpha and Beta (males, 2001)
• cat: CopyCat "CC" (female, late 2001), Little Nicky, 2004, was the first cat cloned for commercial reasons
• mice: over a dozen as of 2002
• rabbit: (March-April, 2003) in France and North Korea independently. Human-rabbit hybrid in China (August, 2003)
• mule: Idaho Gem (male, May 2003) and Utah Pioneer (male, June 2003)
• deer: Dewey (2003)
• horse: Prometea (female, 2003)
• rat: Ralph (male, 2003)
• fruit flies (2004)

Surprisingly, a Chinese scientist, embryologist Tong Dizhou, cloned a fish in 1963, 33 years before Dolly the sheep. Apparently, he published the findings in an obscure Chinese science journal, which was never translated into English. [1] http://www.pbs.org/bloodlines/timeline/text_timeline.html

Health aspects

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 no successful clones have been created at all.

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. Many people believe that attempts to perform human reproductive cloning would be unethical, but some scientists have publicly announced their intention to do so. 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. Meanwhile therapeutic cloning appears to be a promising technology for combating many deadly deseases. For more on these issues, see the article human cloning.

Cloning extinct species

Cloning extinct species has been a dream of scientists for decades. This pursuit was publicized in Jurassic Park, but has come closer to being a reality in recent years (in fact, the dinosaurs in Jurassic Park were not really cloned -- their genomes were reconstructed from bits and pieces of many dinosaur genomes). One of the most anticipated targets for cloning was once the Woolly mammoth , but attempts to extract DNA from frozen mammoths have been unsuccessful.

In 2000, a cow named Bessie gave birth to a cloned Asian guar, an endangered species; this provided hope that similar techniques (using surrogate mothers of another species) might be used to clone extinct species; in anticipation of this possibility, the last bucardo, a Spanish mountain goat, was frozen immediately after it died (from illness after birth). Researchers are also considering cloning endangered species such as the giant panda, ocelot, and cheetah[2] http://www.washingtonpost.com/wp-srv/aponline/20001008/aponline171938_000.htm .

In 2002, geneticists at the Australian Museum announced that they had replicated DNA of the Thylacine (Tasmanian Tiger), extinct about 65 years previous, using polymerase chain reaction (PCR)[3] http://www.cnn.com/2002/WORLD/asiapcf/auspac/05/28/aust.thylacines/ .

One of the continuing obstacles in the attempt to clone extinct species is the need for nearly perfect DNA. Furthermore, if animals were cloned from one individual, the significant problem of lack of genetic diversity would still remain in the attempt to establish a breeding population.

Commercial cloning

While the promise cloning extinct species is an old justification for developing cloning, there are a lot of other applications. One is cloning cattle, horses and other domestic animals. This appears to be a much faster and more efficient way to propagate good genes (as chosen by humans) than traditional breeding.

Another application that has recently became feasible is cloning pets. Little Nicky was the first pet cloned (by the Genetic Savings and Clone company) after the death of the original. The procedure is still very expensive and the demand is in its infancy, though. But there is potential demand in many unexpected areas. For example, some Hollywood movies studios stored genetic samples of some of their "animal actors" to have the possibility of growing a clone that could replace then dead animal in a future sequel [4] http://www.bostonphoenix.com/boston/news_features/this_just_in/documents/0436608
3.asp .

External links

• Cloned Cats in Texas http://news.bbc.co.uk/2/hi/science/nature/3548210.stm
• Cloning News from Genome News Network (GNN) http://www.genomenewsnetwork.org/categories/index/cloning.php
• Discussion of cloning from the Roslin Institute http://www.roslin.ac.uk/public/cloning.html - creators of Dolly the sheep
• Cloning timeline http://edition.cnn.com/interactive/health/0108/cloning.timeline/content.html : from CNN
• Green Light to Cloning in Britain http://education.guardian.co.uk/higher/research/story/0,9865,1280916,00.html
• Cultural and social considerations in therapeutic and reproductive cloning http://samvak.tripod.com/cloning.html