Cell, group of cells, or organism produced asexually from a single common ancestor to which it is genetically identical. Clones may be natural (such as identical twins, or bacteria living in the same colony) or artificial (such as genetically engineered replicas of DNA sequences).
Therapeutic cloning has a medical use. Its greatest potential is the creation of human embryonic stem cells. In theory, these cells can be used to create any tissue type in the human body, potentially allowing replacement organs to be produced using a patient's own genetic material. As cloned tissues would be genetically identical to the original organs, there would be no risk of rejection by the patient's immune system, a common complication in donor organ transplants.
The creation of cloned copies of DNA fragments and molecules has applications to medical research, diagnosis, and treatment. Individual human genes can also be cloned for direct therapeutic use such as gene therapy and for implantation into another organism to produce useful substances (molecular cloning), for example introducing the gene for human growth hormone into a bacterium which then manufactures the hormone.
Reproductive cloning involves the creation of organisms. This is common in nature, with many plants and micro-organisms reproducing from a single parent without exchanging genetic material. A number of plants and animals (for example poplars and coral) form clonal colonies and propagate via detachment of part of the colony in a process known as fragmentation. Some species also produce seeds that do not require fertilization, a process known as apomixis. Cloning has been known in horticulture for many centuries, using techniques such as grafting.
In 1997, UK scientists developed the first viable procedure for creating clones of mammals from non-reproductive adult cell types. Genetic material was transplanted from a donor into an embryo with its original DNA removed (somatic cell transfer). The resulting embryo was transplanted into a surrogate and produced a living clone, Dolly the sheep.
This achievement and the subsequent creation of clones of different mammalian species fired an intense ethical debate, as for the first time there was a method of producing human clones. Many countries, including the USA, Australia, and India, passed legislation banning all forms of human cloning, even for research purposes. In 2000, the non-binding United Nations Declaration on Human Cloning was issued, stating that reproductive human cloning was contrary to human dignity and calling on member states to ban the practice. Reproductive human cloning is explicitly forbidden in the Charter of Fundamental Rights of the European Union, which became legally binding in 2009. Therapeutic cloning of human genetic material, on the other hand, was legal in most countries by 2013.
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