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Cloning

This text is used by the permission of U.S. Department of Energy Human Genome Program: http://www.ornl.gov/hgmis
Site sponsored by the U.S. Department of Energy Office of Science http://www.er.doe.gov/
Office of Biological and Environmental Research, http://www.er.doe.gov/production/ober/ober_top.html,
Human Genome Program http://www.science.doe.gov/ober/hug_top.html

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    The possibility of human cloning, raised when Scottish scientists at Roslin Institute created the much-celebrated sheep "Dolly" (Nature 385, 810-13, 1997), has aroused worldwide interest and concern because of its scientific and ethicalimplications. The feat, cited by Science magazine as the breakthrough of 1997, also has generated uncertainty over the meaning of "cloning" --an umbrella term traditionally used by scientists to describe different processes for duplicating biological material.
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What is cloning?

To Human Genome Project researchers, cloning refers to copying genes and other pieces of chromosomes to generate enough identical material for further study. Two other types of cloning produce complete, genetically identical animals. Blastomere separation (sometimes called "twinning" after the naturally occurring process that creates identical twins) involves splitting a developing embryo soon after fertilization of the egg by a sperm (sexual reproduction) to give rise to two or more embryos. The resulting organisms are identical twins (clones) containing DNA from both the mother and the father. Dolly, on the other hand, is the result of another type of cloning that produces an animal carrying the DNA of only one parent. Using somatic cell nuclear transfer, scientists transferred genetic material from the nucleus of an adult sheep's udder cell to an egg whose nucleus, and thus its genetic material, had been removed. (All cells that are not egg or sperm cells are somatic cells.) 
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Why clone?

One goal of this and similar research is to develop efficient ways to alter animals genetically and reproduce them reliably. Alterations have included adding genes (such as those for human proteins) to create drug-producing animals as well as inactivating genes to study the effects and possibly create animal models of human diseases. Cloning technology also may someday be used in humans to produce whole organs from single cells or to raise animals having genetically altered organs suitable for transplanting to humans.
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    The technique used to produce Dolly and other cloned animals is an extension of 40 years of research using DNA from nonhuman embryonic and fetal cells. Before this demonstration, scientists believed that once a cell became specialized a liver, heart, udder, bone, or any other type of cell the change was permanent and other unneeded genes in the cell became inactive. Dolly's creators demonstrated that nuclei of an adult animal's specialized cells can be made to revert to a nonspecialized, embryonic state, thus restoring the ability to give rise to any kind of cell. Explorations into how cells revert to an undifferentiated state may provide insights into the process by which cells become cancerous.
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    Using the same technique that produced Dolly, researchers have cloned a number of large and small animals including sheep, goats, mice and cows. But scientists remain uncertain about whether genetic changes in the cells used to obtain nuclei will lead to adverse effects on the health of the cloned animals.
 

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