A clone is an organism or cell derived asexually (through mitosis ) from a single ancestor cell. The genetic content of the newer cell (or of any individual cell of the organism) is identical to that of the ancestor cell. In biochemistry a clone is a replica of all or part of a macromolecule, for example, deoxyribonucleic acid ( DNA ).
The word "clone" originated in 1903 when it was proposed by Herbert J. Webber of the U.S. Department of Agriculture as a term to designate the offspring of plants propagated artificially (and asexually) via methods such as "rooting cuttings." Most clones occur naturally, as most organisms reproduce asexually. Most animals reproduce sexually, but many of the less complex invertebrates reproduce asexually, giving rise to clones. Currently, the terms "clone" and "cloning" are used in association with recent animal reproduction procedures (having both in vitro and in vivo stages) and the ability of scientists to alter the genetic makeup of organisms toward the greater benefit of humans in such areas as medicine and agriculture.
Reproductive cloning in higher animals, such as mammals, requires nuclear transfer, in which the entire genetic content of a cell is transferred to an embryonic cell that has been emptied of its genetic material. (Only 1 to 2 percent of these transfers are successful.) The most notable animal clone, the sheep Dolly, was born in 1997. She was the first animal to be cloned from the mature cell of another animal. Dolly was proof that a single, mature (fully differentiated) cell contains workable versions of all the genes necessary to produce an entire organism. In 2001 it was announced that human embryos, at the six-cell stage, had been produced via nuclear transfer.
Therapeutic cloning can involve recombinant DNA technology or molecular cloning. A segment of DNA from one organism (a vector) is introduced into the DNA of a second organism, yielding a hybrid or recombinant DNA molecule. The hybrid DNA molecule is then introduced into a host organism, in which it can be rapidly replicated and expressed to produce proteins of commercial or medical importance. The first such protein to be marketed commercially was human insulin, which is a small protein, having fewer than 100 amino acids, and which was "manufactured" in Escherichia
coli bacteria (the host organism). The first agricultural protein made from recombinant DNA, bovine growth hormone, was approved for use in 1994 and is given to approximately 25 percent of cows in the United States (its use is banned in the European Union). Other recombinant DNA products used in medicine include tissue plasminogen activator (TPA), used as an anticoagulant in the treatment of heart attack and stroke victims; factor VIII, a blood coagulant deficient in hemophiliacs; human growth hormone; interferons, used in the treatment of cancer; interleukins; monoclonal antibodies ; and vaccines, such as hepatitis B vaccine.
Bacterial cells as host cells are not able to execute the read-out of many human genes and the genes of many other eukaryotic organisms, and eukaryotic host cells must be used. Animals that carry genetically engineered heritable genes are called transgenic. Transgenic animals have included lactating mammals such as sheep, goats, cows, and pigs. Most commonly an animal is engineered to produce a large quantity of a particular human protein in its milk. A widely used technique includes injecting a segment of DNA, human or animal, that contains the appropriate human gene (the DNA construct) into an embryo, and then implanting the embryo in a surrogate mother. If the technique is successful, the milk of the mature animal will contain active human protein in sufficient amounts to be of value. A herd of genetically engineered animals could then be produced using this technique, or possibly, eventually, via reproductive cloning.
Plant cells can also be used to mass-produce human proteins. Corn, soybean, and tobacco plants have been used in this way. The yield of protein from plants is generally higher than that from animals; however, human cells link carbohydrates to some antibodies, and these molecules cannot yet be produced in plants. Transgenic crops that are harvested for food are far more common than those used to produce human proteins.
Therapeutic cloning sometimes calls for the use of human stem cells. Stem cells are believed to be cells that have the potential to become any cell that exists in the mature organism; hence, it is felt that they might be used in the future to repair or even regenerate organs, such as the heart or pancreas, or portions of organs. Human stem cells were isolated in 1998. If a patient's stem cells were needed and were not available, cells from existing stem cell lines could be used, but in these cases, nuclear transfer would be required. This is presently legal in Britain but illegal in the United States. An alternative source would be generic stem cells combined with the administration to the patient of any necessary antirejection drugs.
SEE ALSO Genes .
Vivienne A. Whitworth
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