Genetic Engineering Cloning Term paper
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Genetic Engineering: Cloning
Genetic Engineering is defined as the scientific alteration of the structure of genetic material in a living organism, which involves the production and use of recombinant DNA .
A clone is a group of genetically identical cells. For example, tumors are clones of cells inside an organism because they consist of many replicas of one mutated cell. Another type of clone occurs inside a cell. Such a clone is made up of groups of identical structures that contain genetic material, such as mitochondria and chloroplasts. Some of these structures, called plasmids, are found in some bacteria and yeasts. Techniques of genetic engineering enable scientists to combine an animal or plant gene with a bacterial or yeast plasmid. By cloning such a plasmid, geneticists can produce many identical copies of the gene .
The term clone also refers to a group of organisms that are genetically identical. Most such clones result from asexual reproduction, a process in which a new organism develops from only one parent. Except for rare spontaneous mutations, asexually reproduced organisms have the same genetic composition as their parent. Thus, all the offspring of a single parent form a clone .
Strictly speaking a clone refers to one or more offspring derived from a single ancestor, whose genetic composition is identical to that of the ancestor. No sex is involved in the production of clones, and since sex is the normal means by which new genetic material is introduced during procreation, clones have no choice but to have the same genes as their single parent. In the same way, a clone of cells refers simply to the descendants of a single parental cell .
Scientists have long been intrigued by the possibility of artificially cloning animals. In fact, people have known since ancient times that some invertebrates (animals without backbones), such as earthworms and starfish, can be cloned simply by dividing them into two pieces. Each piece re-grows into a complete organism. The cloning of vertebrates, however, was much more difficult. The first leap forward in the cloning of these more complex organisms came in the 1950's with work done on frogs .
Beginning in 1952, Robert Briggs and Thomas King, developmental biologists at the Institute for Cancer Research (now the Fox Chase Cancer Center) in Philadelphia, developed a cloning method called nuclear transplantation, or nuclear transfer, which was first proposed in 1938 by the German scientist Hans Spemann. In this method, the nucleus--the cellular structure that contains most of the genetic material and that controls growth and development--is removed from an egg cell of an organism, a procedure known as enucleation. The nucleus from a body cell of another organism of the same species is then placed into the enucleated egg cell. Nurtured by the nutrients in the remaining part of the egg cell, an embryo (an organism prior to birth) begins growing. Because the embryo's genes came from the body cell's nucleus, the embryo is genetically identical to the organism from which the body cell was obtained .
In their experiments, Briggs and King used body cells from frog embryos. From these cells, they were able to produce several tadpoles. The men used embryos consisting of only a few thousand cells as the source for body cells and nuclei, because at that stage of development an embryo's cells are still relatively unspecialized. As an embryo develops into a completely formed organism consisting of billions of cells, its cells become increasingly specialized. Some cells become skin cells, for example, while others become blood cells. Skin cells can normally make only more skin cells. Likewise, blood cells can normally make only blood cells. By contrast, each of the unspecialized cells of an early embryo is capable of producing an entire body. At the time of Briggs's and King's experiment, researchers were not sure whether specialization occurs because different cells get different assortments of genes or because genes that are not needed in a particular kind of cell become inactive .
Additional research on nuclear transplantation was conducted in the 1960's and 1970's by John Gurdon, a molecular biologist at Oxford University in England. In 1966, Gurdon produced adult frogs using nuclei from tadpole intestine cells. This experiment proved that even cells that have undergone a great amount of specialization remain totipotent--capable, under certain circumstances, of directing the development of a complete organism. Totipotency implied that all of a fully developed organism's body cells contain a complete set of genes and that specialization occurs because certain genes are active in some cells and inactive in other cells .
Despite the demonstrated totipotency of body cells, scientists were repeatedly frustrated in their attempts to use nuclear transplantation with nuclei taken from the cells of adult vertebrates. In the rare cases in which offspring resulted from such experiments, the young never survived to adulthood .
A different and simpler cloning procedure, called embryo splitting, or artificial twinning, was developed in the 1980's and was adopted by livestock breeders. In this procedure, an early embryo is simply split into individual cells or groups of cells, as happens naturally with twins, triplets, and other multiple births. Each cell or collection of cells develops into a new embryo, which is then placed into the womb of a host mother animal, which carries it to a full term. Although this technique permits the production of multiple clones, the clones are derived from an embryo whose physical characteristics are not completely known rather than from an adult animal with known characteristics--a serious limitation for practical applications of the procedure. By the early 1990's, embryo splitting and nuclear transplantation using cells from embryos had been used to clone a number of animals, including mice, cows, pigs, rabbits, and sheep .
Researchers said the cloning of animals, especially those that have been genetically modified in certain ways, could have a number of medical, agricultural, and industrial applications. For example, cloning could result in the mass production of genetically modified cattle that secrete valuable drugs into their milk. But the cloning of animals indicated that it might also be possible to clone humans. Much of the public expressed revulsion toward the prospect of human cloning, and some politicians vowed to outlaw it. Its proponents, however, saw human cloning as a way to help people, such as by allowing infertile couples to have children .
Transgenic animals (animals engineered to carry genes from species other than their own) can be made to produce a wide variety of proteins that could be sold as drugs, as well as other proteins, called enzymes, that could be used to speed up industrial chemical reactions. Although the creation of transgenic animals began in the 1980's, cloning was expected to make it possible for such animals to be mass-produced. Large numbers of transgenic animals could produce vast quantities of needed drugs and other useful substances more efficiently and at much lower cost than is possible with bioengineering methods. As of mid-1998, most genetically engineered proteins were being manufactured in bioreactors, large steel vessels in which billions of genetically modified microorganisms produce proteins that are then extracted and purified .
Researchers involved in cloning envision a number of other practical applications for their work, including the creation of genetically modified animals that could provide organs for human organ transplants; the mass production of faster-growing and leaner livestock; and the perpetuation of endangered species .
The same procedures used to clone sheep and cattle could theoretically be used to clone humans. However, human cloning would probably be more difficult than sheep or cattle cloning, because the cells of human embryos start producing proteins at a relatively early stage. Thus, there would not be as much time for the egg cytoplasm to reprogram a transplanted nucleus. However, the successful 1998 cloning of mice, which also start producing proteins at an early embryonic stage, strongly indicated that this problem could be overcome in humans .
Geneticists foresee a number of practical applications for the cloning of humans. Infertile...
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