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. Uses of Cloning: 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. Early Scientific Experiments of Cloning: Scientists have long been intrigued by the possibility of artificially cloning animals. In fact, people have known since ancient times that just just cutting them into two pieces can clone some invertebrates, such as earthworms and starfish. Each piece grows into a complete organism. The cloning of vertebrates (animals with back bones) is much more difficult to clone.
The first step in the cloning the complex organisms (vertebrates) came in the 1950’s with experiments done on frogs. In 1952, Robert Briggs and Thomas King, 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.
Animal Cloning: Wilmut and his colleagues took mammary-gland cells from an adult sheep and placed them in a solution that essentially starved them of nutrients and caused them to stop growing for a few days. Then, with a spark of electricity, they fused each mammary cell with an enucleated egg cell. The resulting cells were allowed to grow into embryos, which were then transplanted into surrogate mother ewes (female sheep) to complete their development. Nearly 300 attempts at this technique resulted in failure for the scientists.
Some eggs did not accept mammary cell nuclei, embryos that were produced died, and lambs that were born were abnormal and died. But one lamb, apparently healthy, survived the procedure: Dolly, who was born in July 1996. Mouse Cloning: At the University of Hawaii they created more than 50 mice using adult cells in a variation of the cloning procedure used with Dolly (the first clone). There were two major differences between this mouse technique and the Dolly technique that allowed the Hawaii scientists to achieve such remarkable success.
The first difference was that the Hawaii researchers used naturally dormant cumulus cells (cells that surround eggs in ovaries) in their procedure. Because these cells were not growing, they could be easily reprogrammed inside enucleated egg cells without starving them in a special solution, as was necessary with the udder cells used in the Dolly procedure. Secondly, instead of electrically fusing a body cell with an enucleated egg cell, as was done in the Dolly technique; the Hawaii researchers used an extra-fine needle to inject the nucleus from a cumulus cell into an enucleated egg cell.
Because this technique did less damage to the egg than did electrical fusion, it increased the chance that the resulting cell would develop into a healthy embryo. Human Cloning: 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. Practical Purposes of Cloning: The mass production of animals engineered to carry human genes for the production of certain proteins that could be used as drugs; the proteins would be extracted from the animals’ milk and used to treat human diseases.