CLONE

also spelled Clon, population of genetically identical cells or organisms that are derived originally from a single original cell or organism by asexual methods. Cloning is fundamental to most living things, since the body cells of plants and animals are clones ultimately derived from the mitosis of a single fertilized egg. More narrowly, a clone can be defined as an individual organism that was grown from a single body cell of its parent and that is genetically identical to it. Plants that are able to propagate by asexual means produce genetically identical plants that are clones. Cloning has been commonplace in horticulture since ancient times; many varieties of plants are cloned simply by obtaining cuttings of their leaves, stems, or roots and replanting them. A vast array of fruit and nut tree varieties and innumerable ornamental plants represent clones. The body cells of adult animals and humans can be routinely cloned in the laboratory. Adults cells of various tissues, such as muscle cells, that are removed from the donor animal and maintained on a culture medium while receiving nutrients manage not only to survive but to go on dividing, producing colonies of identical descendants. By the 1950s scientists were able to clone frogs, producing identical individuals that carry the genetic characteristics of only a single parent. The technique used in the cloning of frogs consists of transplanting frog DNA, contained in the nucleus of a body cell, into an egg cell whose own genetic material has been removed. The fused cells then begin to grow and divide, just like a normal fertilized egg, to form an embryo. Mice were first successfully cloned in the 1980s, using a procedure in which the nucleus from a body cell of a mouse embryo is removed from the uterus of a pregnant mouse and transplanted into a recently fertilized egg (from another mouse) whose genetic contents have been evacuated. The cell is cultured artificially until it divides and becomes an embryo. The embryo, which is composed entirely of cells derived from the single implanted nucleus, is artificially implanted into the uterus of another mouse that brings it to term. Cloning a new animal from the cells of an adult (as opposed to those of an embryo) is considerably more difficult, however. Almost all of an animal's cells contain the genetic information needed to reproduce a copy of the organism. But as cells differentiate into the various tissues and organs of a developing animal, they express only that genetic information needed to reproduce their own cell type. This tended to restrict animal cloning to the use of embryonic cells, which have not yet differentiated into blood, skin, bone, or other specialized cells, and which can more easily be induced to grow into an entire organism. The first success in cloning an adult mammal was achieved by a team of British researchers led by Ian Wilmut at the Roslin Institute in Edinburgh, Scot., in 1996. After having already produced clones from sheep embryos, they were able to produce a lamb using DNA from an adult sheep. In their procedure, the nucleus of a cell from the mammary gland of an adult sheep was implanted in another sheep's unfertilized egg whose nucleus had been removed. The key to the new procedure is to synchronize the cell cyclei.e., the ordered sequence of events that occur in a cell in preparation for divisionof the mammary cell with that of the egg. To achieve this, before implantation the mammary cell is deprived of nutrients; this stops its cell cycle, thus preventing it from dividing. The nucleus is then implanted into the recipient egg and fused to it, and an electrical current is applied to simulate the burst of energy that occurs during fertilization. The egg begins dividing normally and becomes an embryo, which is implanted into another ewe. The lamb that is born is a clone of the donor of the original mammary cell. The practical applications of cloning are economically promising but philosophically unsettling. Animal breeders would welcome the chance to clone top-quality livestock. Genetically engineered animals could be cloned in large numbers to increase the production of drugs or human proteins that are useful in fighting disease. Clones are also highly useful in biological research because of their genetic uniformity. The cloning of human beings is a subject fraught with ethical and moral controversy. If cloning can ensure the infinite replication of specific genetic traits, a judgment would need to be made as to which traits are desirable and therefore worthy of perpetuation. The persons empowered to exercise such judgment would be in a position to change the course of human development. The use of recombinant DNA technology to manipulate and change genes is sometimes called gene cloning. See genetic engineering.