Cloning

Cloning

Cloning is the production of a group of genetically identical cells or organisms, all descended from a single individual. The members of a clone have precisely the same characteristics, except where mutation and environmentally caused developmental variation have occurred.

EXAMPLES FROM NATURE

Primary Reproductive Mode

In species whose reproduction is strictly asexual, each population consists of one or more clones, depending on the number of ancestral individuals. Such species include all bacteria and blue-green bacteria, most protozoans, algae, some yeasts, and even some higher plants and animals, such as dandelions and flatworms.

Supplementary Reproductive Mode

Some algae (Ulothrix, for example) reproduce sexually and asexually. Those individuals formed by asexual reproduction (zoospores) constitute a clone. In the club mosses and some higher plants, a "runner," or stem, grows horizontally along the surface of the soil and at intervals produces roots and upright stalks. When the sections of stem between stalks disintegrate, the separated individuals constitute a clone.

Some animals have tremendous powers of REGENERATION. If the body of certain starfishes is cut up into its five arms, each arm will regenerate a complete individual. Another type of asexual reproduction found in all animals, human beings included, is the formation of identical twins, triplets, and so on. Identical siblings constitute a clone. Some forms of PARTHENOGENESIS (development of an unfertilized egg into an adult organism) can also produce clones.

Tissue-level Reproductive Mode

The growth of a tumor in the body of an individual is, in effect, the formation of a clone of malignant cells. Another example of clone formation is the proliferation of a single B lymphocyte (a cell type of the immune system), producing identical specific monoclonal antibodies, against a particular antigen (see ANTIBODY).

ARTIFICIAL TECHNIQUES

In one method of artificial cloning used in PLANT BREEDING cells are cut from a plant and placed in a flask with a nutrient medium. The cells grow and divide, forming embryonic tissues that are transferred to soil, where they produce complete plants. GRAFTING is another method of cloning used in HORTICULTURE. Matching cuts are made in the stems of two plants, which are then fitted together so that their transport systems are in contact. The wounded area heals, and the two stems become a single physiological unit. All the McIntosh apple trees now in use and many other fruit varieties have been derived by grafting from single ancestral trees.

"Nuclear transplantation," in which nuclei from cells of one individual are transferred to unfertilized eggs whose nuclei have been removed, is one method of artificial cloning in animals. All the transplanted nuclei are generally identical, and therefore the resultant individuals constitute clones.

In recombinant-DNA formation genetic material from one organism is transferred to a host cell, which then divides many times, forming a clone, all of whose members contain the donor DNA and produce its encoded protein. Thus human genes have been transferred to bacteria whose subsequently formed clones have produced such medically important substances as INSULIN, INTERFERON, HEMOGLOBIN, and GROWTH HORMONE. Of potentially great importance is the recent cloning of human genes in such animals as sheep, goats, and cattle. In this procedure fertilized eggs are removed from normal female animals. The eggs are then injected with a human gene to which has been attached one of the animal's genes, which will activate the human gene only in the animal's mammary gland tissue. The eggs are then implanted in the uteri of "foster mothers."Only some of the animals that are born will carry the human gene in one of their chromosomes and will be able to transmit the gene to their progeny. An animal in which this process has been successfully completed is referred to as "transgenic" because it carries a gene from a foreign organism.

Thus far, the enzyme alpha-1-antitrypsin (AAT), which helps fight emphysema, has been successfully obtained from the milk of transgenic goats, and the gene for lactoferrin, which has both iron-transporting and bacteria-fighting capabilities, has been successfully transferred to a bull. It is expected that some of his female offspring will inherit the gene and these cows will produce lactoferrin in their milk.

Controversy

Great concern has been voiced over the use of GENETIC ENGINEERING for humans and animals. One concern is that transgenic animals carry pathogens of their own that may be transferred to humans with unknown consequences. The 1993 cloning of nonviable human embryos has raised considerable ethical questions about the uses of this technology. The regulatory agencies must establish ground rules for the use of these technologies and products.