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).
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.
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.