Transgenic plants and animals result from genetic
engineering experiments in which genetic material is moved from one organism to
another, so that the latter will exhibit a characteristic. Business corporations,
scientists, and farmers hope that transgenic techniques will allow more
cost-effective and precise plants and animals with desirable characteristics
that are not available using up to date breeding technology. Transgenic
techniques allow genetic material to be transferred between completely
unrelated organisms.
In order for a
transgenic technique to work, the genetic engineer must first construct a
transgene, which is the gene to be introduced plus a control sequence. When
making a transgene, scientists usually substitute the original promoter
sequence with one that will be active in the correct tissues of the recipient
plant or animal.
The creation of
transgenic animals is one of the most dramatic advances derived from
recombinant DNA technology. A transgenic animal results from insertion of a
foreign gene into an embryo. The foreign gene becomes a permanent part of the
host animals’ genetic material. As the embryo develops, the foreign gene may be
present in many cells of the body, including the germ cells of the testis or
the ovary. If the transgenic animal is fertile, the inserted foreign gene
(transgene) will be inherited by future progeny. Thus, a transgenic animal,
once created, can persist into future generations. Transgenic animals are
different from animals in which foreign cells or foreign organs have been
engrafted. The progeny of engrafted animals do not inherit the experimental
change. The progeny of transgenic animals do.
The techniques
for creating a transgenic animal include the following: 1) picking a foreign
gene, 2) placing the foreign gene in a suitable form called a “construct” which
guides the insertion of the foreign gene into the animal genome and encourages
its expression, and 3) injecting the construct into a single fertilized egg or
at the very early embryo stage of the host animal. Much genetic engineering
goes into the choice of a foreign gene and building a construct. The construct
must have promotes to turn on foreign gene expression at its new site within
the host animal genome. By choosing a particular promoter and splicing it in
front of the foreign gene, we can encourage expression of our transgene within
a specific tissue.
One of the most
important applications of transgenic animals is the development of new animal
models of human disease. Transgenic animals can serve as models for many
malignant tumors. Although mice have been the most frequent hosts for
transgenic modification, other domestic animals have also been used. One idea
has been to create transgenic cows which secrete important pharmaceutical
substances in their milk. “Other attempts are being made to express human interferon
in the milk of sheep”.
A transgenic crop plant contains a gene or genes which have
been artificially inserted instead of the plant acquiring them through
pollination. The inserted gene sequence (known as the transgene) may come from
another unrelated plant, or from a completely different species: transgenic Bt
corn, for example, which produces its own insecticide, contains a gene from a
bacterium. Plants containing transgenes are often called genetically modified
or GM crops although in reality all crops have been genetically modified from
their original wild state by domestication, selection and controlled breeding
over long periods of time.
A plant breeder tries to assemble a combination of genes in a
crop plant which will make it as useful and productive as possible.
Depending on where and for what purpose the plant is grown,
desirable genes may provide features such as higher yield or improved quality,
pest or disease resistance, or tolerance to heat, cold and drought.
Combining the best genes in one plant is a long and difficult
process, especially as traditional plant has been limited to artificially
crossing plants within the same species or with closely related species to
bring different genes together.
For example, a gene for protein in soybean could not be
transferred to a completely different crop such as corn using traditional
techniques.
Transgenic technology enables plant breeders to bring
together in one plant useful genes from a wide range of living sources, not just from
within the crop species or from closely related plants.
This technology provides the means for identifying and
isolating genes controlling specific characteristics in one kind of organism,
and for moving copies of those genes into another quite different organism,
which will then also have those characteristics.
This powerful tool enables plant breeders to do what they
have always done — generate more useful
and productive crop varieties containing new combinations of genes — but it expands the possibilities beyond the
limitations imposed by traditional cross-pollination and selection techniques.
Overall, the use of transgenic technology has many advantages
over traditional methods. Transgenic breeding is said to be more specific,
faster, and less costly. Right now research is limited traits involving one or
a few genes. Before scientists can manipulate complex traits, there is going to
be the need for many years of research.