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Genetic Engineering
Introduction to Genetic Engineering
Genetic Engineering, the process of extracting DNA (deoxyribonucleic acid, which makes up the genes of all living things)
from one organism and combining it with the DNA of another organism, thus introducing new hereditary traits into the recipient
organism. The nature and characteristics of every living creature is determined by the special combinations of genes carried by
its cells. The slightest alteration in these combinations can bring about significant changes in an organism and also its progeny.
The science of devising techniques of modifying or controlling genes and genetic combinations is referred to as genetic
engineering. It was practiced in one form or another in the past by farmers and agriculturists trying to create economically
viable species of plants and animals through various breeding techniques Genetic engineering, as a science, was developed in
the mid-1970's primarily to create new strains of microorganisms that produce certain chemicals useful in manufacturing or as
drugs. Genetic engineering is now also applied to improving plants and creating transgenic animals (animals containing foreign
genetic material).
Some persons oppose genetic engineering on religious, ethical, or social grounds. Among the religious questions is whether
humans have the right to transfer traits from one organism to another. A social concern is the possibility of creating harmful
organisms that, if accidentally released into the environment, could cause epidemics.The creation of human clones, for example,
is facing serious opposition especially on moral grounds. Organizations, such as the National Institutes of Health (NIH), are
seeking to control the harmful effects of genetic engineering by imposing guidelines and safety measures for genetic
experimentation. Treatment of hereditary defects through gene transplantation and controlled interchange of genes between
specified species was approved in 1985 and 1987 respectively by the NIH and the National Academy of Sciences. The USDA
has framed regulations for the genetic alteration of plants by plant breeders.
The U.S. Supreme Court ruled in 1980 that genetically engineered microorganisms could be patented. In 1988 the U.S. Patent
and Trademark Office issued its first patent for a higher form of life, a transgenic mouse that is highly susceptible to certain
cancers that appear frequently in humans. This mouse is used in cancer research.
Techniques
Living cells contain minute threadlike structures called chromosomes which contain deoxyribonucleic acid or DNA, shaped like
double helix. The base or rungs of this twisted ladder-like structure constitute the genes which store information determining the
characteristics of an organism. The DNA molecule lends itself for easy replication as enzymes can split up the base sequence of
its double helix thus producing identical pairs. Genetic engineering is performed by technicians using high-powered
microscopes and microsurgical instruments. Genetically engineered microorganisms are created using a technique called gene
splicing, or gene cloning. In this technique, segments of DNA are cut out of a cell (called a donor cell) from an organism and
inserted into the DNA of a vector. A vector is usually a plasmid, a circular structure extracted from a bacterium and containing
some of that bacterium's DNA; in some cases, the vector is a modified virus. The vector, with the combined DNA, is then
inserted into a cell (called a host cell) from a species different from that of the donor cell. Once in the host cell, the DNA makes
exact genetic copies, or clones, of itself. Sometimes, special enzymes called restriction enzymes are used to isolate gene-sized
DNA fragments which break the given molecule by creating a cleavage at particular points on the base sequence of its DNA.
With the help of the enzyme ligase the fragments are then joined or spliced with other DNA structures thus producing
recombinant DNA molecules. When combined with other cells these recombinant DNA molecules transform the latter and lead
to the creation of colonies made up of millions of cells with newly added genetic information. This process leads to the creation
of clones or groups of genetically identical cells.
Producing Chemicals
The host is usually a bacterium; yeast cells or cells from certain other organisms are sometimes used instead. The donor may be
virtually any living organism. The host, like other kinds of organisms, normally produces various chemicals that are used in
carrying out its life processes. Scientists use hosts that reproduce rapidly (by fission) so that large quantities of identical
chemicals can be produced in a short period of time. The type of chemicals an organism produces is determined by its DNA;
modifying the DNA, as is done by gene splicing, modifies the type of chemicals produced. In this way, a chemical ordinarily
produced by, for example, a gland can be produced in large quantities by bacteria.
The bacterium most commonly used as the host is Escherichia coli. E. coli is also the usual source of the plasmids used as
vectors. The plasmids and donor cells are placed in separate test tubes. A restriction enzyme, a chemical extracted from certain
strains of bacteria, is then added to the test tubes. The restriction enzyme chemically cuts specific hydrogen bonds in the DNA
of the plasmids and the donor cells, producing short, matching segments of DNA. There are numerous restriction enzymes and
each cuts the DNA molecule at specific chemical bonds; the type chosen is the one that will cut out from the donor cell a
segment of DNA that will produce the desired trait. Ultrasound, in a technique called mechanical shearing, can also be used to
cut DNA molecules.
Once separated from the rest of the DNA, the donor segment is inserted into a plasmid. The resulting hybrid plasmid contains a
combination of bacterial DNA and donor DNA, called recombinant DNA. In a process called transformation, the hybrid
plasmid is inserted into the host in a culture dish. Inside the host, the hybrid plasmid makes several clones of itself. When the
host reproduces, it creates daughter cells that are clones of itself. These cells manufacture the chemical coded for by the hybrid
plasmid DNA.
Improving Plants
Genetic engineering is used to produce genetically modified (GM) crops with desirable traits in crops. In one technique, DNA
with the desirable trait is spliced into a plasmid obtained from a bacterium that causes galls (growths) on plants. Bacteria with
the hybrid plasmid are then allowed to infect a crop plant and produce galls. The plant cells in the galls receive the desirable
trait from the hybrid plasmid. These cells are removed and placed in a culture dish, where they develop into new plants with the
desired traits. Among the traits introduced into plants in this way is resistance to certain insect pests.
Transgenic Animals
In a process known as transgenesis, an embryo (an unborn organism in its early stages of development) can be altered to carry
specific traits. The most common method of accomplishing this is through the addition of foreign genes, which may come from
any animal possessing the desired trait. Two main methods are used.
The most common method is to inject a desired gene into a zygote (fertilized egg). Under the right conditions, the new gene will
join one of the zygote's strands of genes. The zygote divides, forming an embryo, and then the cells of the embryo repeatedly
divide; each new cell contains a copy of the new gene.
The second method is to incorporate the new gene into a virus that has been modified so it cannot reproduce itself after entering
a cell. The virus delivers the gene to the cell's nucleus.
The offspring of a transgenic animal will normally inherit whatever alteration was made to its parent. One of the first transgenic
animals, created in 1982, was produced when a gene for a growth hormone was injected into a mouse zygote. The resulting
mouse grew to be twice as large as a typical mouse.
Uses
Genetics or genetic engineering began as a science in the early 1900's with the scientific experimentations of the Austrian monk
Gregor Mendel. American geneticists later developed techniques of isolating and altering genes to produce insulin and
interferon in the early 1970's. The Food and Drug Administration (FDA) approved the use of bacterially produced insulin as the
first recombinant DNA drug in 1982. Subsequently, scientists successfully produced mice twice their normal size, made tomato
plants resistant to caterpillars, transferred genes from one species of fruit flies to another, and interchanged genes between
various plant species with remarkable results. A variety of corn with superior nutritional value was patented in 1986 by the U.S.
Patent and Trademark Office, and in 1988 a similar patent was issued regarding genetically engineered mice used in cancer
treatment. The science of genetic engineering has brought immense benefit to industry, agriculture, and medicinGenetic
engineering is used in medicine to manufacture certain drugs and hormones. The first products thus produced were interferon, a
substance used to kill certain viruses and cancer cells, and human insulin, a hormone used to control diabetes. Many human
illnesses are caused by defective genes which get carried over through generations. Diabetes, for example, is caused by the
failure of genes in the pancreas to make insulin. The process of gene splicing can help to produce large amounts of insulin from
cells of the Escherichia coli bacteria, which is then given to needy patients. Urokinase and tissue plasminogen activator,
substances that dissolve blood clots, are produced by genetically engineered bacteria and mice. Human growth hormone, which
corrects a form of dwarfism and helps heal wounds, is produced by genetically engineered bacteria and yeasts. In 1986 the first
genetically engineered vaccines were approved for use against pseudorabies, a fatal herpes infection of pigs, and hepatitis B, a
virus infection of humans. By isolating and studying the genetic structure of cells from unborn babies, scientists can detect
diseases which the babies may later have. Sometimes replacement of disease-causing genes with normal ones can help to cure
problems like cystic fibrosis, cancer, and various liver diseases.
Transgenic mice with certain human genes are commonly used in medical research. Other transgenic animals have been
produced on an experimental basis for agricultural purposes; among these animals are hogs that produce leaner meat and
chickens that are immune to certain diseases. Genetically engineered bacteria help to produce large amounts of a growth
hormone, which increases milk production in cows. The first clone of a sheep named Dolly was created in Scotland in 1996 by
a team of scientists led by British biologist Ian Wilmut. This opened up possibilities of obtaining superior quality milk, meat,
and wool by producing better livestock. The very next year, American and Japanese scientists working in Hawaii successfully
produced clones of mice.
Genetically engineered crop plants have been developed to possess such traits as resistance to disease, drought, insects, and
frost. In 1994, a kind of tomato designed to resist spoilage became the first genetically engineered food marketed in the United
States. During the late 1990's, the number of food products derived from genetically modified crop plants increased. Public
concern over the safety of these products also increased. These concerns were spurred, in part, by such incidents as one in 2000
in which a type of genetically engineered corn intended only for use as animal feed was discovered in food products sold for
human consumption. The U.S. Department of Agriculture supervises the conditions under which plants are genetically
engineered. Many genetically modified plants are used to produce antibodies and some are even employed in the production of
biodegradable plastic.
Genetically engineered bacteria and viruses are used to manufacture a variety of commercial products including food. These
include aspartame (a sugar substitute), a form of rennin (an ingredient in cheese), tryptophan (a feed supplement for livestock),
and bovine somatotropin, or bST (a hormone used to increase the milk production of cows). Genetically engineered
microorganisms can also be used to break down garbage and toxic waste thus helping to reduce pollution. Enzymes genetically
designed to work under harsh conditions like extreme heat or dehydration help in the manufacture of antibiotics and other
specialized products.
The manipulation of human genes in order to treat or cure genetic disorders is a procedure known as gene therapy. In gene
therapy, plasmids or modified viruses are used to deliver genetic material into cells of particular parts of the body; the genetic
material causes the cells to produce substances that help correct the disorder. In the ex vivo method of gene therapy, the cells
are obtained from the body, genetically altered in the laboratory, and then returned to the body. In the in vivo method, the cells
are genetically altered inside the body.
Since 1990, gene therapy has been used experimentally to treat many disorders, including cystic fibrosis, high blood cholesterol,
cancer, and AIDS. The use of gene therapy to treat patients presents a number of problems. One problem is that it is difficult to
insert the genetic material into enough cells to obtain the desired effect. Another problem is that the patient's immune system
sometimes destroys the genetically altered cells.
After the death of a gene therapy research subject in 1999, and the discovery that some researchers were violating federal
guidelines by not reporting side effects, federal health officials instituted closer monitoring of human gene therapy trials.