GENETIC ENGINEERING (Part 2 of 2)   Leave a comment

Genetic engineering had its origins during the late 1960s and early 1970s in experiments with bacteria, viruses, and small, free-floating rings of deoxyribonucleic acid (DNA) called plasmids, found in bacteria. While investigating how these viruses and plasmids move from cell to cell, recombine, and reproduce themselves, scientists discovered that bacteria make enzymes, called restriction enzymes, that cut DNA chains at specific sites. The 1978 Nobel prize for physiology or medicine was shared by the discoverer of restriction enzymes, Hamilton O. Smith, and the first people to use these tools to analyse the genetic material of a virus, Daniel Nathans and Werner Arber.

The action of restriction enzymes is the crux of genetic engineering. DNA is made up of two long intertwined helices. The backbone of each helix is constructed from a chain of organic compounds called nucleotides. DNA is the carrier of genetic information; it achieves its effect by directing the synthesis of proteins. DNA is composed of four different nucleotides, repeated in specific sequences that form the basis of heredity. Restriction enzymes recognize particular stretches of nucleotides arranged in a specific order and cut the DNA in those regions only. Each restriction enzyme recognizes a different nucleotide sequence. Thus, restriction enzymes form a molecular tool kit that allows scientists to cut the chromosome into various desired lengths, depending on how many different restriction enzymes are used. Each time a particular restriction enzyme or set of restriction enzymes is used, the DNA is cut into the same number of pieces of the same length and composition. At least 80 restriction enzymes are now known.

When restriction enzymes are used along with other enzymes that tie together loose ends of DNA, it becomes possible to remove a bit of DNA from one organism’s chromosome and to insert it into another organism’s chromosome. This allows scientists to produce new combinations of genes that may not exist in nature. For example, a human gene can be inserted into a bacterium or a bacterial gene into a plant.

So far, however, there are limits to this ability. Scientists are unable to start with only test tubes full of nucleotides to create a whole new organism. They must start with the complete genetic material of an already existing organism. Thus, genetic engineering allows the addition of only one or a small number of new characteristics to an organism that remains essentially the same. In addition, only characteristics that are determined by one or a few genes can be transferred. The technology of genetic engineering does not enable scientists to transfer behavioural traits, such as intelligence, that are a complex mixture of many genes and the effects of cultural conditioning.

In practice, genes to be inserted into bacteria are first recombined into a plasmid, which replicates and travels independently from its bacterial host. The modified plasmid is then inserted into another bacterium.

At the time that these methods became available, it was not known whether DNA could be replicated and expressed in a foreign organism. Experiments done in the mid-1970s with these techniques, however, showed clearly that a human gene could be reproduced by a bacterium along with its own genetic material and that the bacterium could make the protein coded for by the human gene. This opened the way to making large amounts of human hormones and other significant human substances in the laboratory, a much easier route than isolating these substances from blood or cadaver glands.

One of the first areas in which genetic engineering exerted an influence was the medical field. Genetic engineering techniques allowed the production of large amounts of many medically useful substances, particularly the class of biological compounds called peptides. Peptides are short proteins. Many of the most important chemical messengers in the body are peptides. These include hormones such as insulin, nervous system messengers such as the endorphins, and regulatory messengers from the hypothalamus that control production of hormones by the pituitary gland, the gonads, and the thyroid gland, among others.

Several biologically useful peptides were made and tested in clinical trials during the late 1970s and early 1980s. The first genetically engineered product to be approved for human use was human insulin made in bacteria. Insertion of the human insulin gene into bacteria was accomplished by the pioneer genetic engineering company Genentech. Testing, approval for medical use, and large-scale production of genetically engineered human insulin were carried out, and the first diabetic patient in the world was injected with human insulin made in bacteria in December 1980, making this the first genetically engineered product to enter medical practice. (Genetically engineered products are often identified by the prefix r, for “recombinant.” Thus, genetically engineered insulin is sometimes written, r-insulin.)

The interferons are another medically important group of peptides that became available in abundance only after the development of genetic engineering techniques. Interferon was useful for treating viral infections, and there were strong indications that it might be effective against some cancers. Before the advent of genetic engineering techniques, it took laborious processing of thousands of units of human blood to obtain enough interferon to treat a few patients. And this interferon was not very pure. With the insertion of the interferon gene into bacteria, large amounts of very pure interferon became available. This supply allowed trials of interferon in the early 1980s against more than ten different cancers, including the particularly virulent form of Kaposi’s sarcoma often found in persons with acquired immunodeficiency syndrome (AIDS). The human body makes more than 50 different varieties of interferon. It is thought that some types of interferon may be more effective against cancer than others.

Other medically useful human peptides that have been made widely available because of genetic engineering are human growth hormone, which is used to treat persons with congenital dwarfism and was formerly obtained from cadaver pituitary glands, and tissue-type plasminogen activator (t-PA), which is a promising new treatment for persons who suffer a heart attack.

Vaccines

Genetic engineering techniques have also been investigated as a means to produce safer new vaccines. The first step is to identify the gene in a disease-causing virus that stimulates protective immunity. That gene is isolated and inserted into a harmless virus, such as vaccinia, the virus used to immunize against smallpox. The recombinant vaccinia virus is used as a vaccine, producing immunity without exposing people to the disease-causing virus. In the case of viruses about which little is known, such as the AIDS virus, this extra margin of safety is crucial.

Recombinant techniques may be useful in making vaccines against organisms for which no vaccines could be made by traditional methods. These include possible vaccines against the tropical parasites that cause schistosomiasis and malaria.

Diagnosis, Therapy, and Research

Genetic engineering is also being used in the prenatal diagnosis of inherited diseases. Restriction enzymes are used to cut apart the DNA of parents who may carry a gene for a congenital disorder, and the DNA pattern of cells from the fetus is compared. In many situations the disease status of the fetus can be determined. Currently this technique is applicable to thalassemias, Huntington’s disease, cystic fibrosis, and Duchenne’s muscular dystrophy.

A future medical use of genetic engineering is for gene therapy. Persons who are born with a congenital disorder resulting from a defective gene could have a sound gene inserted into their cells, preventing the manifestations of the disease. The era of gene therapy began on Sept. 14, 1990, when the first therapeutic, genetically engineered cells were infused into a 4-year-old girl with adenosine deaminase (ADA) deficiency, an inherited life-threatening immune deficiency. The infused cells were lymphocytes from the girl’s own blood, into which researchers had inserted copies of a missing gene that directs production of ADA. On Jan. 29, 1991, gene therapy was used for the first time to treat cancer, when two patients with advanced skin cancer were infused with their own white blood cells after the cells had been genetically altered to produce a tumour-killing protein. Many obstacles must be overcome to achieve the promise of gene therapy, but its value could be immense.

Ribozymes, RNA molecules that act like enzymes to cut and splice themselves.

Genetic engineering has allowed discoveries that could not have been made any other way. One of the most important is the discovery of oncogenes, specific genes that play an important part in causing some cancers. The identification and isolation of oncogenes depended on being able to cut cancer-causing DNA into manageable segments and finding the specific segments that were responsible for transforming normal cells into cancer cells. Discovery of ribozymes RNA molecules that act like enzymes to cut and splice themselves gave scientists hope for a new way of destroying the expression of unwanted genes. (RNA, along with DNA, is a carrier of genetic information.)

Agricultural advances are also expected from genetic engineering. Some of the earliest recombinant organisms made were a soil bacterium that was induced to make a toxin against a worm that destroys corn roots, a bacterium engineered to make potato and strawberry crops more frost-resistant, and a tobacco plant bearing a bacterial gene that protects against herbicides. Work on agricultural applications of genetic engineering proceeds slowly because recombinant organisms must be released outdoors in test fields to find whether they work. They cannot be tested only in a contained laboratory. Government regulatory agencies and ecological scientists are wary of the possible adverse consequences of releasing recombinant organisms, particularly fast-reproducing bacteria, into a field plot. No one knows how likely it is that such organisms may escape the field, grow in some situation in which they are not wanted, and cause unexpected effects.

National Institutes of Health, established 1887 at Marine Hospital, New York, N.Y.; present name from 1948.

Public debate about the safety of recombinant organisms began in the 1970s. Government bodies were set up to screen proposed experiments and institute safety guidelines. Chief among them was the Recombinant Advisory Committee of the National Institutes of Health. For many years the public was concerned about the safety of laboratory research with recombinant organisms. As research has continued and no safety hazards have become evident, public concern has abated somewhat.

Assisted by William A. Check.

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Posted 2012/02/25 by Stelios in Education

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