Genetically Modified Organism
Genetically Modified Organism
Genetically modified organism
A genetically modified organism, or GMO, is an organism whose genetic structure has been altered by incorporating a single gene or multiple genes—from another organism or species—that adds, removes, or modifies a trait in the organism by a technique called gene splicing. An organism that has been genetically modified—or engineered—to contain a gene from another species is also called a transgenic organism (because the gene has been transferred) or a living modified organism (LMO). Most often the transferred gene allows an organism—such as a bacterium, fungus, virus , plant, insect, fish, or mammal—to express a trait that enhances its desirability to producers or consumers of the final product.
Plants and livestock have been bred for desired qualities (selective breeding) for thousands of years—long before people knew anything about the science of genetics. As technology advanced, however, so did the means by which people could select desired traits. Modern biotechnology represents a significant step in the history of genetic modification.
Until the final decades of the twentieth century, breeding techniques were limited to the transfer of desired traits within the same or closely related species. Genetically modified organisms, however, may contain traits transferred from completely dissimilar species. For example, before modern biotechnology, apple breeders could only cross-breed apples with apples or other closely related species. So, if a breeder wanted to make a certain tasty apple variety that was more tolerant to the cold, a cold-tolerant apple variety had to be hybridized with a tasty variety. This process usually involved significant trial and error because there was little assurance the cold-tolerance ability would be transferred in any individual attempt to hybridize the two varieties.
Development of modern biotechnology
The characteristics of all organisms are determined by genes—the basic units of heredity. A gene—a segment of deoxyribonucleic acid (DNA)—is capable of replication and mutation , occupying a fixed position on a chromosome (a group of several thousand genes; humans are defined by 22 pairs of chromosomes plus X and Y), and is passed on from parents to offspring during reproduction. A gene determines the structure of a protein or a ribonucleic acid (RNA) molecule. Found in all cells, DNA carries the genetic instructions for creating proteins; RNA decodes those instructions. Proteins perform diverse biological functions in the body, from helping muscles contract, to enabling blood to clot, to allowing biochemical reactions to proceed quickly enough to sustain life. By modifying a protein, particular phenotypic (physical) or physiologic changes—such as the color of a rose or the ability to bioluminesce (glow like a firefly)—are created.
A fundamental aspect of modern biotechnology is the belief that the essential genetic elements of all life are the same. Since the 1950s, molecular biologists have known that the DNA in every organism is made up of pairs of four nitrogen-containing bases, or building blocks: adenine (A), thymine (T), cytosine (C), and guanine (G).
In 1953, scientists James Watson and Francis Crick discovered that DNA is constructed in a double helix pattern—sort of a twisted ladder. Although A always pairs with T, and G with C, there is significant variety in how the pairs stack. The variable sequence of these DNA base pairs constitutes, in effect, the variety of life. So even all organisms are made from the same basic building blocks, their differences are a result of varying DNA sequences. A principle of modern molecular biology and biotechnology is that because these genetic building blocks are the same for all species, DNA can be extracted and inserted across species.
Genetic engineering techniques
The tools of modern biotechnology allow the transfer of specific genes, hence specific traits, to occur with more precision than ever before. The individual gene conveying a trait can be identified, isolated, replicated, and the inserted into another organism. This process is called genetic engineering , or recombinant DNA (rDNA) technology—that is, recombining DNA from multiple organisms.
Through such engineering, the apple breeder who wants the tasty apple variety to be cold-tolerant as well has the potential to find a gene that conveys cold tolerance and insert that gene directly into the tasty variety. Although there is still some trial and error in this process, overall, there is greater precision in the ability to move genes from one organism to another. The gene that conveys a cold-tolerance ability does not need to come from another apple variety; it may come from any other organism. A cold-tolerant fish, for example, might have a suitable gene.
There are multiple methods by which genetic material may be transferred from one organism to another. Before a gene is moved between organisms, however, it is necessary to identify the individual gene that confers the desired trait. This stage is often quite time-consuming and difficult because often it is not clear what gene is needed or where to find it. Finding the gene entails using or creating a library of the species' DNA, specifying the amino acid sequence of the desired protein, and then devising a probe (any biochemical agent labeled or tagged in some way so that it can be used to identify or isolate a gene, RNA, or protein) for that sequence. As of 2002, isolating the desired gene is one of the most limiting aspects to creating a GMO.
Once the desired gene has been identified it must be extracted from the organism, which is usually done with a restriction endonuclease (an enzyme ). Restriction endonucleases recognize particular base sequences in a DNA molecule and cut and isolate these sequences in a predictable and consistent manner. Once isolated, the gene must be replicated to generate sufficient usable material, as more than one copy of the gene is needed for the next steps in the engineering process.
One common method of gene replication is called polymerase chain reaction (PCR). Through the PCR method, the strands of the DNA are broken apart—in effect, the ladder is divided down the middle—and then exact copies of the opposite sides of the ladder are produced, creating thousands or millions of copies of the complete gene.
After replication, the gene must be inserted into the new organism via a vector (an agent that transfers material—typically DNA—from one host to another). A common vector is Agrobacterium tumefaciens, a bacterium that normally inserts itself into plants, causing a tumor. By genetically engineering A. tumefaciens, however, the bacterium can be used to insert the desired gene into a plant, replacing its tumor-causing genetic material.
Another vector, called ballistics, involves coating a microprojectile—usually a heavy metal such as tungsten—with the desired gene, then literally shooting the microprojectile into material from the new host organism. The micro-projectile breaks apart the DNA of the host organism. Then when the DNA reassembles, some of the new, desired genetic material is inserted. Once the DNA has been inserted, the host organism can be grown, raised, or produced normally and tests can be performed to observer whether the desired trait manifests.
Varieties of GMOs
In 2000, of the total land planted with GMO crops worldwide, the United States occupied 68%, Argentina 23%, and China 1%. The United States produces many types of GMOs for commercial and research purposes.
A common type of GMO is the modified agricultural seed. Corn, soybeans, and cotton are a few examples of staple agricultural GMO products grown in the United States. Genetic modifications to these products may, for example, alter a crop's nutritional content, storage ability, or taste.
With the advent of genetic engineering, for example, common hybrid crops such as the tomato were launched into a new era. The first food produced from gene splicing and evaluated by the Food and Drug Administration (FDA) was the Flavr Savr Tomato in 1994. Tomatoes usually get softer as they ripen because of a protein in the tomato that breaks down the cell walls of the tomato. Because it is difficult to ship a quality ripe tomato across the country before the tomato spoils, tomatoes are usually shipped unripened. Engineers of the Flavr Savr Tomato spliced a gene into its deoxyribonucleic acid (DNA) to prevent the breakdown of the tomato's cell walls. The result of adding the new gene was a firm ripe tomato that was more desirable to consumers than the tasteless variety typically found on store shelves, particularly in the winter months.
Genetic modifications may also confer to a species an ability to produce its own pesticide biologically, thereby potentially reducing or even eliminating the need to apply external pesticides. For example, Bacillus thuringiensis (Bt) is a soil bacterium that produces toxins against insects (mainly in the genera Lepidoptera, Diptera, and Coleoptera ).- When they are genetically modified to carry genetic material from the Bt bacterium, plants such as soybeans will able to produce their own Bt toxin and be resistant to insects such as the cornstalk borer and velvetbean catepillar. Researchers at the Monsanto chemical company estimate that Bt soybeans will be commercially available by about 2006.
By means of genetic engineering, a gene from a soil bacterium called Agrobacterium sp confers glyphosate resistance to a plant. Glyphosate (brand names include Roundup, Rodeo, and Accord) is a broad-spectrum herbicide (kills all green plants). As of 2002, Monsanto was the sole developer of all glyphosate resistant crops on the market. These crops (often called "Roundup Ready") include corn, soybeans, cotton, and canola.
Genetically engineered pharmaceuticals are also useful GMO products. One of the first GMOs was a bacterium with a human gene inserted into its genetic code to produce a very high quality human insulin for diabetes. Vaccines against diseases such as meningitis or hepatitis B, for example, are produced by genetically engineered yeast or bacteria. Other pharmaceuticals produced by using GMO microbes include interferon for cancer , erythropoetin for anemia , growth hormone for the treatment of dwarfism, tissue plasminogen activator for heart attack victims. Through genetic engineering, transgenic plants are likely to become a commercially viable source of pharmaceuticals.
Benefits and risks
Although the technological advances that allowed the creation of GMOs are impressive, as with any new technology, the benefits must be weighed against the risks. Useful pharmaceutical products have been created as a result of genetic engineering. Also, GMOs have produced and new and improved agricultural products that are resistant to crop pests, thus improving production and reducing chemical pesticide usage. These developments have had a major impact on food quality and nutrition.
The possibility that biotech crops could make a substantial contribution to providing sufficient food for an expanding world is also a reason given for engaging in the research that underlies their development. However, the debate over GMOs continues among scientists and between consumers and modern agricultural producers throughout the world regarding issues such as regulation, labelling, human health risk, and environmental impact.
In the United States, for example, there has been much controversy over the health risks of canola oil. Canola oil is genetically engineered rapeseed oil or "LEAR" oil (low erucic acid rape), a semi-drying industrial oil used as a lubricant, fuel, soap, and synthetic rubber base, and as an illuminant to give color pages in magazines their slick look. It was first developed in Canada (thus the name "canola oil").
Canola oil is derived from the mustard family and is considered a toxic and poisonous weed. In 1998, the EPA classified canola oil as a biopesticide with "low chronic toxicities," yet placed it on the "Generally Considered Safe" list of foods. Proponents, who tout the oil's health benefits, claim that due to genetic engineering and irradiation, it is completely safe, pointing to its unsaturated structure and digestibility.
Widely used as a cooking oil, and because it is so inexpensive, it is used in thousands of processed foods in the United States and North America. Thus, millions of people have been exposed—most of them unknowingly—to genetically engineered foods. However, there has been little research on the potential adverse effects of these products on humans.
When processed, canola oil becomes rancid very easily. It has been shown to cause health problems such as lung cancer and has been associated with loss of vision, disruption of the central nervous system, respiratory illness, anemia, constipation, increased incidence of heart disease and cancer, low birth weight in infants, and irritability. It has a tendency to inhibit proper metabolism of foods and curbs normal enzyme function. Generally, rapeseed has a cumulative effect, often taking nearly ten years for symptoms to manifest.
The dangers of introducing genes that may cause undesirable effects in the environment is also a concern among many. For example, when farmers spray an herbicide to remove weeds growing among crops, the sprayed chemical often damages the crop plants. If the crop is genetically engineered to be resistant to the chemical, the weeds are killed, but the crop plants remain undamaged.
Although this situation appears to be beneficial, it is likely to lead to greater use of the particular herbicide, which would have several negative effects: the crop is likely to contain greater herbicide residues; and the increased spraying contaminates the rest of the environment. Although not all herbicides are dangerous, the safer choice seems to minimize rather than maximize their use. Also, if genes added to produce pesticide resistance in crop plants jumped to a weed species, then weeds would thrive and be difficult to control.
Thus, as debate over GMOs rages, it is important to note that a wide variety of ecological and human health concerns exist side by side with the new advances made possible by genetic engineering.
[Paul R. Phifer Ph.D. ]
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Human Genome Project Information, Oak Ridge National Laboratory, 1060 Commerce Park MS 6480, Oak Ridge, TN USA 37830 (865) 576-6669, Fax: (865) 574-9888, Email: [email protected], <http://www.ornl.gov/hgmis>