Genetically modified (GM) foods are created using biotechnology to change their genetic material. A variety of techniques is used to introduce the desired genes or ‘inactivate’ unwanted ones. Techniques include benign bacterial or viral infection (bacterial carriers), gene splicing, gene ‘silencing’, biolistics, calcium phosphate precipitation and electroporation.
Genetically modified (GM) foods are created using biotechnology. The genetic material of plants or animals can be changed (modified) through a variety of techniques. Food may be genetically modified to increase its shelf life, make it resistant to pesticides and insecticides, or improve the crop’s nutritional yield.
How genetic modification works
Genes are the blueprints for our bodies and control factors such as growth and development. Within almost every cell of the body, genes are beaded along tightly bundled strands of deoxyribonucleic acid (DNA) called chromosomes. These are encased by a membrane (the nuclear membrane) to produce a nucleus.
Genes use chemical messages to instruct the cell to perform its functions by making proteins or enzymes. Traditional methods of genetic modification have been used for centuries, but this process involves randomly mixing genes and can only occur in related organisms. Today’s techniques of genetic modification are not random. Instead, a specific gene with a desirable trait is inserted into a new plant variety.
Current techniques also allow for genes to be transferred between unrelated plants and other organisms. By introducing a foreign gene, scientists prompt the altered cell to make new proteins or enzymes so that the cell performs new functions. For example, the gene that helps a cold-water fish survive low temperatures can be inserted into a strawberry to make it frost-resistant.
Genes may be taken from an animal, plant or micro-organism. If the genes are inserted into another species, the resulting genetically-modified organism (GMO) is referred to as ‘transgenic’.
Techniques of genetic modification
The GM techniques currently used to transfer foreign cells into animals and plants include:
- Bacterial carriers
- Gene silencing
- Gene splicing
- Viral carriers.
The bacterium Agrobacterium can infect plants, which makes it a suitable carrier for delivering DNA to a new organism.
The bacterium is prepared in a special solution to make its cell walls more porous. The selected gene is inserted into an extrachromosomal DNA molecule (called a plasmid) and dropped into the solution. The solution is heated, which allows the plasmid to enter the bacterium and express the new gene.
The genetically altered bacterium (or recombinant) is allowed to recover (is ‘rested’) and grow and, depending on the plasmid, make extra copies of the new gene. The bacterium is then allowed to infect the target plant cells so it can deliver the plasmid and the new gene into the cells to be transformed.
The selected DNA is attached to microscopic particles of gold or tungsten. Like firing a gun, these DNA-laden metal particles are shot into the target cells using pressurised gas. The microscopic particles release the DNA inside the plant cells.
The prepared target cells are immersed in a special solution with the selected DNA. A short but intense electric shock is then passed through the solution. The result is small pores or tears in the cell walls, which allow the new genetic material access to the nuclei.
The damaged cells are placed into another solution and encouraged to repair their breached walls, locking the ‘donor’ DNA inside the cell. The selected DNA is incorporated as a plasmid into the host, which is then incorporated into the plant DNA to provide the host with a new gene.
This technique is designed to eliminate an unwanted characteristic from an organism. The gene responsible for the organism’s undesirable trait is identified and ‘switched off’.
One method of ‘silencing’ a particular gene is to attach a second copy of the gene the wrong way around. This technique is used to prevent plants like peanuts and wheat from producing the proteins (allergens) commonly responsible for human allergies. Another approach is to insert foreign DNA within a gene to inactivate it.
The concept of gene splicing involves cutting a gene from one organism and inserting it into the DNA of another organism so the desired characteristic can be transferred from one organism to another. Bacteria contain restriction enzymes that form part of their defence against invasion by another organism or bacteriophage (a bacterial virus). The restriction enzymes attack the foreign DNA by cutting it into precise sections and preventing it from being inserted into the bacterium’s chromosome.
Different bacteria produce different restriction enzymes that cut any DNA at different places, making the DNA ‘sticky’ in some cases, which means they can be ‘pasted’ directly onto the target organism’s prepared DNA.
Using these restriction enzymes from bacteria, molecular biologists can genetically engineer the DNA for insertion into target (host) cells to modify gene traits. The molecular biologist then uses another enzyme (DNA ligase) to fuse the new gene sequences into the chromosome.
Alternatively, instead of pasting, the new gene may be inserted into a plasmid that invades the target cell and delivers the gene. An example of this is the invasion of plant cells by Agrobacterium tumefaciens.
The selected DNA is injected into a fertilised ovum (female egg cell) through an extremely slender device called a glass capillary tube. The genetically modified egg is then transplanted into the prepared uterus of a receptive female and allowed to grow to term.
This method ensures that almost every cell in the developing organism’s body contains the new DNA, but not all the offspring will carry the transgene (and be deemed ‘transgenic’).
Some viruses can invade target cells, but not cause cell damage or death. The selected DNA is added to the genetic makeup of the carrier virus and the virus is allowed to infect the target. As the virus invades cells and replicates, the selected DNA is added to the target cells.
Examples of genetically modified foods
Some current examples of GM foods include:
- Crops genetically engineered to be resistant to particular insect pests. For example, toxin genes (Bt toxin) from a bacterium found in soil (Bacillus thuringiensis) are inserted into plant crop DNA so that the plants produce toxins specifically deadly to the larvae of their pest insects. This makes the plants insect resistant (IR).
- Soybeans have been genetically modified to tolerate high levels of weed-killing herbicides that would normally kill them. This makes them herbicide tolerant (HT).
- Plants are genetically modified to ensure longer shelf life or greater resistance to frost.
- Farm animals (such as pigs, cows and chickens) are genetically modified for faster growth rates, leaner muscle-to-fat ratios, better resistance to disease or the ability to produce higher levels of omega-3s in their meat.
- Plants are modified to yield higher protein or nutrient levels or to produce healthier oils containing functional food components such as omega-3 fatty acids.
- GM cows can produce milk that contains higher levels of bioactive milk proteins or human blood-clotting components or a human breastmilk component.
Safety assessment and labelling of genetically modified foods
In Australia, foods or ingredients that have been genetically modified (GM foods) must be subjected to a pre-market safety assessment by the government food regulator – Food Standards Australia New Zealand (FSANZ). Apart from certain exemptions (such as highly refined foods in which the modified DNA is no longer present, or takeaway or restaurant foods), GM foods offered for sale must have their GM status identified on the food label.
Where to get help
- Food Standards Australia New Zealand Tel. (02) 6271 2222
- TechNyou (formerly the Gene and Nanotechnology Information Service) Tel. 1800 631 276
- Office of the Gene Technology Regulator Tel. 1800 181 030
Things to remember
- By introducing a foreign gene, scientists prompt the altered organism to make new proteins or enzymes so that the cell performs new functions. The altered organism is said to be genetically modified (GM).
- Some of the many techniques used to genetically modify foods include introducing the desired DNA via benign bacterial or viral infection.
- If the genes are inserted into another species, the resulting organism is referred to as a transgenic organism. Examples include transgenic cows, pigs and strawberries.
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Last reviewed: July 2011
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