Genetically Modified Crops: How Do They Do It? - January 10, 2001
Jeff Schalau, County Director, Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County

Scientists and farmers have been genetically modifying food crops since the late 1800's through a process known as hybridization or selective plant breeding. This was the pre-biotechnology method of introducing genetic materials from one individual plant to another. Since the mid-1990s, scientists have been genetically modifying crop plants by inserting genes from different organisms (such as bacteria) with desirable traits into the crop plant.

If you follow the national and world news, this issue is undoubtedly familiar to you. Genetically Modified Organisms (GMOs) are a highly emotionalized topic. Production agriculture sees them as an opportunity for increased production while decreasing the uses of pesticides. Companies that have created them see them as good business. However, health-conscious consumers are highly skeptical of them for a variety of reasons. Why not take the time to become better educated on this topic so that each of us can make informed decisions about GMOs. Our first lesson will be: how do they do it?

Scientists and plant breeders first identify a gene of interest. For example, Bacillus thuringiensis otherwise known as Bt is a bacterium that contains a gene for a protein that is toxic to caterpillars and a few other larval stages of insects. Bt is available on the shelves of most nurseries as a "safe" pesticide for caterpillars but due to its fragile nature, it does not last long in the environment. Enter genetic scientists that have the identified the segment of DNA that codes for the specific protein in Bt that kills the caterpillars.

The scientists then isolate the DNA containing the desired gene and purify it. In a long segment of DNA, only a small segment contains the actual gene that must be extracted. The genes are removed from the long DNA strands by restriction enzymes that can located the beginning and end of the gene, tightly bind to it, and snip out the desired gene. The restriction enzymes locate the beginning and end of the gene by finding a specific key sequence for that gene. The small quantity of snipped out gene segments are isolated and removed.

In order to increase the quantity of desired genes, they must be introduced into bacterial cell. Bacteria are used because they reproduce very rapidly. This is done by isolating plasmids from host bacterial cells. Plasmids are small, circular DNA molecules that are reproduced each time a bacterial cell divides. The gene segment is incorporated into the plasmid by using the same restriction enzymes that cut it out of its original host DNA. In order to do this, the plasmids must contain the same key sequence as the host.

The plasmid opens up and the desired segment of DNA inserts itself into the break making the plasmid a circle again. The altered plasmid now has the desired gene in it. Now, the scientists remove the altered plasmids and reintroduce them into live bacterial cells. These designer bacteria now produce the plasmid (and hence the desired gene) every time they reproduce through cell division. In a short time, there are thousands of copies of the designer bacteria. This methodology is used to create large quantities of gene products for the pharmaceutical industry.

These gene products can also be extracted again for insertion into plant cells. By selecting the proper genes, scientists can create crop plants that contain Bt and are therefore resistant to attack by selected insect pests. Other genes can make a plant disease resistant or even resistant to certain herbicides. This is where the public can become frightened of a "Frankenstein" plant.

One such frightening case was a study where GMO corn containing Bt was studied under laboratory conditions. The study showed that when pollen produced by the Bt corn was placed on milkweed foliage, it caused a high percentage of monarch butterfly caterpillar mortality. Recently, the EPA found that monarch caterpillars out in the field simply don't eat enough of the toxic pollen to significantly increase the mortality rate.

There are several other issues of concern surrounding GMO food issues: allergens, nutrition, labeling, and the list goes on. I'll try to cover some of these issues as the data emerges. Until then, keep an open mind and keep informed with sound scientific information.

The University of Arizona Cooperative Extension has publications and information on fruits and vegetables. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at and be sure to include your address and phone number. The Yavapai County Cooperative Extension web site is

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: March 15, 2001
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