by
Ann Taylor
Department of Chemistry
Wabash College
This case study discusses some of the issues associated with the use of genetically modified plants, including ecological risks, resistance, and allergenicity. This case was originally developed for use with a biochemistry course taught in the Chemistry Department at Wabash College. It would also be appropriate for use in a non-majors general, organic, or biochemistry course as well as a general biology course.
The case is presented in two editions. The General Edition is appropriate for non-majors general, organic, and biochemistry (GOB), general biology, or cellular biology courses, and emphasizes the basic biochemistry and scientific ethics issues associated with genetically modified foods. It could also be used at the beginning of a biochemistry course. The Advanced Edition probes the same issues discussed in the General Edition in further detail, and is appropriate for an upper level biochemistry, cell biology, or genetics class.
Both the General and Advanced Editions use the case study as a launching point to read and discuss primary and secondary literature articles. The General Edition set of papers and questions is appropriate for use with non-science majors. The articles require little, if any, prior experience with scientific literature and use very basic methods. All of the articles are available on the Internet (see references for links). The General Edition of the case is appropriate for liberal arts schools, as it touches on many of the ethical issues associated with science, including making decisions in the absence of complete information, balancing the needs of the few versus the many, and economics versus health issues. The Advanced Edition includes more advanced questions regarding the biochemistry of how genetically modified plants “work.”
| Edition | Pre- or co-requisite concepts |
| General | • Central dogma of molecular biology • Gel electrophoresis • Enzymes |
| Advanced | • Transcription and translation • Restriction enzymes • PCR • Protein structure analysis tools* • Enzyme kinetics and inhibition* |
*The depth of this prerequisite knowledge can be altered by changing the questions and/or selected articles.
After completing this case study, all students should:
Additionally, advanced students should:
I use this case study during the DNA unit at the end of the semester in conjunction with a laboratory experiment where we test corn meal samples for the presence of genetic modifications (submitted to Journal of Chemical Education, July 21, 2003). The pairing of the case study with a laboratory activity has worked very well, as it provides a context for the laboratory activity and allows students to productively use the “waiting” times in the laboratory. This approach is also an easy way to add a case to a course without having to take time out of the traditional lecture period.
I have also used the case at the beginning of a biochemistry course by using the general set of papers, then discussing the advanced papers as the various topics they deal with were covered in class (e.g., the glyphosate inhibition of EPSPS during enzyme kinetics—see sample small group exercise at the end of this section). Alternatively, one could return to the case several times throughout the semester, either using the full paper assignments or in-class exercises based on the papers.
When the case is initially handed out, the situation is role-played by several students who take the parts of the various characters in the case scenario, and then the questions at the end of the story (except for the flippant one) are discussed. Students can review what they already know about genetic modifications in answering the first question. The safety question can be followed up with questions such as, “What do we mean by safe? Safe for whom?” This will lead to the environmental issues discussed in the papers. The third question ties in the economic issues discussed in the USDA bulletin, as well as the long-term possibilities of resistance. The groups then work on understanding their assigned primary literature papers. During the discussion period, each group presents their results and then the entire group returns to the initial questions, as well as extensions such as:
The herbicide Round-Up, also known as glyphosate, works by inhibiting 5-enolpyruvylshikimate 3-phosphate synthase, or EPSPS for short. EPSPS catalyzes the following reaction:
Figures A and B display Lineweaver-Burke plots for inhibition of E. coli EPSP synthase by glyphosate. Experiments were performed in a 1-cm path length rectangular cell for variable phosphate (total volume = 1.0 mL) and a 10-cm path length cylindrical cell for variable EPSP (total volume = 4 mL). (Figures adapted from Sammons et al., 1995, copyright © 1995 American Chemical Society. Used with permission.)
The biggest challenge facing farmers is controlling weeds and diseases while maintaining crop yield and quality. It is important that these objectives are met cheaply, as the average acre yields 128 bushels of corn and the average price of corn has been $2.30/bushel over the last 10 years (USDA 2003). That $295/acre needs to pay for the cost of seed, equipment (a new combine costs $100,000), fertilizer, weed control, and the labor of the farmers (salary for the year!). Considering an acre is 4,840 square yards, the most practical methods for achieving good weed and disease control, crop yield, and quality are mass-methods. Historically, this has meant the widespread use of pesticides and herbicides to control unwanted insects and weeds. A new approach to these issues is through the production of genetically modified plants. In fact, 75% of soybeans and 34% of corn planted in 2002 were genetically modified (USDA 2002).
There are two common types of genetically modified crops: insect resistant (Bt) and herbicide resistant (Round-Up Ready). “Bt” is short for Bacillus thuringiensis, a soil bacterium whose spores contain a protein (Cry), which is broken down in an insect’s gut to release a toxin, known as a delta-endotoxin. This toxin binds to and creates pores in the intestinal lining, resulting in an ion imbalance, paralysis and after a few days, death of the insect (Colordao State University n.d.). Several different Cry genes, also known as “Bt genes,” have been identified. They are effective against different orders of insects and affect the insect gut in slightly different ways. The most common herbicide resistance products are the “Round-Up Ready” (RR) plants produced by Monsanto. Roundup® is the commercial name for glyphosate, an inhibitor of an enzyme required for amino acid synthesis in plants. RR plants contain the bacterial version of this enzyme, which is not affected by glyphosate.
To make a genetically modified plant, several components are needed in addition to the gene for the protein of interest. First, a method for getting the gene into the plant is needed. Most commonly a plasmid from Agrobacterium tumefaciens called Ti is used. Alternatively, a “gene gun” can be used. Once the gene is in the plant, it must be correctly transcribed and translated. This requires a promoter sequence and species-specific codon usage. Usually a powerful promoter from Cauliflower Mosaic Virus (CaMV 35 S) is used.
There are several environmental questions about the use of transgenic foods. In the case of Bt plants, there is concern about the effect of the toxin on non-target organisms such as butterflies via pollen. There is also the possibility that insects will become resistant to the Bt toxins. Resistance is also an issue with the Round-Up Ready plants; as glyphosate is repeatedly sprayed on fields, weeds may evolve to not be sensitive to the herbicide. This is already the case with horseweed in the United States and goosegrass in Malaysia.
There are also concerns in regard to human health. Although the human digestive system does not contain the receptors for the Bt toxins nor the conditions for proteolysis into the active toxin form, these proteins are not normally found in the human diet. Consequently, there is concern that Bt toxins might be allergens. The genes which have been approved for use in human food products denature under high heat and are easily digested by gastric enzymes, so there is relative confidence that they will not cause food allergies. Recently, Taco Bell brand taco shells were removed from the market for containing an unapproved Cry protein, Cry9C. This protein is heat stable and resistant to degradation by trypsin, two characteristics of potential food allergens (Hileman 2003). However, these characteristics do not guarantee allergenicity, so sequence-based methods are also being developed.
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Acknowledgements: This case was developed with support from The Pew Charitable Trusts as part of a Case Studies in Science Workshop held at the University at Buffalo, State University of New York, on May 12–16, 2003.
Date Posted: 03/17/04 nas
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