What are Genetically Modified Crops?


We consume GMOs, genetically modified organisms, every day. Currently in the US, 80 percent of processed foods contain GM ingredients (Calandrelli, 2013). And while the average American consumes 190 pounds of GM foods annually, not much research has tested their long term health effects (Stonebrook. 2013). Scientists know they can cause allergic reactions, problems with antibiotic resistance, and pesticide exposure (Stonebrook. 2013). If GM heritable traits should “escape” (breed with native plants), they would alter the gene pool, polluting thousands of years of genetic adaptation. This could cause a snowball affect wreaking ecological havoc on all native organisms.

However, GMOs were created to dramatically reduce the need for pesticides and field tillage. By reducing tillage, labor cost, run-off, and erosion are also reduced, but herbicide must be increased. RoundupⓇ resistant corn was genetically engineered to reduce herbicide use to one to two applications, compared to multiple applications with non-resistant corn.

GMOs also have the capacity to save populations from starvation and malnutrition. For example, “golden rice” was

Figure 1. Golden rice grain compared to white rice. (IRRI, 2011)

genetically engineered to synthesize beta-carotene, which could save 2.5 million children’s lives annually from vitamin A deficiency (Coghlan, 2013). In a world with limited arable land and a growing population, some say GM crops are the answer to world hunger. For these reasons, it’s important to know the truth about the benefits and risks of GM foods.

How are Genetically Modified crops Different from Traditionally Breed Crops?

Traditional or selective breeding chooses the best individuals of the population to breed the next generation. This selection is based on the phenotype, or the expressed genes, of the organism. Breeding to achieve a specific trait can take hundreds of years. Many times the desired trait only exists in specific species, which cannot interbreed with the crop. Genetic engineering is based on knowing the genotype, or the full genetic make-up of the organism. Methods allow scientists to rapidly insert desired genes into a recipient cell’s genome. This can be done across different species, creating “transgenetic” organisms. Genetic engineering is mostly used in agriculture to introduce a desired trait, such as pest, herbicide, or disease resistance, into a crop species.

How are Crops Genetically Engineered?

Figure 2. Use of a restriction enzyme to cut a specific DNA sequence. (Wang, 2013)

The first and most complex step is identifying the DNA sequence of the desired trait. This requires years of DNA research studies. In these studies, mutated species lacking specific traits are compared with the original species with the common traits. This comparison identifies the DNA sequence specific missing trait. Once the sequence of the gene is known, it must be isolated.
Scientists use restriction enzymes to run along the DNA and cleave specific DNA sequences, separating the desired gene from the DNA. Figure 2 shows an example of the restriction enzyme EcoRi. This enzyme is specific for the GAATTC sequence, cutting between the G and A in the 3’ 5’ configuration and A and G in the 5’ 3’.

Figure 3. How Agrobacterium genetically transforms plants. (Gelvin. 2005)

Gene transfer tools are used to insert the desired gene into a plant cell’s genome. The most commonly used is the circular DNA, or Ti-plasmid, of the bacteria Agrobacterium tumefaciens. This is labeled as 1. in figure 3. This is naturally a tumor inducing plasmid, which transfers tumor inducing genes into its plant cell host. The virulence (vir) genes, located in the plasmid (labeled as 2 in figure 3), give the bacteria its ability to transfer part of its DNA. The transferred-DNA section (or T-DNA) is the section of the plasmid which is transferred (labeled as 3). Naturally Agrobacterium tumefaciens‘ T-DNA contains plant tumor inducing genes, however these genes have been removed by the commercially used bacteria species.

The T-DNA region is then cut with the same restriction enzyme that was used before. The desired gene and a verification gene, are inserted into the cell. Antibiotic resistance genes are commonly used for the verification genes. The genes stick to the ends of the cut plasmid and the enzyme DNA ligase binds the plasmid back together. The bacteria are exposed to an antibiotic, killing off any cells missing the genetically modified plasmids. The surviving bacteria divide themselves, producing more bacteria with the same modified genes.

The bacteria are mixed with wounded plant cells. The plant cells produce phenolic defense compounds as a response to wounding. Expression of the Agrobacterium’s vir genes is triggered by the phenolic compounds, produced as a response to plant wounding. The resulting virulence (vir) proteins process the T-DNA section, producing a ‘T-strand’ (Gelvin. 2005). The T-strand and several vir proteins enter the plant cell through a transport channel. This channel forms when the cells attach (Gelvin. 2005), and is labeled as 4. in figure 3. Once inside the plant cell they form a T-complex, labeled as 5. in the figure. This complex guides the way to the nucleus, labeled as 6. in the figure. The T-DNA is integrated into the cell’s genome, and the cell is transformed to be transgenetic. The transgenetic plant cells are reproduced and grown in tissue culture labs. This provides a sterile environment for the cells and tissues to grow.

Work Cited

IRRI. 2011. Golden Rice grain compared to white rice. Flickr. http://www.flickr.com/photos/ricephotos/5516789000/in/set-72157626241604366.
Calandrelli, Emily. 2013. GMO Fears Overblown. The Break Through. http://thebreakthrough.org/index.php/programs/conservation-and-development/gmo-fears-overblown/
Stonebrook, Shelley. 2013. 4 Potential Health Risks of Eating GMO Foods. Care 2. http://www.care2.com/greenliving/health-risks-of-eating-gmo-foods.html
Coghlan, Andy. 2013. Golden Rice creator wants to live to see it save lives. News Scientist. http://www.newscientist.com/article/dn24417-golden-rice-creator-wants-to-live-to-see-it-save-lives.html?page=2#.UvrASrTA5KU-foods.html
Gelvin, Stanton B. 2005. FIGURE 2. How Agrobacterium genetically transforms plants. Agricultural biotechnology: Gene exchange by design. Nature. 433, 583-584. http://www.nature.com/nature/journal/v433/n7026/fig_tab/433583a_F2.htmlmod/g http://artsci.wustl.edu/~anthro/blurb/Backgrounder.htmlmo-examples
Wang, Jane. 2003. RESTRICTION ENDONUCLEASES: MOLECULAR SCISSORS FOR SPECIFICALLY CUTTING DNA. http://www.scq.ubc.ca/restriction-endonucleases-molecular-scissors-for-specifically-cutting-dna/

© Caroline Hooks and “hortshorts,” 2015. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Caroline Hooks and “hortshorts” with appropriate and specific direction to the original content.

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