The way they are created disrupts the plant’s DNA in unintended, potentially harmful ways. In genetic engineering, a single gene is removed from one organism and forcibly inserted into another. First, scientists identify the gene they want and analyze its sequence. (If the source gene is to be taken from bacteria, some of its sequence has to be rearranged because bacteria produce certain amino acids using a code different from the one used by plants).
After figuring out a working gene sequence, engineers add a promoter sequence at one end of the gene to turn it on (the most popular one in GM crops being CaMV 35S, which forces the gene to constantly churn out the protein), and a terminator sequence at the other end (which tells the DNA to stop). Lastly, scientists add a marker gene, usually one that confers antibiotic resistance, so they can later douse the plant cells with antibiotics, killing off normal cells and revealing those that have been genetically modified. This combination of gene sequences – called a “gene cassette” – is then multiplied into millions and inserted into target plant cells via one of two primary methods, both of which trigger a wound response the cell.
One method employs a bacterium (Agrobacterium tumefaciens), which normally infects a plant by inserting a portion of its own DNA into the plant’s DNA and then causing the plant to produce tumors. Genetic engineers remove the tumor-creating section of this bacterium’s DNA and replace it with the desired gene cassette, so the bacterium “infects” the plants with the foreign genes instead.
The second method uses a gene gun. Scientists coat millions of particles of tungsten or gold with gene cassettes and blast them into millions of plant cells, only a few of which incorporate the foreign gene cassette.
In either of the two delivery forms, the next step is the application of the antibiotic to which the gene cassette confers resistance. Most of the plant cells die, but a few – the ones in which the transgene has inserted – survive. These are developed into plants that researchers can duplicate by making clones through tissue culture or harvesting the seeds.
Each plant grown from a gene insertion is unique because where the transgene ends up integrating itself into the host DNA is uncontrolled and cannot be reproduced. For this reason, the possible consequences to the plant’s DNA are different with each insertion, so all plants developed from a specific insertion are collectively referred to as an “event.”
In sum, genetic engineering artificially combines genes from different species and forcibly inserts them into unknown and random locations on the host genome. The procedure, which disrupts the precise orchestration of thousands of genes that has evolved over millennia in the normal plant’s genome, is highly mutagenic. (We now know that genes, like nutrients, do not work singly, but as part of highly integrated networks.) Plus it introduces bacterial genes for drug resistance along with strong promoters to express the foreign proteins at high levels in all parts of the plant.