The PCR technique amplifies (copies) the DNA billions of times in order for detection and quantitation to be possible. Specificity for the inserted DNA (transgene) is accomplished by utilizing short pieces of DNA (primers) which are complementary to the transgenic DNA sequence. Amplification is accomplished by using the enzyme DNA polymerase which catalyzes DNA synthesis using the primers as templates for new DNA synthesis to occur. Primers bind to an area of the DNA near or within the GMO sequence such that only this area of the DNA genome is amplified. Because the primers are specific to the transgenic DNA sequence and because the DNA is amplified many billion-fold in this process, PCR is a highly sensitive and specific GMO detection method.
There are various ways to detect GM DNA in a sample, based on the structure of the foreign DNA sequence, or transgene. A typical transgene contains the DNA sequence for the desired trait (e.g., glyphosate tolerance), viral and/or bacterial genetic elements that serve to regulate the expression of the trait gene in the plant, and often a marker for antibiotic resistance that allows for initial selection of the transformed plant cells from those cells that do not contain the transgene. In addition, insertion of the transgene occurs randomly into the plant DNA. Because of this, a PCR test that identifies the junction sequence between the transgene and the plant DNA will be specific for that GM event. Such tests are called event-specific tests because they identify the specific insertion event.
- Broad-spectrum GMO tests: The same viral and bacterial genetic elements are often incorporated in transgenes to regulate expression of the trait gene in the plant allowing these DNA sequences to be targeted and used as broad-spectrum (screening) GMO tests. These tests are not specific to a particular GM crop or GM event; instead, each of these broad-spectrum tests can detect many – but not all – GM crops/events. If the intent is to determine if GM DNA is “detected” or “not detected” in a sample, then PCR test(s) targeting one or more broad-spectrum DNA sequences may be sufficient.
- Event-specific and construct-specific GMO tests: Event-specific and construct-specific PCR assays may be used to identify specific GM events.
- Combination of broad-spectrum and specific GMO tests: If the intent is to quantitate the GMO level of the sample, then a combination of broad-spectrum and specific PCR assays may be most efficient, depending upon the particular GM crop to be tested.