How does PCR work?
The polymerase chain reaction (PCR) is
a method to find the genes/DNA that make a particular
plant genetically modified. The goal is to amplify specific
sequences of DNA, for example the inserted herbicide
gene. DNA is amplified by using specific short DNA sequences
(called primers) that flank the region of the
gene that is to be amplified. In other words, the primers
allow us to find the "gene of interest"
by latching onto the gene. At the onset of every PCR
reaction we add DNA extracted (DNA
extraction) from the GMO crop, primers, DNA polymerase
and bases to a vial.
Remember from our biology classes that
DNA is made up of two tightly bonded strands, turning
in a helix. For our gene specific primers to
work, we need to open or separate this DNA. A PCR thermocycler
is a machine that heats the DNA helix so it opens (denatures).
Once the DNA is separated into two strands, the temperature
is dropped rapidly so that the primers can latch (hybridize)
onto a single, complementary strand of DNA. This is
the annealing step. The temperature of the
thermocycler is raised again so that the DNA polymerase
(an enzyme) produces more DNA. This is the extension
step. A typical PCR program consists of 30-45 cycles
of denaturing, annealing, and hybridizing
steps. We then have enough of the gene/DNA synthesized
Analysis: After the PCR
reaction, the DNA from each vial is carefully pipetted
onto a agarose gel. The gel is already stained with
ethidium bromide so that the added DNA lights up under
UV light. The charged DNA molecules "run"
down the agarose gel and separate according to their
size. For example, large DNA fragments stay on top of
the gel, while smaller DNA fragments travel the bottom
of the gel. A GMO plant would show a particular DNA
fragment indicative of the gene, while a non-GMO plant
would not show that DNA fragment on the gel. Please
see a picture of a gel below: