What is PCR (polymerase chain reaction)?
Polymerase chain reaction (PCR) is a technique that is used to amplify trace amounts of DNA (and in some
instances, RNA) located in or on almost any liquid or surface where DNA strands
may be deposited. The key to understanding PCR is to know that every human,
animal, plant, parasite, bacterium, or virus contains genetic material such as
DNA (or RNA) sequences (nucleotide sequences or pieces of DNA or RNA) that are
unique to their species, and to the individual member of that species.
Consequently, if a sample contains segments of DNA or RNA, PCR is a method used
to amplify (make many more identical copies) of these unique sequences so they
can then be used to determine with a very high probability the identity of the
source (a specific person, animal, or pathogenic organism) of the trace DNA or
RNA found in or on almost any sample of material.
PCR amplification is only part of the identifying test, however. Once the
amplification is done (see below), the amplified segments need to be compared to
other nucleotide segments from a known source (for example, a specific person,
animal, or pathogenic organism). This comparison of unique segments is often
done by placing PCR-generated nucleotide sequences next to known nucleotide
sequences from humans, pathogens, or other sources in a separating gel.
Electrical current is run through the gel and the various nucleotide sequences
form bands that resemble a "ladder" according to their electrical charge and
molecular size. This is termed gel electrophoresis. Bands or
"ladder" like steps
that migrate to the same levels in the gel show identity of nucleotide
sequences. This method is one of the most popular ways PCR tests are completed
(See Fig 1).
Figure 1, Bands or "ladder" like steps of PCR produced DNA of
Mycobacterium (courtesy of the CDC)
Figure. Repetitive element (Rep)–PCR (A) and pulsed-field gel electrophoresis (PFGE) (B) patterns of Mycobacterium cosmeticum isolates from 2 patients in Ohio and 1 patient in Venezuela. Rep-PCR was performed by using BOXA1R primer (3), and PFGE was performed with restriction enzyme AseI. Lanes 1, 2, Ohio isolates OH1 and OH2; lanes 3, 4, control strains ATCC BAA-878T and ATCC BAA-879; lane 5, Venezuelan isolate VZ1. DNA size standards are 100-bp (S1) and 48.5-kb marker (S2).