Originally, PCR was employed to produce usable amounts of small- and medium-sized DNA segments. Many other important techniques quickly developed. PCR is used to identify bacteria and virus infections. It is not only very sensitive but is also selective enough to distinguish closely related strains. For example, this procedure has been adapted to detect genetically modified crops to help ensure they are utilized only in approved ways.
A widely known application of PCR is DNA fingerprinting. Certain regions of the human chromosome have short sections that are repeated up to several hundred times. Different individuals have different numbers of repeats in each section. If several sections are tested, a unique pattern is observed. Initially, these regions were cut out of sample DNA
and tested for size. PCR allows the variable regions to be copied millions of times, greatly increasing the sensitivity and speed of the technique. Because of the technique's sensitivity, extreme care must be taken to avoid any contamination of the samples. Forensic scientists continue to make even greater use of this method.
One of the latest applications of PCR is the DNA microarray or gene chip. It allows medical personnel to test a cancer cell before and after chemotherapy with different drugs. If two patterns look the same, the two drugs administered are likely working on the same pathway and may not be as useful as treatment with two drugs that work by different pathways.