The polymerase chain reaction (PCR) is an invaluable tool in modern molecular biology. It also serves an important purpose in law enforcement as a step in a process called DNA fingerprinting.
The polymerase chain reaction uses two primers (short single-stranded sequences of DNA) and a special high-temperature stable polymerase (an enzyme that copies DNA) to make many copies of a sequence in a DNA sample. PCR enables researchers to make lots of copies of a specific region of the genome from a DNA sample and thereby isolate it for analysis.
PCR's main advantage in forensics is that forensic scientists can use it to amplify or make copies of regions of the genome that vary widely between different individuals, called VNTRs (variable number tandem repeats). By comparing the length of different VNTRs they can determine whether the sample may be a match with the suspect's DNA.
Other advantages of PCR in forensic science are that scientists can use it to amplify VNTRs from the sample, even if only trace amounts of DNA are present initially. Often forensic scientists must work with very small amounts of DNA, so the ability to use a small or partially degraded sample is vital.