Question

How does PCR relate to crime scene investigation?

Answer 1

Hint: Currently, the tool of choice for the detection of human remains in forensic coursework is polymerase chain reaction (PCR). Samples of DNA from crime scenes also contain PCR-inhibiting co-purified impurities. When sufficient copies of DNA are present, PCR inhibition is the most common cause of PCR failure. Inhibitors have been frequently detected in forensic studies of DNA derived from a variety of templates.

Complete answer:
The amount of DNA available for use in criminal investigations is limited to whatever can be extracted from a few strands of hair, skin cells, blood, or other bodily fluids left behind at the scene.
Isolating adequate quantities of intact DNA from such material can be tedious, thereby impeding the use of advanced DNA technologies to classify offenders and eventually solve crimes. In that it can rapidly amplify specific sequences from the isolated DNA, increase the amount of material and pave the way for further study, PCR represents a simple, cost-effective, and relatively easy solution to this problem.
For many modern DNA technologies, PCR is, therefore, a prerequisite and is an integral component of genetic fingerprinting. Genetic fingerprinting is based on identifying people according to their genetic material variations. This is possible because, while people share 99.9% of their DNA, they vary by 0.1%.
In just around 35-40 cycles, PCR can be used to amplify DNA approximately a billion times. DNA collection and analysis is an important part of CSI, and the collected samples are mostly extremely low in concentration and low quality.
PCR is then regularly used to amplify this DNA to quantities that can be further evaluated by methods such as fingerprinting, sequencing, etc. to classify individuals involved in the crime.

Note:
These repeated sequences are called variable number tandem repeats in modern genetic fingerprinting (VNTRs). Among closely related individuals, the genomic regions containing these repeats are very similar but sufficiently different to ensure that unrelated individuals are extremely unlikely to have the same VNTRs.