Tuesday, April 19, 2011

The Polymerase Chain Reaction

The original DNA-typing method, the one Jeffreys used in the Narborough case, was too cumbersome and time consuming to be useful for casework and database construction. A different, faster technology was needed. It was developed in 1985.

The polymerase chain reaction is one of the most stunning breakthroughs in all DNA science. The idea is attributable to Kary Mullis, who worked for Cetus Corporation at the time; he received the Nobel Prize in chemistry in 1993 for his discovery. Cetus scientists commercialized PCR, and it has been extensively used in research and application laboratories, including forensic science labs, all over the world.

Essentially, PCR copies a segment of DNA multiple times using an enzyme called a DNA polymerase. The segment that is to be copied is specified, or defined, by two primers. In this way a small amount of DNA can be used to make a very large number of copies of a desired small portion of the total molecule. This process has been extremely useful in basic and applied biomedical research and as the basis for clinical and forensic tests.

Forensic DNA typing is based on a simple principle: A person’s DNA type is a statement about how many times the head-to-tail repeat sequences are repeated. Chromosomes are paired, and on these pairs one chromosome has a repeat number inherited from the mother, while on the other chromosome is a repeat number inherited from the father. In the majority of people the numbers are different. Combining information about these numbers of repeats at several regions in the DNA can generate a profile that is very unlikely to be found in more than a single individual.