HISTORY OF DNA ANALYSIS

• Jurisdiction: Australia

The Polymerase Chain Reaction .................................................................. [80.300]

Short tandem repeats ................................................................................... [80.320]

Validation of DNA analysis methods ............................................................. [80.340]

[80.300] The Polymerase Chain Reaction

Non-coding regions of DNA mutate more readily than coding regions as there are generally no fatal consequences of mutations in non-coding regions. This lack of restriction against mutation allows the repetitive DNA to mutate, spread and diversify, increasing the number of polymorphisms throughout the genome (Ridley, 1985; Cooper et al, 1985). This is further illustrated by the high prevalence of repeat regions scattered throughout the genome. The detection of these regions and their use in forensic DNA analysis is facilitated through a process known as the Polymerase Chain Reaction (PCR).

PCR is a method of amplifying or copying the DNA (Saiki et al, 1985). The double-stranded DNA is first heated to a specific temperature to separate the strands (denature) and then cooled to a second specific temperature to allow the short sections of manufactured DNA known as primers to bind (anneal) to specific sequences on the DNA. For each section of DNA being analysed, two primers are required for attaching to each side of the site of interest on the DNA molecule. At a third specific temperature, an enzyme known as Taq polymerase facilitates the copying of the DNA by adding the components of the DNA to make each single strand a double strand (extend) using the G-C, A-T pairing code. It was the discovery of Taq polymerase that was the key factor in the development of PCR. Taq polymerase is a chemical found in a bacterium (Thermits aquaticus) that functions at high temperatures and essentially drives the replication process. At the end of the Taq polymerase extension process, the copying is complete and all DNA fragments are double stranded again.

By repeating the above heating and cooling process of denature, anneal and extend, the DNA can be replicated in each cycle exponentially. In this way just a few copies of DNA can be made into millions or billions of exact copies. For example, if you started with just one copy of the genome, after the first cycle you would have two copies, then 4, 8, 16, 32 and after 28 cycles you would have around 130,000,000 copies. PCR represents an enormous advance in DNA analysis.

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