Biology Reference
In-Depth Information
5
Polymerase chain reaction
In 1985 a new method - the polymerase chain reaction (PCR) - was reported [1 - 3].
PCR can amplify a specific region of DNA by copying the locus exponentially. The
method has revolutionized all areas of molecular biology, including forensic genetics,
as extremely small quantities of DNA can be analysed. Under optimal conditions,
DNA can be amplified from a single cell [4, 5]. The increased sensitivity of DNA
profiling using PCR has had a dramatic effect on the types of forensic sample that
can be used, and it is now possible to analyse trace evidence and highly degraded
samples successfully - albeit with less than 100% success.
The evolution of PCR-based profiling in forensic genetics
PCR technology was rapidly incorporated into forensic analysis. The first PCR-based
tool for forensic casework amplified the polymorphic HLA-DQ
α
locus (the
α
subunit
of the DQ protein is part of the major histocompatibility complex) [6]. It was used
for the first time in casework in 1988 to analyse the skeletal remains of a 3-year-old
girl [7, 8]. The DQ
α
system's major drawback was that it had a limited power of
discrimination.
Minisatellites (VNTRs) were widely used in casework but required a relatively
large amount of DNA. In an attempt to overcome this limitation, PCR technology
was applied to the analysis of VNTR loci, and alleles between 5 kb and 10 kb could
be faithfully amplified from fresh biological material [9]. However, it was of limited
value for many forensic samples, which often contained small amounts of DNA
that was highly degraded. To overcome the problems caused by degradation, tandem
repeats, called amplified fragment length polymorphisms (AMP-FLPs) that were
smaller than 1 kb were selected for PCR based analysis [10 - 15]. However, as with
VNTRs, their use was limited in forensic contexts because of the size of the larger
alleles, which were difficult to analyse in degraded samples. By the early 1990s,
a large number of STRs had been characterized [16]. The STR loci were simpler
and shorter than VNTRs and AMP-FLPs, and were more suitable for the analysis
of biological samples recovered from crime scenes [17]. The STR markers were
not individually as discriminating as the VNTR and AMP-FLPs, but had a major
advantage that several of them could be analysed together in a multiplex reaction.
