Examination of Firearms Review: 2007 to 2010 - Interpol

can be used. PCR is a cyclic process in which target DNA, for example an STR, is
being duplicated in vitro. The PCR process was developed in 1985 37 . PCR is widely
used in DNA profiling in circumstances where there are only minute quantities of
starting material. One cycle of the PCR process consists of denaturation of the DNA,
binding of the primers, and elongation of the new DNA strands 38 . Because this cycle
can be repeated multiple times the concentration of DNA will increase exponentially.
The amount of amplicons (target DNA sequences) after a certain amount of PCR
cycles can be defined by 2 n – (2n+2), where n is the number of cycles. In forensic
cases somewhere between 28 and 34 cycles are used 28 . When the amplification
process is finished the PCR mixture will almost exclusively consist of amplicons,
therefore only the amplicons will be visible during further analysis. Every PCR
process requires at least two primers, one on each site of the target DNA. By
selecting suitable primers, highly variable regions or STRs can be amplified from very
small amounts of DNA. However the complementary primer sequence has to be very
specific, to ensure that only the target DNA is amplified. Furthermore PCR primers for
STR loci have to bind as close as possible to the tandem repeat to decrease the
amplicon length, because a shorter template STR has more chance of surviving
degradation, and therefore increasing DNA profiling success. Current PCR
techniques are able to amplify multiple STRs at the same time (this process is called
multiplexing), meaning that only a small amount of DNA is needed and consumed for
DNA typing (for more information on multiplexing readers should refer to Budowle
and van Daal 34 ).
In forensic investigations typically at least ten different loci are examined 26,28 ,
including amelogenin for sex determination. The loci that are examined are the same
in every sample. All loci have their own specific code, for example D21S11, FGA and
TH01. Every locus (singular of loci) has its own set of DNA-features, if all the features
of the different loci are found it is said to be a full DNA-profile. The information from a
DNA-profile is used to decide, whether any two samples of DNA are from the same
individual or not 29 . The usage of the same standard set of loci by different countries
provides the ability to compare a DNA-profile with different DNA databases on an
international level. The Interpol standard set of loci (ISSOL) currently preferred is
given in Table 2. Recently a DNA match in an unsolved murder, which occurred in
the Netherlands in 1996, has led to the arrest of a German man. The DNA profiles
were exchanged on the basis of the Treaty of Prüm. The Netherlands is affiliated with
the Treaty of Prüm, which has now been transposed into European Union legislation.
Because of this, DNA profiles of different countries can be compared on a daily basis.
The Netherlands can compare DNA profiles with Germany, Austria, Spain,
Luxembourg, Slovenia, Finland, and France. This has already led to a total of 1103
international matches, of which 629 occurred in 2009.
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