Biology Reference
In-Depth Information
site for some inhibitors and can competitively remove or reduce the concentration
of the inhibitor [33, 36].
The action of inhibitors can be detected, for example by spiking a PCR with
a known amount of DNA; this alerts the analyst that further purification steps are
required [51]. PCR inhibition can also be detected using real-time DNA quantification
(see Chapter 4).
Sensitivity and contamination
The great advantage of PCR is that it will amplify DNA from a template of only
a few cells. This high level of sensitivity can also be a potential disadvantage, as
DNA from incidental sources can be present and contamination can be introduced.
Throughout the handling and analysis of DNA samples, extreme care needs to be
taken to minimize the chance of introducing this extraneous DNA.
When samples are collected from the scene of an incident, there may be cellular
material from persons who had been present at the scene prior to the incident and
hence DNA profiles will be generated from people unconnected with the incident.
This type of DNA can be termed as incidental as it is not a contamination of the
samples. At the time of the incident there is an opportunity for transfer from the
perpetrator, and it is this cellular material that is pertinent to the investigation. Con-
sider an event such as theft from a house. Prior to the incident there will be cellular
material from the owners and from any recent visitors. At the time of the break in
there may be transfer from the burglar. If the incident is discovered by a neighbour,
then they will introduce their cellular material after the incident and prior to the
scene being secured. When the police are called they have the potential to intro-
duce their cellular material. Once the scene is secured then those entering should
be wearing full protection to minimize the opportunity for transfer of their cellular
material [52, 53]. If there is any introduction of DNA from those at the crime scene,
during collection and transportation, or from laboratory staff, then this is considered
as contamination.
The PCR laboratory
Once evidential samples have reached the forensic laboratory there is further potential
to introduce contamination. A fundamental feature of PCR laboratories, to reduce
the possibility of introducing contamination, is that they are clearly divided into pre-
and post-PCR areas (Figure 5.5).
Pre-PCR
Once the samples reach the laboratory, potential contamination comes from the
reagents, equipment and the forensic scientists undertaking the analysis. To pre-
vent contamination being introduced from the scientist, protective clothing is worn,
