Biology Reference
In-Depth Information
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Forensic
Biology
DNA
Extraction
and PCR
Set-up
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Sample
Reception
Pre-PCR
Changing Area
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Movement of
samples
PCR
Amplification
Door
Hatch
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Post-PCR
Changing Area
Negative air
pressure
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Positive air
pressure
Analysis of
PCR Products
Figure 5.5
The PCR laboratory is designed so that the work flows through the different processes
in one direction starting with sample reception and forensic biology and finishing with the
post-PCR analysis. The samples are passed through air-lock hatches to minimize the possibility of
any material being transferred from post-PCR to pre-PCR areas. Access to the pre- and post-PCR
laboratories is through different changing areas and dedicated staff will work in either pre- or
post-PCR areas. Positive air pressure in pre-PCR areas and negative pressure in post-PCR rooms also
reduces the possibility of introducing any contamination into the pre-PCR areas
including a lab coat, gloves, a face mask, safety glasses/visor and a head cover. Even
with these precautions it is still possible to get the scientist's DNA profile showing
up; a database of all the people who enter/work in the laboratory can be used to
detect when contamination could have been introduced within the laboratory.
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When laboratories are engaged in analysing both samples from suspects and from
crime scenes, it is important to have dedicated areas for the two classes of sample;
this prevents any potential cross-contamination of the crime scene and suspect DNA.
Special dedicated facilities may also be used when dealing with samples that contain
very small amounts of DNA, such as hair shafts.
DNA extraction and PCR set-up are commonly carried out in specialized clean
hoods that provide a very controlled environment. The hoods have stainless steel
surfaces and are easy to keep clean; they have filtered air to prevent any dust or
other contaminant getting into the reaction. The pipetting of any liquids involved in
the extraction and PCR set-up is performed using pipette tips with barriers to prevent
any DNA carry-over contamination.
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Note: Databases may also contain the DNA profiles of Police Officers and Crime Scene Investigators who could potentially
contaminate crime scenes with their own DNA.
