Biology Reference
In-Depth Information
3
Biological material -
collection, characterization
and storage
The sensitivity and evidential power of DNA profiling have impacted on the way
in which crime scenes are investigated. Because only a few cells are required for
DNA profiling, crime scene investigators now have a much wider range of biological
evidence to collect and also have a much greater chance of contaminating the scene
with their own DNA.
Sources of biological evidence
The human body is composed of trillions of cells and most of these contain a nucleus,
mature red blood cells being a notable exception. A wide variety of cellular material
can be recovered from crime scenes (Table 3.1).
Each nucleated cell contains two copies of an individual's genome and can be used,
in theory, to generate a DNA profile under optimal conditions [1 - 3]. In practice,
15 or more cells are required to generate consistently good-quality DNA profiles
from fresh material [4, 5]. Forensic samples usually show some level of degradation,
and with higher levels of degradation, more cellular material is required to produce
a DNA profile. If the material is very highly degraded then, even with the high
sensitivity of DNA profiling, it may not be possible to generate a DNA profile.
The biological material encountered most often at scenes of crime is blood
(Figure 3.1). This is mainly because of the violent nature of many crimes and also
because it is easier to visualize than other biological fluids such as saliva.
Other frequently encountered samples include seminal fluid, which is of prime
importance in sexual assault cases; saliva, which may be found on items either held
in the mouth, such as cigarette butts and drinking vessels, or on bite marks, or in close
proximity to the mouth when speaking, such as the inside of masks or phones; and
epithelial cells, deposited, for example, as dandruff and in faeces. With the increase
in the sensitivity of DNA profiling the recovery of DNA from epithelial cells shed
on touching has also become possible [6]. Door handles, steering wheels and knife
