BIOLOGICAL MATERIAL – COLLECTION, CHARACTERIZATION AND STORAGE
Table 3.1 scene. The DNA profiles generated from crime scene material are compared against reference profiles that are provided by suspects, and in some cases, the victims
very little follicle attached and are not a good source of DNA, whereas plucked hairs or hairs removed due to a physical action often have the root attached, which is a rich source of cellular material. The four most common nucleated cell types that are recovered from scenes of crime are white blood cells, spermatozoa, epithelial cells and hair follicles (Figure 3.2).
Figure 3.1 Blood is the most common form of biological material that is recovered from crime scenes. (a) Large volumes of blood can be collected using a swab, if the blood is liquid then a syringe or pipette can be used (picture provided by Allan Scott, University of Central Lancashire) (b) Blood on clothing is normally collected by swabbing, or cutting out the stain (picture provided by Elizabeth Wilson) (see plate section for full-colour version of this figure)
Figure 3.2 (b) spermatozoa; (c) epithelial cells (from saliva); (d) a hair shaft with the follicle attached (the cells have been stained with haematoxylin and eosin)
Collection and handling of material at the crime scene
The high level of sensitivity that makes DNA profiling an invaluable forensic tool can also be a potential disadvantage. Contamination of evidential material with biological material from another source, such as an attending police officer or scene of crime officer,isaveryrealpossibility.Itisvitalthattheappropriatecareistaken,suchasmain- taining the integrity of the scene and wearing full protective suits and face masks during the investigation of the scene [3-5] (Figure 3.3). Improper handling of the evidence can have serious consequences. In the worst cases, it can cause cross contamination, lead to sample degradation, and prevent or confuse the interpretation of evidence.
Identification and characterization of biological evidence
Searching for biological material, both at the crime scene and in the forensic laboratory is performed primarily by eye. In the laboratory, low power search microscopes may help to localize stains and contact marks. Alternative light sources have been found to assist with finding biological material both in the field and in the laboratory. Epithelial cells, saliva and semen stains may fluoresce at different wavelengths of light compared with the background substrate and therefore may become visible [6-8]. A range of light sources is available and these can either operate at fixed wavelengths or a variable number of wavelengths that are suitable for detecting different types of stain. Searching a crime scene or items recovered from a crime scene for blood can be aided by the use of luminol (3-aminophthalhydrazide). This chemical can be sprayed onto a wide area and will become oxidized and luminescent in the presence of haemoglobin, which is found in red blood cells. It is necessary to be able to darken the area that is being searched in order that the luminescence can be detected. Luminol can also be used