Storage And Transport Of Dna Evidence

Carelessness or ignorance of proper handling procedures during storage and transport of DNA from the crime scene to the laboratory can result in a specimen unfit for analysis. For example, bloodstains should be thoroughly dried prior to transport to prevent mold growth. A recovered bloodstain on a cotton swab should be air-dried in an open envelope before being sealed for transport. DNA can be stored as non-extracted tissue or as fully extracted DNA. DNA samples are not normally extracted though until they reach the laboratory.

Most biological evidence is best preserved when stored dry and cold. These conditions reduce the rate of bacterial growth and degradation of DNA. Samples should be packaged carefully and hand carried or shipped using overnight delivery to the forensic laboratory conducting the DNA testing. A nice evidence collection cardboard box was recently described for shipping and handling bloodstains and other crime scene evidence (Hochmeister et al. 1998). Inside the laboratory, DNA samples are either stored in a refrigerator at 4°C or a freezer at —20°C. For long periods of time, extracted DNA samples may even be stored at —70°C.

PRESUMPTIVE TESTS FOR BLOOD, SEMEN, AND SALIVA

Forensic evidence from crime scenes comes in many forms. For example, a bed sheet may be collected from the scene of a sexual assault. This sheet will have to be carefully examined in the forensic laboratory before selecting the area to sample for further testing. Prior to taking the effort to extract DNA from a sample, presumptive tests are often performed to indicate whether or not biological fluids such as blood or semen are present on an item of evidence (e.g., a pair of pants). Locating a blood or semen stain on a soiled undergarment can be a trying task. Three primary stains of forensic interest come from blood, semen, and saliva. Identification of vaginal secretions, urine, and feces can also be important to an investigation.

Serology is the term used to describe a broad range of laboratory tests that utilize antigen and serum antibody reactions. For example, the ABO blood group types are determined using anti-A and anti-B serums and examining agglutination when mixed with a blood sample. Serology still plays an important role in modern forensic biology but has taken a backseat to DNA in many respects since presumptive tests do not have the ability to individualize a sample like a DNA profile can.

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