DNA extraction from hair, bodily fluids, and tissues
DNA extraction from hair, bodily fluids, and tissues (Forensic Science)
When biological materials that have been found at crime scenes—such as hairs, bodily fluids, and tissues—are submitted for DNA (deoxyribonucleic acid) analysis, the samples must first undergo DNA extraction procedures. The most common methods for extracting DNA from such materials are organic extraction, Chelex extraction, extraction using preservation paper, and extraction using silica-based columns. The common organic extraction uses detergent and proteinase to break open (lyse) cells, followed by introduction of an organic solvent to separate proteins and other cellular debris away from the DNA. Chelex extraction is a quick and easy procedure, but the purity of the DNA extracted is low. Chelex binds metal ions that could otherwise lead to poor DNA typing results, but little other purification is done. Preservation papers provide another quick method for extracting DNA. Bodily fluids are applied directly to the paper, where the cells are immediately lysed. Once the sample is dried, a small portion can be punched out, washed briefly, and then moved directly to DNA amplification. Silica-based columns bind DNA following cell lysis, allowing cellular debris to be washed through. The DNA is then eluted in relatively pure form.
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Hair (Forensic Science)
Given their ubiquity, hairs are often found at crime scenes. Hairs with root tags attached are excellent sources of DNA, whereas hairs without their roots are not good sources of nuclear DNA for short tandem repeat (STR) testing. Shed hairs, which are the kind most often found at crime scenes, generally do not have root tags and so require mitochondrial DNA testing.
Hair roots are processed like other tissues. A questioned shed hair is first cleaned to remove any exogenous DNA; an enzymatic detergent such as Terg-A-Zyme is often used for this purpose. The keratin (protein) of the hair is broken down with proteinase, detergent, and dithiothreitol, releasing any trapped DNA. Prior to this, the hair may be ground or homogenized, which further helps to free DNA.
An alternative method for extracting DNA from hair is alkaline extraction. The hair is washed and then is exposed to a strong basic solution (such as sodium hydroxide), which destroys the protein without harming the DNA. The solution is neutralized and filtered, leaving the DNA ready for analysis.
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Bodily Fluids (Forensic Science)
The bodily fluids most commonly processed in crime laboratories (generally in dried form) are semen and blood; these are followed in frequency by saliva and urine. Other fluids, including vaginal secretions and perspiration, are also sources of DNA. In blood, the abundant red blood cells do not have nuclei and therefore do not contain DNA. In contrast, white blood cells, which make up less than 1 percent of cells present in blood, harbor a full DNA component. This means that blood is a valuable source of DNA, although not an ideal one.
Semen, which contains huge numbers of spermatozoa, each containing its complement of DNA, is considered one of the richest sources of genetic material. Owing to the strength of sperm cell walls, isolation of DNA from semen requires treatment similar to that of hair shafts. The other bodily fluids contain DNA somewhat by chance, in that epithelial cells are shed into them, such as from the mouth (saliva) or urinary tract (urine). DNA from these sources can be isolated from bodily fluids using several of the procedures noted above.
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Skin, Bone, and Other Tissues (Forensic Science)
Shed skin cells are a viable source of DNA and may potentially be collected from any item that has come into contact with skin, such as clothing, keys, or backpacks. Generally, organic extraction of such DNA is performed after a cutting or swab is collected from the item that has had skin contact. It should be noted that individuals can shed cells at very different rates; thus two pieces of evidence that seem similar may produce variable levels of DNA typing success.
Bones are frequently included in forensic investigations, in general owing to their relative longevity. Fresh skeletal material is a rich source of DNA, but the longer bones are in contact with the environment, the more degraded the DNA becomes. Likewise, inhibitory chemicals such humic acids can leach from soil into bone, making DNA analysis difficult. This is particularly true of spongy bones, where rain, soil, and microorganisms easily enter and destroy DNA. In contrast, skeletal materials with more cortical (compact) bone (such as the femur, or thighbone) resist DNA degradation. The DNA is extracted as described above after the bone is cleaned and then ground or drilled to create a powder. In some instances, bone may be decalcified using EDTA (ethylenediaminetetraacetic acid), which helps to augment its breakdown.
Other tissues are also potential sources of DNA. Generally, tissues that do not harbor degradative enzymes (as does the...
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Further Reading (Forensic Science)
Belgrader, P., et al. “Automated DNA Purification and Amplification from Blood-Stained Cards Using a Robotic Workstation.” BioTechniques 19 (September, 1995): 426-432. Describes the automated isolation of DNA from paper matrices.
Butler, John M. Forensic DNA Typing: Biology, Technology, and Genetics of STR Markers. 2d ed. Burlington, Mass.: Elsevier Academic Press, 2005. Provides a detailed overview of DNA methodologies used by forensic scientists.
Deedrick, Douglas W. “Hairs, Fibers, Crime, and Evidence: Part 1—Hair Evidence.” Forensic Science Communications 2 (July, 2000). Presents an overview of the uses of hairs as forensic evidence.
Graffy, Elizabeth A., and David R. Foran. “A Simplified Method for Mitochondrial DNA Extraction from Head Hair Shafts.” Journal of Forensic Sciences 50 (September, 2005): 1119-1122. Describes the alkaline method for DNA extraction from head hairs.
Nagy, M., et al. “Optimization and Validation of a Fully Automated Silica-Coated Magnetic Beads Purification Technology in Forensics.” Forensic Science International 152, no. 1 (2005): 13-22. Discusses the automated isolation of DNA using silica.
Walsh, P. S., D. A. Metzger, and R. Higuchi. “Chelex 100 as a Medium for Simple Extraction of DNA for PCR-Based Typing from Forensic Material.” BioTechniques 10 (April, 1991): 506-513. Reports on the...
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