DNA extraction from hair, bodily fluids, and tissues
DNA extraction from hair, bodily fluids, and tissues is a critical process in forensic science, allowing for the analysis of genetic material found at crime scenes. Various methods are employed to extract DNA from these biological materials, including organic extraction, Chelex extraction, preservation paper techniques, and silica-based column extraction. Hair, particularly those with root tags, provides a strong source of nuclear DNA, while shed hairs generally require mitochondrial DNA testing. Bodily fluids such as blood, semen, saliva, and urine can also yield DNA, with semen being particularly rich in genetic material due to the abundance of sperm cells.
Skin cells, which can be found on items that have come into contact with an individual, are another viable source of DNA, while bones and other tissues can be processed to extract genetic material, although the success of extraction can vary based on environmental factors and the condition of the samples. Each extraction method has its advantages and limitations, influencing the purity and quality of the DNA obtained. Understanding these processes is essential for those interested in forensic applications and the role of DNA in criminal investigations.
Subject Terms
DNA extraction from hair, bodily fluids, and tissues
Definition: Techniques used to obtain DNA from different kinds of biological materials so that the DNA can be analyzed.
Significance: DNA comparison has become a critical tool for identifying victims and suspects in a variety of crimes, and biological evidence such as hairs, bodily fluids, and tissues can provide the DNA needed for comparisons.
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.
Hair
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.
Bodily Fluids
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.
Skin, Bone, and Other Tissues
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 digestive tract, for instance) are favored. Small pieces of tissue may be homogenized to disrupt cells before the tissue is incubated in detergent and proteinase. Organic extraction is the most common method of DNA purification, but DNA has been extracted successfully using the Chelex and silica-based column methods.
Bibliography
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