DNA analysis (Forensic Science)
DNA (deoxyribonucleic acid) is a small biological molecule that contains the genetic material that dictates individual traits. It is contained in every cell in the human body except mature red blood cells. Because DNA is the basic unit of all life, the cells of nonhuman living organisms, such as plants, microbes, and animals, also contain it.
In addition to identifying potential suspects, the results of DNA analysis have been used to exonerate innocent persons who have been wrongly convicted, to establish paternity, and to identify the victims of mass murder and natural catastrophes. In some cases, DNA analysis of canine samples has been used when dogs have been involved in crimes or to establish the presence of persons at crime scenes through the matching of canine hairs on their clothing or other personal belongings. DNA analysis has also been performed on microbial samples, to trace the origins of deadly pathogens, and on botanical samples, to gather information about the origins of certain plants (such as marijuana) and about plant specimens found at crime scenes.
Biological evidence obtained at crime scenes can provide investigators with little or no information unless the DNA contained within the evidence samples is analyzed. A forensic scientist cannot compare a sample of saliva, for example, with a sample of a suspect’s blood or a sample of the suspect’s saliva just by looking at the swab on which the saliva was...
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DNA Extraction (Forensic Science)
After biological evidence samples are submitted to the laboratory, DNA analysts carefully obtain subsamples from the original samples and then use one of the available extraction (also known as isolation) methods to isolate the DNA in the subsamples. Just as there are different biological sources from which scientists must obtain DNA samples, different methods have been developed for this purpose. The methods vary in complexity and the amounts of time they take as well as in quality and sample throughput, but the same principles underlie all of these extraction technologies.
The sample is first subjected to a lysis (dissolution) step, which can be mechanical, chemical, or a combination of both, depending on the material. During this step, the cells are broken open to allow their contents to be released. Following this, the DNA in solution is bound to a membrane or magnetic beads and the remaining of the cell’s contents are subsequently subjected to a series of washes that usually contain ethanol. Because DNA is insoluble in ethanol, it remains attached to the membrane or it is pelleted during centrifugation while the remaining components are either dislodged from the membrane or stay in solution (the supernatant) and are discarded. In the last step of the extraction, the DNA is resuspended in distilled water or an elution buffer.
The process of obtaining high-molecular-weight DNA from known mixed samples, as is often...
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DNA Quantification (Forensic Science)
After the DNA has been isolated, the analyst needs to quantify the DNA and dilute it to a predetermined amount in order to amplify it. An analyst can estimate DNA concentrations using agarose gels, fluorometry, or spectrophotometry, but the most accurate means of determining the DNA concentration of a particular sample is through real-time polymerase chain reaction (qPCR). This process follows the same principle as the traditional PCR, but it quantifies the sample as it is amplified using fluorescence. Specific primers are added so that only the type of DNA of interest is quantified.
The technology of qPCR works with fluorescent dyes that are introduced into the double strands of the DNA and emit a signal or with gene-specific probes that fluoresce when they find a complementary DNA strand with which to bind. The emitted signals are recorded and transferred to a computer, which provides the output and transforms it into a numerical value. After the DNA is quantified, it is diluted to the required concentration for that type of sample and the corresponding PCR amplification “recipe.”
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DNA Amplification (Forensic Science)
The final step of the analysis is the amplification of the DNA sample. Amplification is the exponential copying of a gene fragment or fragments of interest. The sample is subjected to cyclic increases and decreases in temperature while in a mixture of (fluorescent) primers that demarcate the fragments of interest, an enzyme that extends the newly created fragments, and other reagents that provide the optimal conditions for in vitro replication of the specific genetic marker or markers of interest.
The steps described above are only the first portion of a series of procedures performed on a biological evidence sample. When DNA analysis is complete, the forensic scientist can begin the task of DNA typing, which allows the comparison of an evidence DNA sample to a known DNA standard.
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Further Reading (Forensic Science)
Budowle, Bruce. “Genetics and Attribution Issues That Confront the Microbial Forensics Field.” Forensic Science International 146 (Fall, 2004): S185-S188. Discusses the potential for using microbial DNA as a means of attribution in forensic scenarios.
Gehrig, Christian, and Anne Teyssier. “Forensic DNA Analysis.” CHIMIA International Journal for Chemistry 56 (March, 2002): 71-73. Describes the various techniques available for DNA analysis of human samples.
Hunter, William. DNA Analysis. Philadelphia: Mason Crest, 2006. Presents an interesting discussion of the different applications of DNA analysis in crime scene investigations.
Miller, Heather. Nonhuman DNA Typing. Boca Raton, Fla.: CRC Press, 2007. Introduces the field of nonhuman DNA typing and its applications to forensics. Includes interesting case examples with information on legal decisions.
Rudin, Norah, and Keith Inman. An Introduction to Forensic DNA Analysis. 2d ed. Boca Raton, Fla.: CRC Press, 2002. Uses very simple language to describe all the basic principles of DNA analysis in the forensic sciences. Includes a useful glossary.
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