DNA profiling and matching of physical data, such as fingerprints, are used in solving all crime types ranging from housebreaking and car crime to assaults, murder and rape. The forensic scientists will look for suitable samples at a crime scene, examining such items as weapons, clothing, hair or anything else from which they can obtain body cells for DNA profiling, or fingerprints or "marks" for use in fingerprint matching.

Before the DNA can be analyzed, it must be extracted from the cells and purified. The cell and nuclear membranes need to be broken up to allow the DNA to be free in solution. Once the DNA is free, it can be separated from all other cellular components. After the DNA has been separated in solution, the remaining cellular debris can then be removed from the solution and discarded, leaving only DNA.

DNA is collected from cells and cut into small pieces using a restriction enzyme. This generates DNA fragments of differing sizes as a consequence of variations between DNA sequences of different individuals. The fragments are then separated on the basis of size using gel electrophoresis. The separated fragments are then transferred to a nitrocellulose or nylon filter; this procedure is called a Southern blot. The DNA fragments within the blot are permanently fixed to the filter, and the DNA strands are denatured. Radiolabeled probe molecules are then added that are complementary to sequences in the genome that contain repeat sequences. These repeat sequences tend to vary in length among different individuals and are called variable number tandem repeat sequences or VNTRs. The probe molecules hybridize to DNA fragments containing the repeat sequences and excess probe molecules are washed away. The blot is then exposed to an X-ray film. Fragments of DNA that have bound to the probe molecules appear as fluoresent bands on the film. The Southern blot technique requires large amounts of non-degraded sample DNA. PCR greatly amplifies the amounts of a specific region of DNA.

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Journal of Forensic Toxicology and Pharmacology

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