Unlike the incomplete data provided by fingerprints, blood types, and protein profiles used as evidence in criminal investigations, DNA provides a much stronger case against suspects. DNA can be found in a variety of sources often present at crime scenes, including semen, blood, hair, saliva, and cells from underneath the fingernails. The DNA found in substances such as hair or bone is called mitochondrial DNA. It is not as useful as DNA found in the nucleus of a cell, but can still be useful when, for example, studying fossilized creatures. Since DNA is absolutely unique to each person (except when looking at that of twins), it can be used to catch criminals as well as keep the innocent out of jail. DNA is also useful because it takes such a long time for it to decay.
Though miniscule biological samples are enough to find a sufficient amount of DNA, a polymerase chain reaction can be used to create a large number of copies of a sample so that the DNA can be analyzed. This is done by using restriction enzymes-used to "chop the DNA up"- to find certain sequences in the DNA. These are then arranged by how long the chains are and what their charges are. With a certain treatment, the DNA can be placed on nylon, creating “DNA bands” similar to a bar code. These DNA fingerprints can be compared side to side, for example placing the DNA fingerprints of a suspect next to samples found at a crime scene.
It's interesting to read about what they're really doing on those crime shows. It sounds like it's not as easy as plopping the samples in a machine and letting the computer do all the hard work.
Baird, Colin.Chemistry in Your Life. W.H. Freeman and Company. New York, 2006.
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