Tuesday, February 27, 2007

Criminal Profiling Topic of the Day: “DNA Evidence”

Almost everyone is familiar with the term “DNA evidence” used in criminal investigations. But just what is DNA, and why is it used for evidence?

DNA is found in the cells of every living thing. In humans, a DNA molecule contains 23 pairs of chromosomes. One chromosome from each pair comes from your mother, and the other comes from your father. Because no two individuals’ DNA is exactly alike (except for identical twins), it can be used to identify perpetrators and victims of a crime, and it can link them to each other or to a specific crime scene. DNA extracted from the cells of blood, body fluids such as saliva or semen, or tissue are some of the samples that can be used for identification of an individual. Since a single piece of DNA is too small to be analyzed, a process called Polymerase Chain Reaction, or PCR, allows us to make enough exact copies of the same piece or sequence of DNA until we have a sample large enough to identify the person it came from. This is referred to as DNA amplification and output analysis.

In PCR DNA amplification, several types of materials and equipment are necessary to copy a specific segment of DNA. In the first step, a DNA mixture is created by adding materials to the extracted DNA and applying heat to unwind and separate the two strands of the ladder like structure of the DNA double helix. The “rungs” of the ladder are complimentary paired together in a sequence unique to only that particular piece of DNA so that each strand then becomes a template used to make an exact copy of its matching strand. Approximately 3 million base pairs of DNA vary from person to person. The remaining steps consist of heating, cooling, and adding enzymes and other materials to the DNA mixture in order to synthesize the correct complimentary base pairs, creating a new corresponding strand that will bind with each of the original separated strands. This cycle is repeated approximately 30 times with each cycle doubling the number of template DNA produced from the previous cycle. Now there is enough DNA in the PCR output to examine and identify as belonging to a specific person. This is done by injecting the amplified DNA into small wells containing a special gel and applying electrical current. This causes the DNA fragments to be separated by size, which forms distinctive bands on the gel. The exact sequence of the bands produced by PCR from the target DNA is then compared to the DNA sequence extracted from a separate sample known to belong to a particular person.
Besides the valuable contribution DNA amplification and output analysis has made to forensic science, it is responsible for significant advances in other scientific areas as well, such as to diagnose inherited disorders, treat disease, and genetically modify crops.

Donna Weaver

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