Untangling the helix: law enforcement and DNA.

Author:Wilson, Robin S.

DNA evidence is one of the most powerful crime-fighting tools since the advent of latent fingerprint technologies. It has the ability to convict the guilty and free the innocent. The exoneration of 28 individuals whose cases are detailed in the National Institute of Justice (NIJ) publication, Convicted by Juries, Exonerated by Science: Case Studies in the Use of DNA Evidence to Establish Innocence After Trial, prompted U.S. Attorney General Janet Reno to establish the National Commission on the Future of DNA Evidence. The Commission will make recommendations to maximize the value of DNA evidence in the criminal justice system. After more than a year of meetings with criminal justice professionals, policymakers and scientists, the commission recognizes that, to fully realize the potential of DNA as criminal evidence, law enforcement will need to have the capability of using DNA evidence at every level, from the crime scene to the courtroom.

What is DNA?

DNA, or deoxyribonucleic acid, a double-helix molecule found in the nuclei of cells, is the basic building block of life. based on the arrangement of four chemicals, called bases, our DNA determines each of our individual characteristics. The arrangement of the three billion pairs of bases in each DNA molecule is different for everyone (except identical twins). An individual's DNA is the same in every cell, from the moment of conception to death. A person's DNA in scraped skin cells will be the same as the DNA in his blood, saliva, organs, semen or hair.

Solving Crimes With DNA

Currently, the majority of criminal cases use DNA either to confirm or exclude a suspect as the source of evidence left at a crime scene. Because a person's DNA is the same in every cell, biological evidence from a crime scene can be compared to known samples from those involved in or suspected of a crime. Once a suspect is identified, a blood or cheek swab sample is collected and sent to the laboratory with the crime scene evidence for DNA analysis. The laboratory analyst compares the DNA profiles from the evidence and the suspect to determine if there is a match. Blood and semen are the traditional sources of DNA evidence, but law enforcement officers around the country are discovering that other evidence can be analyzed for DNA: saliva left on a rape victim, chewed tobacco or gum left behind at a murder scene, mucous on clothing or tissues, or skin cells left on the end of a weapon used to deliver forceful blows. DNA evidence must be properly identified, preserved and collected at the crime scene to maintain its integrity and to maximize its value as an investigatory tool. NIJ is trying to establish acceptable procedures for the safe handling of evidence. Additional training of law enforcement personnel may help to ensure that proper procedures are followed.

Traditionally, DNA has been used to develop criminal cases and, during the...

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