DNA Profiling

By Jones, Phillip | Law & Order, August 2004 | Go to article overview

DNA Profiling


Jones, Phillip, Law & Order


The DNA analysis results disappointed the Leicestershire constabulary. Dr. (now Sir) Alec Jeffreys, a Research Fellow at the University of Leicester, had performed his new technique to compare DNA extracted from a suspect's blood sample with DNA from a killer-rapist's semen. The DNA profiles didn't match. But the police devised another tactic to take advantage of the new technique: flush out the killer by collecting blood samples from the local male population for DNA analysis. It worked.

As chronicled in Joseph Wambaugh's book, The Blooding, a bakery manager learned that a coworker had donated blood for an acquaintance, a man named Colin Pitchfork. The manager informed the police about the switch, and they arrested Pitchfork.

Interrogation yielded a confession. Later, Jeffreys' analysis confirmed that Colin Pitchfork's DNA and the killer-rapist's DNA shared identical profiles.

Almost 20 years ago, this case marked the first use of DNA testing in a criminal investigation. Since then, advancements in molecular biology and computer technology have forged DNA analysis into an essential tool of law enforcement.

DNA Typing: Analyzing the Nucleotides

In the original DNA typing method, Alec Jeffreys digested DNA samples with a restriction enzyme that cleaves DNA at a specific nucleotide sequence. The enzyme's target nucleotide sequence occurs many times in a cell's DNA, so enzyme digestion produces a collection of DNA fragments. After enzyme treatment, DNA fragments are sorted by size, producing a series of lines that looks like a bar code.

Since nucleotide sequences vary among the genomes of individuals, the number and length of DNA fragments acquired by enzyme digestion also vary from person to person. Consequently, these DNA bar code patterns can distinguish one individual from another.

The Colin Pitchfork case garnered international attention, and DNA typing was hailed as the most significant development in forensic science since fingerprinting. A modification of Jeffreys' Restriction Fragment Length Polymorphism (RFLP) test became the first accepted protocol in the United States for forensic characterization of DNA. The RFLP technique made U.S. headlines after the Federal Bureau of Investigation used it to analyze stains on Monica Lewinsky's infamous blue dress.

But RFLP analysis has a drawback for forensic scientists: the technique requires large amounts of intact DNA. This requirement can exclude DNA samples from old evidence or thwart DNA analysis if a crime scene yields only minute amounts of biological material.

Today, most crime labs use a DNA typing system that takes advantage of the polymerase chain reaction (PCR), a technique that duplicates short segments of DNA. Human DNA contains conserved regions and variable regions. In the conserved regions, nucleotide sequences are the same or similar among individuals, whereas variable regions have nucleotide sequences that vary among individuals.

PCR is used to make copies of the variable regions to create a DNA profile. Using a PCR-based technique, a scientist can copy a DNA target sequence one million times or more, and as little as one-billionth of a gram of DNA may be required for analysis. Since PCR targets small segments of a cell's DNA, the technique can be used with partially degraded samples.

The PCR technique's impact on forensic DNA analysis is reflected by the revolution in biological evidence collection practice. Traditional biological samples include tissues and fluids, such as blood and semen. Now, PCR enables DNA analysis with biological samples from dental molds, cigarette butts, beverage cans, eating utensils, chewing gum, postage stamps, ski masks, licked envelopes, toothbrushes, razor shavings, band aids and clothing. Even fingerprints can contain sufficient DNA for genetic profiling. As the National Institute of Justice proclaims, DNA evidence can be collected from virtually anywhere. …

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DNA Profiling
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