Philadelphia Inquirer
Thursday, April 25, 2002

Unique codes held in DNA

By Faye Flam,
INQUIRER STAFF WRITER

The coiled molecules of DNA in human cells carry a unique chemical code that can match a trace of blood, semen, skin or hair to the person who left it.

DNA (deoxyribonucleic acid) carries its code in four characters denoted by the letters A,T, C, and G. But no microscope is powerful enough to see how this code is arranged on a given DNA molecule, so science must use more indirect methods to read it.

Forensics laboratories use several different methods for determining whether two samples indeed carry identical stretches of code.

In the method used in the O.J. Simpson case, scientists take samples of DNA and add a chemical called a restriction enzyme. The enzyme cuts the DNA every time it sees a particular combination of code letters.

The fragments of DNA that come out of the test form a specific pattern of sizes depending on the code they hold. If two samples match after being cut up 10 different ways, then scientists say they have near certainty that they came from the same person.

Recently, scientists have developed a much faster way to do essentially the same thing. The cut fragments are labeled with a fluorescent marker and placed on something called a DNA chip. The fragments pass through a channel in the chip, past a laser that records their various sizes.