In North Carolina, DNA has played a key role in releasing innocent prisoners. After being held for 19 years, Darryl Hunt was freed in December 2003 and later pardoned by the governor after tests showed that Hunt's DNA did not match evidence at the Winston-Salem crime scene where a woman was raped and murdered in 1984. DNA also cleared Ronald Cotton, who was released in 1995 after serving 11 years for a rape and robbery he did not commit.

Q. What is DNA?

A. Deoxyribonucleic acid, or DNA, is a thin, chainlike molecule found in every living cell on Earth. It directs the formation, growth and reproduction of cells and organisms.

Q. How does DNA analysis work?

A. A DNA profile is created first by collecting a sample of an individual's cells, often from blood, tissue or saliva. In the Duke case, it was taken from cheek swabs. A DNA molecule is removed from a cell, purified, and then cut and processed to reveal a unique pattern. That profile is then compared with a DNA sample taken from a crime scene.

Dean A. Wideman, a forensic biologist and independent expert who performed forensic DNA analysis for state crime labs in New York City and Houston, and Bruce Weir, chairman of the University of Washington's department of biostatistics and a former N.C. State University professor of statistics and genetics, discuss how DNA works and what it can mean:

Q: Could a woman be sexually assaulted with the perpetrator leaving no trace of his DNA?

WIDEMAN: "That's not impossible. There are a lot of factors that go into whether you get negative DNA results, like there's no ejaculate. It depends on what activities the female did after the incident and before the evidence collection. If she went to the restroom, showered or changed clothes, then that's another reason you may not find anything. It also depends on how evidence was collected. ... It does not mean [a rape] did not occur. ... If they found no semen, you can conclude that it wasn't there or it just was never collected properly or it was so low an amount it wasn't detected through analytical work. It also depends on how well [the tester] swabbed and where they swabbed and how much of the swab was cut to do the test -- whether they used every bit of evidence and if they made a thorough collection."

Q: If the perpetrator used a condom, would that block his DNA?

WIDEMAN: "If they did, you could find traces of the chemicals that are on condoms on the swab. You can't immediately deduce that because you didn't find something, it didn't occur. ... What's interesting to me is if they found anything foreign to her."

Q: Can a prosecutor get a rape conviction without DNA evidence?

WIDEMAN: "There could be -- with bruises on her neck, arms or legs."

Q: What else are you curious about?

WEIR: "But what did they find? Did they find only from the victim or is there anybody else's DNA there?"

(COMPILED BY STAFF WRITER CINDY GEORGE AND NEWS RESEARCHER LAMARA WILLIAMS-HACKETT)

Get it all with convenient home delivery of The News & Observer.