How 'DNA' testing works

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The tools for solving rapes and murders have improved rapidly. Five years ago DNA tests couldnt link suspects to hair or semen found on a victim. Today a crime lab can identify unique DNA patterns in a tiny sample of just 100 to 200 cells. The steps scientists take to implicate or exonerate suspects:

Collect biological materials from the crime scene and the suspect under investigation, such as blood, hair, semen or saliva. Every cell is a unique library of DNA sequences. The goal is to find out if the forensic and suspects samples match.

Isolate pure DNA by mixing the sample with chemicals that break down other cellular material. DNA molecules consists of paired filaments that interlock like zippers, and each filament is made up of chemicals bases (A, C, T and G) aligned in unique sequences.

Amplify the DNA by separating paired filaments and mixing them with short fragments known as primers. When a primer locks onto a particular site on a sample DNA molecule, it triggers production of a longer fragment that matches a piece of the sample.

Segregate the resulting DNA strands. A sample mixed with 13 primers multiplies into millions of distinctive molecules. Exposed to an electrical current, the molecules a sorted into color-coded bands on a gel.

Compare the crimescene samples with the suspects. Scientists say its virtually impossible for unrelated people to match up perfectly on 13 different levels. If samples do, odds that theyre from one person are overwhelming.

Helped prove the innocence of Anthony Porter, who at one point had been just two days shy of lethal injection for a pair of 1982 murders. Once again, the issue in Illinois wasn't the morality of death sentences, but the dangerously sloppy way in which they were handed out. Once again a confession from another man helped erase doubt that the man convicted of the crime, who has an IQ of 51, had committed it.

By last fall the list of men freed from death row in Illinois had grown to 11. That's when the Chicago Tribune published a lavishly researched series explaining why so many capital cases were suspect. The Tribunes digging found that almost half of the 285 death-penalty convictions in Illinois involved one of four shaky components: defense attorneys who were later suspended or

disbarred, jailhouse snitches eager to shorten their own sentences, questionable "hair analysis" evidence or black defendants convicted by all-white juries. What's more, in the weeks after those stories appeared, two more men were freed from death row. That pushed the total to 13 - one more than the number of inmates Illinois had executed since reinstating the death penalty in 1977.

The Porter case and the Tribune series were enough for Governor Ryan. On Jan. 31, he declared a moratorium on Illinois executions, and appointed a commission to see whether the legal process for handling capital cases in Illinois can be fixed. Unless he gets a guarantee that the system can be made perfect, Ryan told NEWSWEEK last week, "there probably won't be any more deaths," at least while he's governor. "I believe there are cases where the death penalty is appropriate," Ryan said. "But we've got to make sure we have the right person. Every governor who holds this power has same fear I do.

But few are acting on it. In the wake of the Illinois decision, only Nebraska, Maryland, Oregon and Nrw Hampshire are reviewing their systems. The governors of the other states that allow the death penalty apparently think it works adequately. If they want to revisit the issue, they might consider the following factors:

: 7/01/2010