How DNA testing works - Реферат

HOW “DNA” TESTING WORKS The tools for solving rapes and murders have improved rapidly. Five years ago DNA tests couldn’t 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: 1) 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 suspect’s samples match. 2) 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. 3) 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. 4) 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. 5) Compare the crimescene samples with the suspect’s. Scientists say it’s virtually impossible for unrelated people to match up perfectly on 13 different levels. If samples do, odds that they’re 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 wasnt 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. Thats when the Chicago Tribune published a lav­ishly researched series explaining why so many capital cases were suspect. The Trib­une’s 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. Whats 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 wont be any more deaths, at least while hes governor. I believe there are cases where the death penalty is appropriate, Ryan said. But weve 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: Race: The role of race and the death penalty is often misunderstood. On one level theres the charge of institutional racism: 98 percent of prosecutors are white, and, according to the NAAGP, Legal Defense Fund they are much more likely to ask for the death penalty for a black-on-white crime than when blacks are the victims. Blacks convicted of major violent offenses are more likely than white con­victs to end up on death row. But once they get there, blacks are less likely than white death-row inmates to he executed because authori­ties are on the defensive about seeming to target African-Americans. The re­sult is both discrimination and reverse discrimination - with deadly consequences. The risk of errors: The more people on death row, the greater chance of mis­takes. There are common ele­ments to cases where terrible errors have been made: when police and prosecutors are pressured by the community to solve a notorious mur­der; when theres no DNA evidence or reliable eyewitnesses; wnen the crime is es­pecially heinous and draws large amounts of pretrial publicity; when defense at­torneys have limited re­sources, if authorities were particularly vigilant when these issues were at play, they might identify problematic cases earlier. Deterrence: Often the first argument of death-penalty supporters. But studies of the subject are all over the lot, with no