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Dusting for DNA

DNA profilino is based on the discovery that the DNA of one person differì from that of another in specific ways. The FBI analyses 13 places on a person’s DNA to produce a DNA profile and find whether it matches that of a known criminal. Here are the steps in the process.

1.Collection: blood,semen,saliva,skin or hair is labelled and shipped to a forensic lab. Only minute amounts-a single hair root,for example-are required.

2.Isolation:the sample is mixed with detergent and enzymes,which break open the cells and let out their DNA. The cell fragments are removed, and the remaining mixture is spun in a centrifuge tube. That makes pure DNA settle to the bottom.

3.Amplification:the DNA, a double helix, is separated into two strands. Technicians add 26 short pieces of DNA,called primers:sequences of the chemicals C,A,T and G that link to the beginning and end of each of the 13 sites.

4.Replication: when a primer attaches to the beginning of one of the 13 sites,it acts like the “start” button on a photocoopying machine,turning on cellular machinery that makes 1 million copies or more of each site.

5.Identification:copies of the 13 sites,each about 100 to 600 chemical letters long,are separated by size through gel electrophoresis. In this process a drop containing millions of DNA fragments is placed at one end of a sheet of gel. Electric current pulls the fragments across the gel; the larger a fragment, the slower it moves. The fragments, tagged with dye,show up as colored bands under ultraviolet light.

6.Matching: the crime lab feeds the data on the length of the 13 markers into a database. The computer searches for a match. The odds are trillions to one that the length of each of the 13 strands in one person is identical to all the lengths in another.