User:Trèspacifique

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.