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 consist of paired filaments that interlock like zippers, and each filament is made up of chemical “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 are sorted into color-coded bands on a gel.
5 Compare the crime-scene 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.
