There are two main ways DNA is used to solve crimes: the DNA gathered from a crime scene is compared to the DNA of potential suspects, or the DNA gathered from the scene is run through a database in hopes of finding a match. The use of DNA is an integral part of an investigation.
DNA collection techniques are very exact. Investigators must use gloves and special tools in order to prevent evidence contamination. Small amounts of DNA are taken from weapons found on the scene, doorknobs, windows and other surfaces. Once collected, the DNA is sent to a lab for analysis.
Different methods are used to analyze DNA, determine its pattern, and compare it to the DNA of a suspect. Short Tandem Repeat (STR) analysis focuses on how often base pairs repeat on a particular strand of DNA. Each person has their own number of STR's in their genetic composition. Using this information, analysts can compare the STR's in the DNA found at the scene to the DNA of a suspect to make a match.
After analyzing the data, the next step is to confirm the match. In general, DNA is used to either include or exclude a suspect. If the profile from the crime scene matches the profile from the subject, then the suspect is included as a possible source of the DNA. From there, further testing is done to get an exact match. In cases where there are multiple suspects, DNA matches can be built on exclusion. If five men are suspected in a crime, and the DNA profiles do not match four of them, then they are excluded as possible perpetrators.