DNA Testing Methods

Three methods are currently used to analyze DNA evidence. While these methods are very reliable, sometimes results cannot be obtained or are inconclusive if the quantity of the evidence is insufficient to analyze or if the evidence has been contaminated or improperly preserved. The technology used in analyzing DNA evidence is increasing in sophistication and in its ability to distinguish individuals, so it may be possible to test evidence in the future in ways that are not possible today.

The most common form of DNA analysis is called polymerase chain reaction (PCR). The development of PCR testing has greatly advanced the field of forensic DNA testing by increasing the success rate of the analysis of old, degraded, or very small biological evidentiary samples. PCR testing has allowed investigators to analyze evidence samples that previously could not be tested because the quality or the amount of starting material was insufficient for previous DNA analysis techniques.

The PCR process works by taking very small amounts of DNA from biological evidence and making millions of copies of it. This process, often referred to as PCR amplification, creates enough DNA to allow a laboratory analyst to generate a DNA profile. The process also allows laboratory technicians to analyze degraded biological material. A group of 13 different locations is used for the analysis of evidentiary samples and to generate DNA profiles from convicted offenders for the CODIS database.

Because of the capability of PCR testing to amplify very small quantities of DNA, extreme care must be exercised to prevent contamination when identifying, collecting, and preserving biological evidence. For this reason, investigators and laboratory personnel should always wear disposable gloves, use clean instruments, and avoid touching other objects when handling the evidence.

The other two methods used to analyze DNA evidence are restriction fragment length polymorphism (RFLP) testing and PCR testing on DNA from the mitochondria of the cell. RFLP testing usually requires a sample that has 100,000 or more cells (such as a dime-sized bloodstain) and contains DNA that is not degraded or broken into smaller fragments. RFLP has been widely used since the late 1980s and is able to exclude wrongly accused individuals. PCR testing on DNA from the mitochondria of the cell is conducted on samples that are unsuitable for RFLP or PCR nuclear DNA testing (such as dried bones or teeth, hair shafts, or samples that contain very little or highly degraded nuclear DNA). Mitochondrial DNA testing is available only in a limited number of laboratories primarily because of the time it takes to perform the tests.

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Understanding DNA Evidence: A Guide for Victim Service Providers
April 2001