Knowledge of DNA and laboratory procedures will help Officers of the Court prepare for and participate in criminal cases involving DNA evidence. Without DNA evidence, the cold case depicted in the case study would likely not have been reopened.

It is important to understand how proper collection and storage techniques can affect DNA evidence and how results of DNA testing can impact the outcome of a criminal case.

DNA Defined

All organisms are made up of cells. A human has about 100 trillion cells; all of them (except for red blood cells) contain genetic material known as deoxyribonucleic acid (DNA). DNA is sometimes called a "genetic blueprint" because it contains all of the instructions that determine an individual's genetic characteristics.

Within an individual, the DNA profile obtained from all tissue types (blood, saliva, tissue, hair, semen, etc.) will be the same.

DNA is used to differentiate individuals, but nearly all human DNA is the same; less than 0.1% is highly variable. This variability is referred to as polymorphism. These highly variable regions of the DNA (those with a high degree of polymorphism) are tested in forensic DNA analysis.

In the case study, critical biological material was present at the scene of the crime (the rape and murder), even though much of it was not visible to the naked eye. This material, which was carefully collected and stored, became key evidence for the reopening of the case and the filing of formal charges against a former person of interest.

Nuclear STR Testing

The most common method used for nuclear DNA testing is called short tandem repeat (STR) technology. There are segments of DNA that have sequences that repeat in short, adjacent, or tandem segments. STR testing has a very high degree of discrimination and can be used on partially degraded, small-quantity DNA evidence.

Mitochondrial DNA Testing

Mitochondrial DNA can be useful for samples that either do not contain a sufficient quantity of nuclear DNA or are not of sufficient quality for nuclear DNA testing. Shed body, head and pubic hairs with no cellular material (hair follicles) and aged skeletal remains are the samples that are most commonly analyzed for mtDNA.