How is DNA Testing Done

The entire DNA chain is incredibly long, much to long to examine all of it. Human DNA is made up of about 3.3 billion base pairs. The differences between DNA samples occur only in small segments of the DNA--the rest of the DNA is pretty much the same. DNA testing focuses on those segments that are known to differ from person to person.

As DNA testing has evolved over time, the testing methods have become more precise and are able to work with much smaller DNA samples. Early DNA testing was done using dime-size drops of blood. Today's tests can extract DNA from the back of a licked stamp. The DNA must be extracted from whatever sample is provided. DNA must be isolated and purified before it can be compared. In essence, it has to be "unlocked" from the cell in which it exists. The cell walls are usually dissolved with a detergent. Proteins in the cell are digested by enzymes. After this process, the DNA is purified, concentrated, and tested.

DNA testing is done most often today using a process called "short tandem repeats," or STR. Human DNA has several regions of repeated sequences. These regions are found in the same place on the DNA chain, but the repeated sequences are different for each individual. The "short" tandem repeats (repeated sequences of two to five base pairs in length) have been proven to provide excellent DNA profiling results. STR is highly accurate--the chance of misidentification being one in several billion.

Starting in 1996, the FBI established a national database known as CODIS (Combined DNA Index System). CODIS includes thirteen standard locations on the DNA chain. Genetic sequences at these thirteen locations (or loci) provide unique human identification.

DNA testing takes time and care. Real life DNA testing (as opposed to television DNA testing) usually takes at least three to five days.