As has already been mentioned here at Forensics FAQ, there are many reasons for performing DNA testing.

These include the following -

1. Linking a suspect to a crime scene
2. Establishing paternity or maternity
3. Proving a sibling or half sibling relationship
4. Determining genealogical roots or ethnic origin

The DNA testing methods employed vary and depend largely upon the types of results being looked for as well as the types of samples that are available.

Complex

The entire DNA chain is far too long to examine completely - human DNA, for example, has about 3.3 billion base pairs. However, most of the DNA sequence is common between humans. Therefore, only the smaller, variable part, is used within the testing framework.

The earliest forms of DNA testing required a sample of blood the size of a coin. As processes have evolved and developed over time the required sample size has shrunk considerably, such that a DNA sample can now be taken from traces of saliva on the back of a postage stamp that has been licked.

Before a DNA comparison can take place the DNA must be isolated and purified from the host sample, i.e. removed from the surrounding cell into which it has been placed. This is achieved by disolving the cell walls with a detergent, digesting proteins in the cell with enzymes. Following this the DNA can be purified, concentrated and tested.

Short Tandem Repeats (STR)

The most common DNA testing method today is STR - short tandem repeats.

Human DNA comprises several regions of repeated sequences. These are always located at the same place in each chain, however, they vary from individual to individual. Short tandem repeats are repeated sequences of two to five base pairs in length. Their use in DNA profiling has proven to be excellent with chances of misidentification being one in the billions.

The CODIS database set up by the FBI holds thirteen standard locations of the DNA chain per individual to aid in identification.

Incidentally, DNA testing is not as quick or as straightforward as the forensic shows on television portray it to be - testing typically takes as long as three to five days.