DNA is the blueprint that encodes all the data for building a human body, along with instructions on how it should operate. Every cell in a person's body contains a copy of this DNA. Most human DNA doesn't differ from person to person, but there are about 3 billion base pairs (pairs of amino acids that form the base of the gene) that do vary. This sounds like a lot, but that's only 1/100 of the entire genome.
DNA typing is based on an unusual feature found in the human genome. There are multiple copies of certain short sequences, 3 to 30 base pairs long, that are repeated one after another as many as 100 times. These groups of repeat sequences are widely scattered through the genome. Everyone has these repeat units, but the number varies from person to person. Only identical twins will have the same numbers and patterns of these sequences. These genetic data aren't instructions to make anything; scientists think they might exist to get mixed up in the regular genes and provide some variety for evolution.
First, DNA is isolated from a sample, such as blood, saliva, semen, tissue or hair. Sometimes samples must be treated to clean them up, since dirt and other debris can contaminate them – particularly if they have been lifted from a crime scene. A special kind of bacterial enzyme is then used to cut up the large DNA genome into shorter fragments ranging from 100 to more than 10,000 base pairs long. The fragments are then sorted by size. This is done by putting them into a slab of material resembling Jell-O and applying electricity. DNA carries a negative charge and is pulled through the gel towards a positively charged electrode.
The separated DNA fragments in different samples are then compared. But finding just one matching data point isn't enough for positive identification, any more than having one digit of someone's social security number will enable you to discover their identity. That's why the FBI insists on 13 different probes, each targeting a different site on the genome. By that standard, apart from identical twins, a person is 2,000 times more likely to win the Publisher's Clearinghouse sweepstakes than to have a DNA profile that matches anyone else.
The most common DNA sequencing techniques require large amounts of high-quality DNA. This has become much easier now that scientists have discovered how to amplify small samples using something called the polymerase chain reaction (PCR). They can make as many as one billion copies of DNA in about three hours.