Your DNA contains information on how to build the many thousands of different types of proteins that build your body and make it work. The information to build those proteins is contained in regions of DNA called genes. But your DNA also has regions that aren't genes. Some of those regions are similar enough in all people so that they can be analyzed the same way, but different enough so that everyone's DNA profile is unique. Any tissue with cells that contain DNA can be used to create a DNA fingerprint.
Short Tandem Repeats
Short tandem repeats, or STRs, are non-coding sections of DNA. Non-coding sections of DNA are also called introns. STRs are useful because of two important characteristics. First, each STR consists of a repeated set of three- or four-base sequences. For example, one could be guanine-adenine-thymine-adenine (GATA) repeated say, four times. The number of times an STR sequence is repeated is one of your personal characteristics. The second important characteristic is that the start and end of an STR is the same in everyone. That is, while you might have three repeats and your brother might have 11 repeats, you'll both have the same DNA sequence at the beginning and end of that STR.
If one of your STRs has a length of six repeats, that will always be the measurement your DNA gives, but your neighbor might also have six repeats of that specific STR. But if someone analyzed 13 different STRs from your DNA, it would be extremely unlikely that anyone else would have the exact same pattern. It's even more specific, though, because you inherit DNA from both parents. So you'll have two lengths for every STR. So you could have one STR where your pattern is 3, 9; another with 6, 8; and another with 2, 13. That makes the chances of finding an unrelated person with your same pattern very small.
The process of measuring STRs starts with gathering some cells. The next step is dissolving the cell walls and separating out the DNA. If necessary, extra copies of your DNA are made. Then the DNA is snipped into the STR regions by a kind of molecular scissors called a restriction enzyme. Restriction enzymes work by cutting DNA strands at a particular sequence. Since everyone's STRs have the same beginning and ending sequences, the same restriction enzyme can be used to cut out everyone's STRs. Then the STRs are "weighed" in a technique called gel electrophoresis. The weights of the whole set of STRs is the DNA fingerprint.
The Right Tissue
Since all you need to produce a DNA fingerprint is some DNA, any tissue with cells that have DNA will do the job. Almost every tissue in your body will do -- red blood cells are just about the only cells that won't work. Hair follicles, white blood cells and random cheek cells that make their way into saliva are common sources of DNA, but skin cells, muscle tissue even something like liver cells will work. A tiny sample of any of those sorts of tissues will do, because a common lab technique can make many copies of a DNA sample.
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