Recent story about Colin Pitchfork (first person convicted of rape using DNA analysis) got me wondering how this works. How is DNA coded in a way which allows you to match it up with another sample? When you look at it under a microscope or something there aren’t exactly letters and numbers for each part of it.
In: Biology
The most common technique is to use various enzymes that split up the genes at specific points. Then the genes are forced through a gel by an eletrical field. Shorter genes go through the gel faster then the longer genes. If you power it off when the genes are half way through the gel and then use some fluorizing light to identify the genes they will form certain bands depending on their lengths. The theory here is that different genetic materials will be different enough that it is cut in different locations and therefore be of different lengths. By using different enzymes you get a different pattern again. Each set of DNA gives a unique fingerprint that you can compare.
It should be noted that this system is not perfect by any means. Even though no two non-twins have the same DNA this test can make two different sets of DNA look the same by pure chance, just like two fingerprints may look very similar to each other just by chance. These tests also use DNA amplification techniques first so it is possible that a small sample like a single dead skin cell may just happen to be the one to get amplified and tested against. So a DNA match might mean that the suspect were on the same subway as either the victim or the criminal within the last few days.
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