Even more than a finger's print, each person's "DNA fingerprint" is unique. While old-fashioned fingerprints record the wavy, whorled ridges on fingertips, DNA fingerprints record the chemistry of small sections of DNA in chromosomes. A genome -- a complete set of human chromosomes -- is a string of DNA with about 3 billion chemical modules, or bases, divided into roughly 3,000 genes. If you think of the genome as a giant reel of videotape, each gene corresponds to one TV show. The thousands of bases in a gene correspond to thousands of thrilling seconds of comedy or drama, in our analogy.
DNA stores information in a four-letter code: Each base, or nucleotide, is labeled C, G, A or T. Four letters may seem primitive compared to our 26-letter alphabet, or our 10-digit counting system. Still, the DNA in every one of your cells codes for all of the thousands of proteins you need from conception to death.
Each position on the long molecule can store twice as much data as one bit of computer code, and that only improves once you consider multiple locations. While three bits of binary code can store 8 (2 to the third power) unique combinations, three
bases of DNA can record 64 (4 to the fourth power) combos. Three is a magic number, since three adjacent bases specify one amino acid, the building block of proteins. (DNA's entire job is to serve as a pattern for proteins.)
Making sense of nonsense DNA
Having read this far, you're probably expecting a discussion of genes and proteins, but as they snarl on NYPD Blue, "Fugeddaboudit, buddy." DNA fingerprinters ignore the DNA in genes in favor of "junk DNA" between the genes.
(Here's a good place to distinguish DNA fingerprinting from DNA sequencing, the decoding of genetic instructions for humans or other species. DNA fingerprinting is much simpler because it looks only at short strands of DNA, in places where one person will likely vary from another.)
Those places are called "junk DNA," or "filler DNA" or "nonsense DNA." Technically, these "introns" separate the "exons," which serve as protein patterns.
This poorly understood junk is generally considered a heckuva lot less interesting than the DNA that codes for proteins. We like to compare nonsense DNA to the ads that foul up sitcoms, say, or cop shows. Just as most people ignore those ads, most geneticists ignore junk DNA; the Human Genome Project, for example, has spent billions reading the sequence of DNA on genes but largely ignores the nonsense.
But DNA fingerprinters are obsessed with junk.
Here's why. Genes make proteins, but proteins work only when patterned on a good DNA template. So the DNA in genes must stay essentially the same generation after generation. Changes are called mutations, and most mutations are harmful.
Say you had a tissue sample from a 1962 strangling in Boston and decoded the bases in the gene that makes serotonin (a molecule that helps nerve cells communicate). The gene would be almost identical to your own serotonin gene -- or O.J. Simpson's, for that matter.
The reason? Most genes for defective serotonin are (like sitcoms that flame out for being too intelligent in the pilot stage), immediately culled from the gene pool. But the genetic similarity wouldn't make you -- or the juicy guy either -- the Boston strangler, who murdered 11 women.
Once was boring enough!
Fortunately, junk DNA is exempt from the need to remain identical. Like an ad on Ally McBeal, it can change, and nobody will notice much.
The characteristic change in junk DNA is called a "repeat." In repeats, short sections are duplicated. In repeats, short sections are duplicated. Like this. Like this.
If a TV ad underwent a repeat, "What_do_you_want_the_Internet_to_be?" would become "What_do_you_want_the_Internet_to_be? What_do_you_want_the_Internet_to_be? What_do_you_want_the_Internet_to_be? What_do_you_want_the_Internet_to_be?"
Terrifying. In Latin, that's called ad nauseum.
In English, it's called boring -- and too familiar -- but who's paying attention?
Similarly, in junk DNA, the sequence "CGGT" might form the repeat "CGGT CGGT CGGT CGGT CGGT CGGT." But since the protein-patterning apparatus ignores junk DNA, the repeat would likely be meaningless.