Opinion ID: 7918859
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Heading: The Science of DNA

Text: Deoxyribonucleic acid or DNA is the genetic material that provides the instructions for all human characteristics, from eye color to height to blood type. P. Hartman & S. Susskind, Gene Action 2 (1965). Many types of cellular material carry DNA, including some types of blood cells, semen, and hair follicles. R. Lewontin & D. Hartl, Population Genetics in Forensic DNA Typing, 254 Science 1745, 1746 (1991). DNA is a “double helix” molecule, similar to a spiral staircase or a twisted rope ladder. Lee et al., supra, at 270. Each strand or “side” of the ladder is composed of four types of building blocks known as nucleotides, which can be connected in any order to form a DNA chain. Id. at 270-71. It is the sequence of the nucleotides that conveys the information, in effect “spelling out” the genetic instructions. G. Beadle & M. Beadle, The Language of Life 193-94 (1966). A strand of DNA contains an estimated 50,000 to 100,000 genes, each of which directs the construction of a specific protein. Hartman & Susskind, supra, at 37. In addition to this “meaningful” DNA, the chain also includes “spacer” or “junk” DNA between the genes. The total amount of DNA composing all of an individual’s genetic information includes over three billion individual nucleotides, and a typical gene for an individual characteristic may be made up of tens of thousands of nucleotides. Lee et al., supra, at 270. While each individual’s total DNA profile is unique, large segments of DNA are common to everyone. Out of the three billion nucleotides making up a complete DNA strand, there will likely be about three million differences in the DNA sequence between two randomly selected individuals. NRC Report, supra, at S-7. Many of these differences will be found in the “spacer” DNA areas, particularly in the number of times a spacer sequence is repeated. These highly variable areas in the DNA strand are known as VNTR’s, for “variable number of tandem repeats.” Typically, a VNTR will contain between twenty and one hundred repeats of the same nucleotide sequence. See Lee et al., supra, at 272 (Fig. 4); Lewontin & Hartl, supra, at 1745-46. In criminal investigations, DNA profiling is typically used to compare a suspect’s DNA with a sample of DNA taken from the crime scene. DNA profiling does not compare every nucleotide of the suspect’s DNA with every nucleotide of the sample DNA, but rather compares the two at selected sites that are likely to vary from person to person. It is possible, however, that sections of DNA taken from different people will match. To avoid this type of “random matching” error, comparisons are made at multiple sites or loci along the DNA chain. Typically, laboratories analyze four or five loci in conducting DNA comparisons, reducing the probability of random matches across all loci to a low level. See L. Roberts, Fight Erupts Over DNA Fingerprinting, 254 Science 1721, 1721-22 (1991). 8