Opinion ID: 1160857
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Heading Rank: 1

Heading: DNA Theory and RFLP Analysis

Text: Virtually each one of the trillions of cells in the human body, with the exception of red blood cells, has a nucleus containing the DNA that underlies the person's entire genetic makeup. [7] The DNA is organized into 23 pairs of homologous chromosomes, 1 chromosome in each pair being inherited from the mother and the other from the father. (1996 NRC Rep., supra, pp. 60-61.) A chromosome is a long DNA molecule in the shape of a spiral staircase. (1992 NRC Rep., supra, p. 33.) It consists of two parallel spiral sides (i.e., a double helix) composed of repeated sequences of phosphate and sugar. The two sides are connected by a series of rungs, which constitute the steps in the staircase. Each rung consists of a pair of chemical components called bases. There are four types of bases  adenine (A), cytosine (C), guanine (G), and thymine (T). A will pair only with T, and C will pair only with G. ( Barney, supra, 8 Cal.App.4th at p. 805, 10 Cal.Rptr.2d 731.) There are over 3 billion base pairs in the 46 chromosomes of a single human cell. [8] When a cell reproduces, the parallel sides, or strands, of its DNA separate, and the bases of each strand pair off with the complementary bases of a new strand. (1996 NRC Rep., supra, p. 63.) A person's individual genetic traits are determined by the sequence of base pairs in his or her DNA molecules. That sequence is the same in each molecule regardless of its source (e.g., hair, skin, blood, or semen) and is unique to the individual. Except for identical twins, no two human beings have identical sequences of all base pairs. [¶] In most portions of DNA, the sequence of base pairs is the same for everyone. Those portions are responsible for shared traits such as arms and legs. In certain regions, however, the sequence of base pairs varies from person to person, resulting in individual traits. A regionor locusthat is variable is said to be polymorphic. ( Barney, supra, 8 Cal.App.4th at pp. 805-806, 10 Cal.Rptr.2d 731.) The DNA sequences that determine a person's genetic traits are contained in the 50,000 to 100,000 genes making up his or her genetic code. (See 1992 NRC Rep., supra, p. 33.) Human DNA also includes other sequences that are noncoding, i.e., they serve no known genetic function. Compared to the genes, the noncoding sequences are more likely to be polymorphic since their individual variation is less constrained by forces of selection. ( Id. at p. 34; 1996 NRC Rep., supra, p. 63.) Because there is no practical way to sequence all three billion base pairs in a person's DNA, forensic scientists seek to identify individuals through variations in their base-pair sequences at polymorphic DNA locations (loci). Each variation in sequence is called an allele. [9] The greatest variations are found at noncoding loci containing variable number tandem repeats (VNTR's) in which the same sequence of base pairs is repeated successively for numbers of times that differ from person to person. [10] This variance is what makes DNA analysis possible. In effect, the lengths of sets of multiple (usually eight) polymorphic fragments (or VNTR alleles) obtained from a suspect's DNA and from crime scene samples are compared to see if any sets match.... ( Barney, supra, 8 Cal.App.4th at p. 806, 10 Cal.Rptr.2d 731.) In the absence of a nonmatch that conclusively eliminates the suspect as the source of the crime scene sample, each match between alleles from the suspect and from the crime scene may be accorded statistical significance. There are three discrete steps in [RFLP] analysis as performed by the FBI... and by [private laboratories like Cellmark and also the OCSD crime laboratory in this case] ...: (1) processing of DNA from the suspect and the crime scene to produce X-ray films [autorads] which indicate the lengths of the polymorphic fragments; (2) examination of the [autorads] to determine whether any sets of fragments match; and (3) if there is a match, determination of the match's statistical significance.  ( Barney, supra, 8 Cal.App.4th at p. 806, 10 Cal.Rptr.2d 731, original italics.)