Until the late 1980s, serological testing in cases of rape involved traditional enzyme studies. DNA typing has virtually eliminated such testing. DNA "fingerprinting," i.e., DNA typing for forensic purposes, was developed by
Dr. Alec Jeffreys in 1985.6-8 He determined that, in each strand of DNA, there are thousands of identical DNA sequences. The length, constitution, and number of the repetitive sequences are different for each person. The identification and demonstration of the sequences of nucleotides possessed by an individual cell is the basis for DNA identification.
The importance of DNA fingerprinting is that any tissue containing nucleated cells can potentially be linked to an individual, usually to the statistical exclusion of all other individuals. DNA can be obtained from sperm, nucleated blood cells, cells from soft tissue, teeth, bone, fingernails, saliva, urine and hair — essentially, any tissue in which there are
nucleated cells. DNA can be extracted from these specimens, chemically divided into fragments, and formed into a pattern that will serve as an identification profile. This pattern can then be compared with a DNA pattern obtained from a suspect's blood specimen. If these match, and the test is done with sufficient probes, there is virtually no doubt that the suspect is the source of the tissue to the exclusion of all other individuals, except for an identical twin. If the patterns do not match, then the suspect is not the perpetrator.
In addition to the absolute nature of DNA identification, there are other advantages. DNA is much more stable than the enzymes and proteins formerly used in blood identification. There are no false positives caused by degradation. If the DNA is altered, it will fail to form a pattern. DNA is fairly resistant to degradation and analysis has been made on human remains thousands of years old.
In addition to its application in criminal matters, DNA identification can be used in paternity suits. A child inherits half its DNA from each parent. If the half of the child's DNA pattern that is different from the mother's pattern matches that of the alleged father, then there is no doubt that he is the biological father.
Two basic concepts must be understood in dealing with DNA analyses:
1. If the DNA profile of the evidence DNA is different from that of the suspect in any aspect, then the suspect is absolutely excluded.
2. If the evidence DNA and the suspect DNA match then there are three possibilities:
a. The evidence DNA came from the suspect.
b. The evidence DNA came from another individual who has the same DNA profile. This is possible if the second individual is a monozy-gotic twin, or because an insufficient number of tests were performed to differentiate the suspect from the other individual.
c. An error was made in either the collection or analysis of the specimen DNA.
Many individuals do not know what is meant by a DNA match. Except for monozygotic twins, the DNA pattern is unique for each individual. DNA profiling, however, does not compare the whole DNA pattern of an individual and that of the evidence DNA, only a minute portion. A match is made by the statistical exclusion of all other individuals. To determine this, the frequency of occurrence of selected alleles in the major population groups is determined, and testing is performed to determine the presence of these selected alleles. If the evidence DNA and the suspect's DNA are tested for an allele that occurs in one in ten individuals, and they match, then 9 out of 10 people in the population are excluded as sources of the DNA. If the second allele tested for also occurs and this matches, then 99 out of 100 people are excluded. If sufficient alleles are tested for, the probabilities for exclusion go into the millions or even billions. Thus, a match is made on statistics.
All nucleated cells in the body contain 23 pairs of chromosomes except for sperm and ova, which contain 23 chromosomes rather than 23 pairs. Each chromosome consists of a double spiral of deoxyribonucleic acid (DNA) in the shape of a twisted ladder, the double helix. The sides of the ladder consist of alternating sugar (deoxyribose) and phosphate molecules; the rungs of the ladder consist of nitrogen bases. The weakest part of the helix or ladder is the rungs, where the nitrogen bases are weakly linked by hydrogen bonds. DNA is composed of units called nucleotides, which consist of a sugar, a phosphate group, and a base. Millions of these nucleotides form a single strand. Despite the millions of nucleotides, only four different bases are used. Two of these are purines (adenine and guanine) and two are pyrimidines (thymine and cytosine). In forming the rungs of the ladder, guanine always binds to cytosine and adenine always binds to thymine. These are the only two possible combinations and these are called complementary base pairs. Because of the millions of nucleotides forming a single strand and the fact that there are 23 pairs of chromosomes in each cell, there is an almost infinite variety in the arrangement of the nucleotides. The order or sequence of the bases in a DNA molecule forms a code of the genetic information of the cells.
A gene is a series of these bases that occupies a specific location (locus) on a chromosome, producing a specific product. There is usually more than one form of a gene for each locus. These are called alleles. Most of the chromosome, however, serves no known function. These areas consist of multiple copies of identical base sequences, 50-60 base pairs in length, arranged one behind the other, that is, in tandem. The repeated sequences are known as tandem repeats, with the areas made up of tandem repeats known as VNTR (variable number of tandem repeats). Just like a gene, a loci of VNTR can have multiple alleles. The VNTR loci were the first areas used in DNA typing because of the multiple alleles.
The original method of DNA analysis was RFLP (restriction fragment length polymorphism). This was a prolonged, complicated method that took a minimum of 6-8 weeks to conduct. DNA was chemically extracted from the submitted biological specimen and purified. It was then cut into fragments by a restriction enzyme. These substances cut the DNA molecule at specific base sequences. The number of DNA fragments and their length as produced by a particular restriction enzyme depend on how often the enzyme's base sequence occurs in the DNA specimen. Because every individual's DNA sequence is different, the fragments in the DNA specimen from one individual to another are different in number and length from those in a DNA specimen from another individual.
The DNA fragments were then separated into bands by electrophoresis. DNA carries a negative charge and thus travels toward the positive pole in electrophoresis. The distance that each fragment travels depends on its length. The longer the fragment, the slower its rate of migration. This creates a number of DNA bands in the gel. These are transferred to a nylon membrane by a technique known as Southern blotting. The membrane is then exposed to a DNA probe having a radioactive tag. The probe looks for a complementary sequence of bases. Thus, adenine looks for thymine, and guanine looks for cytosine. The probe then binds to the DNA pattern. X-ray film is placed next to the membrane to detect the radioactive patterns that appear as a series of bands similar to the bar code on items in grocery stores. Each dark stripe or band represents a point where the DNA probe is bound to its complementary base sequences. The pattern of bands is unique for each individual (excepting identical twins), just as a fingerprint is. While two individuals may have identical patterns for one or two probes, if a sufficient number of probes are used, at some point, the probes will begin to not match and the individual is ruled out. If the DNA being tested came from the individual it is being tested against, and a sufficient number of probes are run, then statistically, the probability that the individual was the source of the DNA approaches 100%. With the use of sufficient DNA probes, the probability that an identification is positive can be as great as 30 billion to one.8
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