Opinion ID: 1506658
Heading Depth: 1
Heading Rank: 35

Heading: pcr

Text: When, as here, the quantity or quality of genetic material recovered from a crime scene is insufficient to allow RFLP analysis, forensic scientists have used the PCR process to amplify the DNA to produce an amount suitable for testing. The PCR process can copy a segment of DNA millions of times. NRC Report, supra, at 22-23. With the resulting genetic product, scientists can conduct allele-specific probe analysis. Fleming, supra, 84 A.L.R. 4th at 322-23. The PCR process copies DNA fragments similar to the way DNA replicates itself during mitosis. Through heating the DNA sample in a thermal cycler, the process separates the helix into separate strands. Primers composed of short DNA segments are added to define the target sequence of DNA. Then, a basic solution containing the enzyme DNA polymerase and the four basic nucleotides are added to the primed DNA sample. The added nucleotides pair with the exposed nucleotides on the separated target-strands in accordance with the G-C, A-T pairing rule. From the original DNA segment, two identical segments result. The thermal cycler runs through its cycle approximately thirty-two times, amplifying the original sample by a factor of two billion. Currently, PCR technology effectively amplifies only small regions of DNA. Accordingly, PCR cannot be used to amplify longer VNTRs for RFLP testing. NRC Report, supra, at 69-70. PCR-based testing methods have several advantages over RFLP analysis. They are relatively simple processes and can yield results within a short period of time, often within twenty-four hours. Of particular importance to the present case, the PCR process also makes possible DNA tests on small amounts of genetic material. A disadvantage of PCR-based tests, however, is that the identified genes have fewer alleles than VNTRs. Hence, scientists must examine more loci to produce the same amount of information about the likelihood that two individuals share a profile. Id. at 71. Also, some of the loci examined by PCR-based tests are functional genes. Unlike non-functional VNTR markers, functional genes are more susceptible to natural selection, a susceptibility that might undermine their usefulness in matching DNA samples. Ibid. Contamination also is of concern in PCR testing. The technology is so efficient that even small contaminants can be replicated along with the targeted DNA. Ibid. Cellmark used two types of PCR-based tests in defendant's case: the HLA DQ Alpha (DQ Alpha) and polymarker (PM) tests.