To date, DNA (deoxyribonucleic acid) typing is the most significant forensic breakthrough since the discovery of fingerprints. Thirty-seven percent of the cases received by the FBI for DNA analysis result in the exclusion of the primary suspect.
Previously, men have spent years, even decades, in prison until DNA tests or updated DNA tests were performed, exonerating them. In the O.J. Simpson trial, the DNA tests were positive, and still he was acquitted.
How reliable is DNA evidence within our justice system? What DNA is, how it is collected, typed, analyzed, and presented at trial may provide some answers to this provocative question.
The Discovery of DNA
Biochemist Phoebus Levene first discovered nucleic acid in individual cells as early as 1911. Ever since then, scientists have attempted to probe the exact genetic implications of nucleic acid.
It took until the 1940s for scientists to discover that DNA forms the building blocks of life and is the substance that dictates not only our hair and eye color, but also everything about our physical makeup. In the 1950s scientists realized that DNA issued its instructions by code and that each code was unique to each individual
Chromosomes in every body cell are composed of genes, individual units of DNA that carry genetic information. DNA also synthesizes the proteins which comprise the enzymes fundamental to cellular growth and function.
Located primarily in the nucleus, the DNA molecule consists of two tightly coiled helical chains. The backbone of each chain consists of alternating units of deoxyribose (a 5-carbon sugar) and phosphate.
A mixture of four nitrogenous bases-adenine (A), guanine (G), cytosine (C), and thymine (T)-form the links of each chain. The sequential arrangement of these four bases represents the inherited genetic code of a particular trait. From chain to chain, adenine (A) is always paired with thymine (T), and cytosine (C) with guanine (G).
A typical section of DNA code might look like this:
G A C A C T
C T G T G A
These codes provide the basis for comparing crime suspects' specimens with those found at a particular crime scene.
DNA Collection
Collection methods are very important at the scene of a crime or when a victim comes forth. Police officers and laboratory personnel are urged to collect as much specimen as possible.
Collection methods are very precise and detailed. It is imperative that samples be collected as quickly after a crime has been committed to avoid being washed away or spoiled since fungi and bacterial decay can destroy the long sections of DNA needed for Restriction Fragment Length Polymorphism (RFLP) analysis. Typically, semen stains and saliva do not last as long as blood or tissue samples.
The sensitivity of DNA tests puts a premium on quality throughout the process and on the need to eliminate human error. At trial, one forensic scientist described his laboratory's high standards; however, when it was discovered that a serious error had been made in labeling samples sent by the defense, he responded that this fell outside the scope of quality assurance monitoring.
Once the crime lab has the sample(s), it goes through a decontamination process to lift foreign matter and debris from the sample. In any of the typing processes, it is possible to produce an incorrect result because an invisible skin flake, such as dandruff, borne on the air, gets into the test tube. Proper labeling of specimens is also of paramount importance. An evaluation is then performed to determine how much DNA is available for typing. “If there is not enough specimens collected, and therefore, not enough DNA to adequately type, forensic teams must then rely on medical or physical evidence and the reliability and truthfulness of witnesses,” says Gretchen at the Milwaukee Crime Lab.
DNA Typing
No matter what methodology is used, all DNA tests are designed to isolate certain nucleotide sequences-the segments of DNA carrying marked distinctions that can identify or exonerate a suspect. To date, there are two basic analytical methods used to detect the recurring nucleotide sequences occurring in DNA: RFLP (Restriction Fragment Length Polymorphism) and PCR (Polymerase Chain Reaction).
The RFLP method of typing DNA finds the fragments in a DNA chain which contains the nucleotide sequences sometimes called polymorphic segments, produces a “print” of the fragments, and measures the fragment lengths. This is accomplished by extracting DNA from a specimen and mixing it with a restriction enzyme that cuts the DNA chain at particular sequences. The fragments created by this process are placed in a gel, to which a high-voltage electrical current is applied. Shorter fragments move through the gel more quickly than longer fragments, and after a short time, the fragments will have lined up according to size. These are then lifted from the gel by a nylon membrane called a blot.
