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DNA Casework

Until 1986, developments in DNA research were of seemingly peripheral interest to forensic scientists. In 1986 in the village of Narborough, England a woman had been raped and murdered. The circumstances lead investigators to believe that case was related to a 1983 unsolved sexual assault and homicide. Sir Alec Jeffreys and his colleagues at Leicester University, England, while investigating the structure of a human gene discovered that non-coding regions of the DNA structure are as different among individuals as coded traits such as hair color and eye color. Jeffreys was called upon to use his newly discovered "DNA fingerprinting" system to learn that the sperm samples from both cases appeared to have come from the same man. The police secured approximately 5500 samples from young men fitting the profile of the suspect. Jeffreys' applied his DNA fingerprinting technique; however, the results were discouraging because none of the donated samples matched the sperm from the rape victims. Unexpectedly, a conversation at a local pub led to a comment by a patron that he had donated two blood samples because a coworker, Colin Pitchfork, was "unable" to donate. Colin Pitchfork was eventually apprehended, typed, matched and later confessed to killing the two women.

Since 1986, there have been many variations to the original Jeffreys' technique; the term DNA typing or DNA profiling has come to be applied to describe these new technologies. DNA profiling has far surpassed conventional serological means to associate the origin of biological evidence such as blood, semen, hair, or tissue to a single individual. Although conventional testing procedures had gone a long way in narrowing the source of biological materials, the discovery of DNA markers has proven to be a much more effective and discriminating way to associate individuals to crime scene evidence. In the United States, courts have overwhelmingly admitted DNA evidence and accepted its reliability.

The Missouri State Highway Patrol Crime laboratory began conducting DNA analysis in 1990 and since has gone through several iterations. RFLP (Restriction Fragment Length Polymorphism) analysis was in use by the Patrol laboratory from 1990 until 1999. PCR or the polymerase chain reaction was added to the Patrols arsenal in 1995 introducing an analysis technique known as DQ-alpha (or DQA1) and PM. The most recent use of the PCR has been with the application of STR's or Short Tandem Repeats, which has been used by the patrol since 1999 and are still used today.

The Missouri State Highway Patrol Crime Laboratory DNA section participates in the COmbined DNA Index System (CODIS). CODIS is a DNA Database containing thousands of DNA profiles from convicted offenders and unsolved forensic cases from local and state labs. It is very effective in associating unsolved cases and in associating unsolved cases with recidivist offenders. When profiles are uploaded to the national level the combined local and state data total over one million samples. These samples are searched routinely against unsolved cases that contain DNA evidence. Currently Missouri collects all offenders convicted of any felony. There are currently over 63,000 offenders in the Missouri CODIS database. In 2005 after the inception of the all felon bill, we had 175 hits on the database. In the first half of 2006 we have had 271 hits.

PCR, or the polymerase chain reaction, is the technology of using DNA polymerases to copy a DNA strand isolated from a living cell. PCR is the outgrowth of understanding how DNA strands naturally replicate within a cell. The most important feature of PCR is knowing that the enzyme called DNA polymerase can be directed to synthesize a specific region of DNA. Small quantities of DNA or broken pieces of DNA found in crime-scene evidence can be copied with the aid of a DNA polymerase. The copying process can be accomplished in an automated fashion using a DNA Thermal Cycler. Each cycle of the PCR technique results in a doubling of the DNA. Within a matter of a few hours, 30 cycles can multiply DNA a million fold. Once the DNA fragments are copied they can be analyzed.

In addition to its role in traditional biological evidence such as blood, semen, and saliva, PCR has been useful in aiding the analyst in obtaining DNA profiles from hair, the smoker of a cigarette, the drinker of a glass, or the wearer of clothing items.

STR's Short Term Repeats

The latest method of DNA typing, short tandem repeat (STR) analysis, may prove to be the most useful technique of all. Not only does this process have the potential for high discrimination, but it also reduces the amount of time to obtain results from a sample and requires less sample than needed for other typing methods.

STR's are locations (loci) on the chromosome that contain short nucleotide sequences that repeat themselves within the DNA molecule. They serve as helpful markers for identification because they are found in abundance throughout the human genome. STR's are length polymorphisms (similar to the VNTR's analyzed for RFLP). However, the repeating sequence in STR's, as the acronym implies, is relatively short in length, generally 3 to 7 bases. In turn, the entire strand of an STR is also very short, less than 400 bases in length. This means that STR's are much less susceptible to degradation than other methods of DNA or forensic biological analysis and may often be recovered from substrates such as soda bottles, articles of clothing, or even single hairs. The size of the theoretical repeat sequence C-A-T can describe STR analysis whereby a person having an allele with the sequence CATCATCAT has a smaller repeat than a person having an allele with the sequence CATCATCATCATCAT and hence this difference would preclude these two individuals from being identified as the same person. The repeat sizes are copied or amplified using the PCR and separated according to size.

The electrophoresis is conducted in an instrument called a 310 Genetic Analyzer and is carried out in a capillary. The 310 Genetic Analyzer detects the amplified repeat sequences as they move through the capillary. The 310 is coupled to a computer which compares the amplified repeat sequences to known DNA sequences and interprets the number of repeats that are contained in the amplified sequence. The amplified repeats are manifested as peaks on a computer generated print out.