DNA Fingerprinting & Crime Retrial

Genetic Sequencing, DNA Forensics & Criminal Trial Verdict Reversal

Mar 29, 2008

What is DNA fingerprinting and how does this type of genetic evidence factor into criminal retrials and verdict reversals?

Robert Caulley

Fourteen years ago, Ohio’s Robert Caulley was convicted of killing his parents. The convicted man now requests that new DNA tests be conducted to prove his innocence.

DNA evidence had been presented during the trial with respect to some of the evidence in this case, but the results were inconclusive. Still, prosecutors doubt that reexamining the evidence will impact the verdict. During the trial Caulley confessed to killing his parents, and is now serving 25 years to life for murder.

DNA Evidence and Retrial

Robert Caulley’s case is certainly not unique. Criminal cases are routinely tried and retried based on DNA fingerprinting analysis. Although it is very uncommon, occasionally a conviction is reversed due to DNA evidence. Reversals are more often the case in retrials in which the original conviction occurred before DNA fingerprinting technology was available.

What Is DNA Fingerprinting?

Everyone, except identical twins, has a one-of-a-kind DNA genome. The DNA molecule, a nucleic acid, is made of monomers called nucleotides, and the specific order in which nucleotides occur in a DNA molecule represents each individual’s unique genetic code.

DNA restriction fragment analysis, or DNA fingerprinting, uses special proteins called restriction enzymes to cut up DNA. A specific restriction enzyme will only cut the DNA when it encounters a certain combination of nucleotides. This results in lengths of DNA, called restriction fragment length polymorphisms (RFLPs). For each individual, the size of these fragments is unique, like the person's DNA.

When DNA samples are taken from a suspect and crime scene, they are shipped to a lab and each exposed to the same array of restriction enzymes. Once the DNA is cut up, RFLPs from each sample are placed into small wells within a gel matrix, that looks much like a rectangular slab of Jell-o.

This Jell-o-like slab containing the DNA is placed in an electrophoresis chamber which contains liquid and positive and negative electrodes. Current is then run through the chamber.

DNA is negatively charged (-), and therefore attracted to anything that is positively charged (+). When current is run through the chamber, the DNA begins to migrate towards the positive pole. Large fragments will move through the gel more slowly than smaller fragments, and, after a short period of time, the RFLPs separate based on their size.

Ultimately bands of DNA are visible in “lanes” that have moved straight out of each well. Each band in a lane represents many DNA fragments that are similar in size. Since the DNA from different individuals will have been cut at different places by the restriction enzymes, each person's DNA will form a unique pattern after being run through the gel.

The electrophoresed DNA pattern of suspect and the pattern of samples obtained from the crime scene are then compared. If the crime scene samples contain RFLP patterns identical to those of the suspect a stong case can typically be made for the suspect’s involvement in the crime.