February - 2003
Vol# 19 - Issue# 70
The Future of DNA Evidence
-Jeff Wise and Dr. Richard Li (Special to CJI)
Law enforcement has many tools at its disposal in the pursuit of criminals. One of the most recent developments is the use of DNA profiles, sometimes referred to as “fingerprints.” DNA evidence is useful for both convicting guilty suspects and acquitting the innocent. However, a new technology is only useful to law enforcement if it can survive the rigorous scrutiny that defense attorneys put it through. DNA evidence boasts a long history of acceptance and success in the courtroom since its introduction in the mid 1980’s. A new technique in DNA analysis called Low Copy Number (LCN) DNA claims to offer unheard of levels of detection. Whereas traditional DNA evidence comes from blood, semen or hair samples, LCN DNA or “trace DNA” can be obtained from as little as a fingerprint or residue from the lip of a drinking glass. LCN DNA demands a close look to determine if the merits of this novel laboratory technique are worth pursuing.
Forensic DNA Evidence
DNA, or deoxyribonucleic acid, is the genetic material inherited from both parents that provides the code for all living processes. DNA is useful for determining identity because it is found in nearly every cell in the human body. Forensic DNA analysis is based on examination of the sequence of the DNA code. The sequence of DNA can be thought of as a string of letters that when combined in a certain order form words, paragraphs and even chapters of instructions for the cell. The entire DNA alphabet has only four letters (A, T, C and G) but the length of the DNA sequence is three billion letters long. Most of the DNA between individuals is the same, which makes sense because we all have similar characteristics. For example, we all have ten fingers and toes, digest food the same way and require oxygen to breathe. However, some of the DNA between individuals is different and these differences allow DNA analysts to determine beyond a reasonable doubt if a sample matches that of a suspect.
Current Methods vs. LCN DNA
James Watson and Francis Crick discovered the structure of DNA in 1953 but it took thirty years for the uniqueness of DNA to be used in the courtroom. Since then the methods of DNA analysis have changed a number of times. As DNA analysis evolved, the amount of DNA needed to determine a person’s identity decreased. Current methods allow an analyst to determine a DNA profile from less than a drop of blood, saliva or semen. Small amounts of DNA can be used because of the development of the polymerase chain reaction or PCR. PCR is a method of creating multiple copies of DNA in the laboratory. With PCR, small amounts of DNA, such as that from a blood sample, can be copied to give the analyst an amount of material that is easier to work with. Typically, in forensic applications the number of “cycles” of PCR amplification is 28. With each cycle, the number of DNA molecules doubles. In 28 cycles a single DNA sequence is copied over 250 million times. Once the DNA is copied, the unique sequences are analyzed and the profile of a suspect is created.
In contrast to current methods, Low Copy Number (LCN) DNA involves increasing the number of PCR cycles from 28 to 34. This may seem like a small adjustment but while 28 cycles yields approximately 250 million copies, 34 cycles increases that number to over 17 billion copies. This increase of DNA allows researchers to determine DNA profiles from previously unusable sources such as fingerprints or clothing simply worn by the suspect.
Forensic Science Service (England)
In 1999, the Forensic Science Service (FSS) in England began using LCN DNA in casework. The increased sensitivity of LCN methods allowed researchers with the FSS to determine DNA profiles from objects that were simply touched by the suspect. Their use of LCN DNA required a number of changes in the way investigators handled evidence collection. Since LCN DNA requires only a handful of individual cells, they are not visible to the naked eye. For this reason, stand