DNA

Although the discovery of DNA dates back to 1869 with Frederich Miescher's study of a phosphorus-containing substance isolated from the nuclei of cells found in discarded bandages of wounded soldiers, it was not until 1953 that Watson and Crick published its molecular structure. Many studies since then have established that DNA is the material responsible, in part, for the inheritance of virtually all of our traits. Exceptions to this rule include one's fingerprint (ridge) pattern, which is established during the third to fourth month of fetal development. The discovery that DNA is actually an antiparallel double helix helped to explain why this unique molecule, with the assistance of “helper” molecules, is able to replicate within the cell. Molecular biologists and biochemists continue to study DNA to better our understanding of the processes of transcription (transfer of information from DNA to RNA) and translation (the use of RNA in the synthesis of cellular proteins). These activities are essential to cell function and life.

The development of DNA analysis technology for the purpose of human identification in the mid-1980s marked a turning point in the practice of forensic science and has created a revolution in criminal justice. The human genome, coding for approximately 30,000 to 40,000 genes, consists of approximately 3.1 billion nucleotide subunits or building blocks of DNA distributed unevenly on 46 chromosomes that are located in the nucleus of each of our cells. Of the 3.1 billion nucleotide subunits within the human genome, only a small portion (0.1%) makes one person different from another. It is this exceedingly small fraction of DNA that can be used by the forensic scientist to individualize biological samples left behind by perpetrators of violent crimes. Forensic DNA analysis of this evidence is used to obtain identifying information by determining which alleles (genes) are present at a number of specific genetic sites (loci) within the human genome. In each case, a distinct genetic profile is constructed. With this information, a comparison can be made with DNA obtained from the victim and suspect(s). Different profiles indicate that the suspect is excluded as the source of the evidence, whereas identical profiles indicate that the suspect could be the source. This is analogous to identifying and comparing points of minutiae in a latent fingerprint with an inked print to determine if a match exists. In forensic investigations, exemplar and questioned specimens are compared to determine if there is a relatively high [Page 140]probability or even scientific certainty that there is a common origin.