They next place the DNA fragments in a gel, and then pass an electrical current through the gel. The fragments move downwards, either quickly or slowly, depending on their size. The final steps involve making them radioactive, and filming them by X-ray. Experts compare the banding from the crime scene evidence with a specimen from the suspect and look for a match. DNA analysis is regarded as definitive by juries.
All forensic tools possess limitations and DNA analysis is no exception.
The laboratory work takes place over months prior to a case coming to court. The result will be closely scrutinized by scientific and legal experts before it is ruled as admissible. Experts agree band shifting can still be a challenge, but exacting quality control, including replicating tests, makes It easy to spot and eliminate. DNA analysis can also be imprecise if the sample is too small or has been damaged by environmental exposure, as may often be the case in forensic investigations. Canada's three forensic laboratories already have PCR technology, and are perfecting it for use in criminal investigations.