Before jumping into this topic, it will be helpful to have a basic understanding of what DNA is – for that you can check out our last post. Armed with that knowledge, this post is focused on where DNA evidence can be found and the testing methods typically used in a criminal investigation to find a “match” with a suspect or victim.
As you probably guessed, DNA is primarily used to answer the “Who?” question in a criminal case. Because DNA is unique to each individual, it is capable of providing an answer to this question, under the right circumstances. The reliability of that answer has increased over time as science and technology have advanced our understanding of DNA and available testing methods.
Where can DNA be found at a crime scene?
Because DNA is present in every cell, it can be left behind by a perpetrator or victim in a variety of ways. Saliva on the butt of a cigarette. Blood on a murder weapon. Skin cells under a victim’s fingernails. Even sweat and oils from a finger or palm that transferred to a door handle or murder weapon (called “touch DNA”). DNA found on physical evidence connected to the crime is then compared to known DNA samples of suspects and victims.
How does modern DNA testing work?
So what are examiners actually doing in the lab? How do they know that DNA collected from a crime scene “matches” a suspect’s DNA? First, if law enforcement has not done so already at the scene, typically analysts will swab the surface of an item or take a cutting (for fabrics) to collect any DNA present – creating an “unknown” sample. Because the quantity of DNA in these samples is usually very small, analysts will use polymerase chain reaction (PCR) amplification to copy it millions of times (If you watched the Khan Academy video mentioned in our last post, it explains how replication of DNA can be done quite easily.).Oftentimes, examiners use a DNA testing method called Short Tandem Repeat, or STR, analysis.A short tandem repeat is a segment of the DNA strand that contains a sequence of two to five base pairs. The number of times this segment is repeated at that location varies from person to person. Examiners look at specific sections of the DNA strand, called loci, and make note of how many times the segment is repeated. This generates what is known as a DNA “profile” for the sample. The same procedure is done with the victim’s and the suspect’s known samples, and then the profiles are compared to see if there is a match.
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