Who (really) dunnit?
Almost all cells in our body contain DNA, a molecule that stores all the information we need to function. Though everyone’s DNA is made using the same building blocks (or the DNA ‘bases’ C, G, A and T), how these blocks are combined, and in what order, can be very different from one person to another, making everyone unique.
As we go about our daily lives, we leave microscopic pieces of our DNA in the world. A few skin cells here on a discarded coffee cup, a speck of saliva there on a table, a tiny droplet of blood on the floor as you flee the scene of the crime…
To discover who was at the crime scene, forensic scientists collect and analyse samples, searching for traces of DNA evidence to link suspects to the crime. Back in the lab, rather than using all of the suspects’ DNA, forensic scientists look at select regions[1]. These chosen regions are called Short Tandem Repeats (or STRs), and vary greatly person-to-person, producing a visibly distinct pattern in each individual. Forensic scientists can make millions of copies of the DNA from the crime scene using a technique called Polymerase Chain Reaction (or PCR), which requires an enzyme called DNA polymerase. Afterwards, all of the STR patterns found at the scene are imaged and can be compared to samples collected from potential suspects.
Unfortunately, as the name implies, STRs are also very repetitive. STRs appear as short combinations of DNA bases that can repeat themselves several times in a row. For example, an STR could be made of TAAG-TAAG-TAAG (3 repeats of the same) in one person and TAAG-TAAG (2 repeats of the same) in another[2]. For the DNA polymerase, the repetitive nature of these regions can cause it to make a mistake (i.e., miss one or more of the bases in the sequence) when it tries to copy the DNA, creating a shorter DNA fragment than expected: a stutter fragment. This phenomenon can make analysing STR profiles complicated, especially if the samples contain a mix of DNA from different individuals. These stutter fragments can give false verdicts of who was at the crime scene or make it harder to distinguish one person from another.
Very recently, scientists based at the Promega Corporation in the USA engineered a polymerase that is much better equipped to overcome this problem of repetitive STRs. By improving the ability of the polymerase to interact with DNA, they reduced DNA stuttering in their test samples from ~20% to as little as ~2%[3]. For forensic scientists, this achievement represents a significant advance in the accuracy of STR profiling, and when commercially available could enhance their crime-busting powers!
[1] https://nij.ojp.gov/topics/articles/what-str-analysis
[2] What are Tandem Repeats in DNA? Short Tandem Repeats (STR) and its applications
[3] https://mms.businesswire.com/media/20240920660502/en/2251132/1/
Edited by Hazel Imrie
Copyedited by Cameron McKeddie
