DNA has an awesome power for identification. The genetic sequences that you carry around (and drop) daily are unique. When it comes to identifying an individual from a biological sample, DNA is the gold standard.

There are a number of ways in which DNA can be analyzed as forensic evidence, from simply finding DNA to extensive sequencing. Typically DNA evidence is something between the extremes. A brief history of DNA as evidence was published in McDonald J and Lehman DC (2012) Forensic DNA Analysis Clin Lab Sci 25 109–13.

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One of the large themes of biochemistry is that the vast majority of organisms are similar in molecular composition and central metabolism. With such an overlap in function, there must also be a sizable overlap in the DNA which codes for those functions.

There is.

Often this is expressed along the lines of humans share X percentage of genes with the banana. Largely this sort of statement is addressing the observation that both organisms have the genes for the enzymes of glycolysis along with a variety of other genes—ortholog genes. If that is the case, how could you ever distinquish the difference between two humans without going to full sequencing?

The solution is in the details. And it isn't the details of the coding regions (genes).

Short Tandem Repeats (STRs), also known as microsatellites, are short sequences, typically one to six nucleotides, which are repeated several times, five to ten repeats. This produces areas in the non–coding regions of genome with variable lengths between individuals. When you look at more than one of these regions at the same time, you are able to identify an individual with a high degree of accuracy. This process is known as STR analysis.

But we are getting ahead of ourselves by just a bit, particularly on the technical front.

The polymerase chain reaction (PCR) is a method in which a small section of DNA (the template) is amplified. The specificity is generated by a pair of oligonucleotides (the primers) which are designed to be complimentary to the DNA to be amplified. The primer pair consists of a left primer (or forward or plus, there are different names for the same thing, biochemistry, go figure) and a right primer (reverse or minus). The primers define the beginning and the end of the product to be produced in the reaction. The PCR product will always be primer to primer, inclusive.

With template and primers, the other reagents necessary for the polymerase chain reaction are the deoxynucleotide triphophates (the monomer units of DNA) and DNA polymerase (a DNA polymerase originating from a thermophilic organism which is undenatured at high temperatures).

The reaction then is a thermal cycle in which three temperature are repeated twenty to thirty times. Those temperature, in order of appearance, are denaturation (hot), annealing (cold) and extension (just right). In the denaturation step, all DNA denatured to single strands. During the annealing step, as the temperature cools, primers match with their compliments in the template. In the extension phase, DNA polymerase extends the primer. As this sequence is repeated, the product quickly becomes the dominant DNA molecule in the reaction as illustrated in the figure below.

In order to identify multiple STRs in one reaction, the PCR experiment used is a multiplex. Multiplex is the description of amplifying two or more (many more in the human identification experiments, the current standard is twenty loci in CODIS) products in one reaction.

So, how does it really work in a law enforcement setting?

The short answer is they typically use an all in one system. Like the Applied Biosystems RapidHIT ID System or a more complete catalog.

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