New SRM Allows Accurate Accounting of Forensic DNA
For Immediate Release: October 25, 2007
Michael E. Newman
Thanks to the polymerase chain reaction (PCR), a method for making multiple copies of DNA fragments, even tiny bits of biological evidence (in the form of hair, tissue, bones, teeth, blood or other bodily fluids) from a crime scene can be used to isolate genetic material that eventually can identify a suspect or victim. A new Standard Reference Material from the National Institute of Standards and Technology (NIST), SRM 2372 (Human DNA Quantitation Standard), is available to help ensure the success of this identification process, known as DNA profiling.
One profiling method popular with forensic experts uses short tandem repeats (STRs are short identical sequences of DNA found in specific regions of a chromosome) to compare samples of DNA from a crime scene to DNA from a suspect or victim. Commercial PCR systems that amplify STRs work best if the amount of DNA—measured in nanograms per microliter of solution—fed into the system is within a narrow range. Too concentrated a solution overwhelms the detection apparatus; too dilute yields poor results or none at all.
DNA quantitation—assessing the amount of DNA present in a crime scene sample—is the necessary precursor to making a suitable solution for profiling. A widely used method to achieve this is quantitative PCR (qPCR); however, current commercial qPCR kits may produce varying values for the DNA concentrations in the kit’s reference samples, rendering these standards less reliable for assaying the quantity of extracted evidential DNA. SRM 2372 can be used by qPCR manufacturers to calibrate their systems in the factory so that measurements made with the kits in forensic laboratories are consistently accurate.
The SRM contains samples of human genomic DNA from three sources—an individual male, multiple female donors and a mix of male and female donors. Each sample has been prepared to yield an optical density (OD) of 1.0 on a spectrophotometer when examined using a 260-nanometer wavelength of light. Scientists have determined that for a solution of double-stranded DNA, an OD of 1.0 at 260 nanometers corresponds to 50 micrograms of DNA per milliliter of solution.
More information about SRM 2372, including purchase data, may be found at https://srmors.nist.gov/view_detail.cfm?srm=2372.