Forensic DNA Analysis: Capillary Electrophoresis Workflows

Promega provides complete solutions for analysis of forensic DNA samples by capillary electrophoresis (CE). These include reagents and kits for all steps in CE analysis workflows.

Products include preprocessing reagents for swabs and card punches prior to direct amplification of database samples, and DNA isolation systems for casework samples. We also provide the PowerQuant® System for accurate, human-specific DNA quantitation, and PowerPlex® Systems for STR amplification of Y-specific or autosomal DNA that meet global and regional database requirements. For sample analysis, the Spectrum family of CE instruments provides both low- and high-throughput sample processing options.

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CE Analysis Basics

The vast majority of the human genome is identical among diverse individuals. However, there are small regions of variation, especially in areas with repetitive sequences. One example is short tandem repeats (STRs) that can vary in length among individuals, with each repeat consisting of three to five nucleotides of DNA. Analysis of these repeats reveals a characteristic pattern for each individual that is inherited from paternal and maternal DNA. Forensic DNA databases use these patterns of STRs for human identification, and each national database has requirements for specific STR loci that are included. 

To characterize these STR patterns, or DNA profiles, human genomic DNA is amplified using the polymerase chain reaction (PCR) with sets of primers for each STR being tested. The primers are labeled with fluorescent dyes, and the amplified products are separated according to size by capillary electrophoresis (CE). Fluorescence imaging by the CE instrument results in a set of peaks for each dye channel, which together constitute an electropherogram that serves as a DNA “profile” for that individual. Statistical analysis of the DNA profile and comparison to reference databases can predict whether the individual matches a known DNA profile, with a high power of discrimination.