Finding DNA: Using fluorescent in situ detection

Abstract

It is known that DNA can be deposited onto a surface by touch yet few means have been developed for its in situ detection. Collecting touch-DNA samples can be difficult as likely locations rather than the DNA is targeted leading to many samples that are submitted to a forensic laboratory containing little or no DNA. A range of dyes are available that bind to DNA at high specificity for application within the laboratory and here we report on the use of these dyes to detect latent DNA on various substrates and within biological samples. Six common nucleic acid-binding dyes were selected due to their increase in fluorescence when in the presence of double stranded-DNA; four of the six dyes are permeable to cell membranes. The fluorescence from dye/DNA complex was detected using a high intensity light source, the Polilight® (PL500), an excitation wavelength of 490nm and emission observed/recorded through interference filters centred at 530nm or 550nm depending on the dye emission. The samples were visualised under a fluorescent microscope (Nikon Optiphot) using a B2A filter cube. The detection limit of DNA was determined for the selected dyes along with the optimal conditions, such as buffer composition and dye concentration for a range of surfaces. The ability for the dyes to detect DNA within biological samples such as saliva, hair, skin, fingermarks, and hair follicles was also determined.