Quantitative measurements of fluorescently‐stained DNA in gels from photographic images

Photography under ultraviolet illumination is used routinely to record the results of gel electrophoresis of DNA when a fluorescent dye, which binds to DNA, is present in the gel. The distribution of DNA in the gel can be determined accurately from the photographic negative when the absorbance is interpreted according to the nonlinear film darking function, when the contrast of the film is calculated, and when the images are corrected for uneven ultraviolet transillumination. The contrast can be calculated from the images of a fluorescent plastic intensity standard. The local intensity of the unbound‐dye fluorescence can be used to correct images for uneven illumination. Using this approach, relative abundances of DNA fragments, compared within each electrophoretic track, were measured within an average error of 6.6% of the expected value with no evidence of systematic error. Quantitative comparisons were valid anywhere on the film for a 50:1 range of intensities, with errors distributed normally with a standard deviation of ± 10 % of the expected value.