Abstract: To sequence DNA automatically, fluorescently marked DNA are electrophoresed in a plurality of channels through a gel electrophoresis slab; wherein the DNA samples are resolved in accordance with the size of DNA fragments in the gel electrophoresis slab into fluorescently marked DNA bands. The separated samples are scanned photoelectrically with a laser and a sensor, wherein the laser scans with scanning light at a scanning light frequency within the absorbance spectrum of said fluorescently marked DNA samples and light is sensed at the emission frequency of the marked DNA. The light is modulated from said laser at a predetermined modulation frequency and fluorescent light emitted by said DNA bands at said modulation frequency is detected, whereby background noise from the medium through which the light is transmitted is discriminated against.