Excitation under the scanning electron microscope of DNA-associated fluorescence from chicken erythrocyte nuclei,polytene chromosomes and adenovirus 2 virions

Biological structures not seen by conventional light microscopy, such as longitudinal striations in polytene chromosomes, and, at the limit of sensitivity, virions of adenovirus 2, have been detected via DNA-associated fluorescence excited under the scanning electron microscope. The maximum sensitivity realized, about 1 detected photon per 700 base pairs, falls short by about an order of magnitude of that required to achieve, in unreplicated specimens, the 2 nm intrinsic resolution of the method. A combination of D₂O-H₂O substitution with freeze-drying provides the best unquenching procedure found for in situ DNA. DNA-associated fluorescence for light microscopy can be created by moderate exposure of the specimen in the electron microscope.