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.