Improved visualization of single- and double-strained nucleic acids by STEM.

Annular dark field STEM images have been used to visualize nucleic acid molecules positively stained with uranyl acetate. The regular distribution of uranium clusters makes clearly visible the existence of segments of double and single strands on partially denatured RNA molecules. Unpaired regions, as short as about 8 nm, are detectable by this combination of a highly efficient imaging mode and a well adapted preparation technique.