Electron microscopy of sister chromatid exchanges.

BrdU-substituted chromosomes of Don cells were differentially stained with ammoniacal silver carbonate and analyzed by electron microscopy. The results obtained led us to the following conclusions: (1) the differential impregnation of BrdU-substituted chromatids with silver is mainly due to a preferential extraction of non-histone proteins caused by UV-irradiation and incubation in SSC at 55 degrees C prior to silver staining; (2) the core-like structures detected after silver staining may well represent an artifact due to the effects of severe hypotonic pre-treatment on chromatin dispersion; (3) in each sister-chromatid exchange, the border of exchange clearly defines the course of DNA fibers within the chromatid. This border of exchange tends to run perpendicular to the chromosome axis but may vary slightly depending on the degree of chromosome condensation.