Rapid irradiation procedure for obtaining permanent differential staining of sister chromatids and aspects of its underlying mechanism.

When fixed metaphase preparations of lymphocytes cultured in the presence of BrdU during two cell cycles are subjected to a 1-min simple irradiation treatment with near-ultraviolet light (radiation dose 3X10(5) J/m2), subsequent Giemsa staining produces differential staining of sister chromatids irrespective of previous exposure to a photosensitizer. The effects of this procedure were analyzed by irradiating single metaphases under the microscope, thus allowing precise dosage of radiation. Metaphase were subsequently stained with Giemsa and then subjected to the Feulgen-Schiff procedure. Whereas in the presence of DAPI as a photosensitizer a differential breakdown of BrdU-containing DNA in the chromatids under the influence of irradiation appeared to be the cause of sister chromatid differentiation, alterations presumably in the higher order structure of chromatin, not accompanied by removal of DNA, induced sister chromatid differentiation without DAPI.