TEM observation of compacted DNA of Synechococcus elongatus PCC 7942 using DRAQ5 labeling with DAB photooxidation and osmium black

To visualize the fine structure of compacted DNA of Synechococcus elongatus PCC 7942, which appears at a specific time in the regular light/dark cycle prior to cell division, ChromEM with some modifications was applied. After staining DNA with DRAQ5, the cells were fixed and irradiated by red laser in the presence of 3,3ʹ-diaminobenzidine and subsequently fixed with OsO
4
. A system with He–Ne laser (633 nm) was set up for efficient irradiation of the bacterial cells in aqueous solution. The compacted DNA was visualized by transmission electron microscopy, in ultrathin sections as electron dense staining by osmium black.