DNA are visible. By staining with the Feulgen stain, which specifically stains DNA, the nucleoid can also be seen under a light microscope. The DNA-intercalating stains DAPI and ethidium bromide are widely used for fluorescence microscopy of nucleoids. It has an irregular shape and is found in prokaryotic cells. DNA damage and repair Changes in the structure of the nucleoid of bacteria and archaea are observed after exposure to DNA damaging conditions. The nucleoids of the bacteria Bacillus subtilis and Escherichia coli both become significantly more compact after UV irradiation. Formation of the compact structure in E. coli requires RecA activation through specific RecA-DNA interactions. The RecA protein plays a key role in homologous recombinational repair of DNA damage. Similar to B. subtilis and E. coli above, exposures of the archaeon Haloferax volcanii to stresses that damage DNA cause compaction and reorganization of the nucleoid. Compaction depends on the Mre11-Rad50 protein complex that catalyzes an early step in homologous recombinational repair of double-strand breaks in DNA. It has been proposed that nucleoid compaction is part of a DNA damage response that accelerates cell recovery by helping DNA repair proteins to locate targets, and by facilitating the search for intact DNA sequences during homologous recombination. See also Plasmid Homologous recombination DNA repair References Bacteriology Cell anatomy