DNA lesions within the heterochromatin elicit fast damage response and are re-located to euchromatic regions

DNA is highly compacted by wrapping around histones and folding into higher order structures building chromatin. This chromatin can be subdivided in two classes: the highly transcriptional active, sparse euchromatin and the densely packed heterochromatin. The heterochromatin is transcriptionally inert but essential for the regulation of the gene expression and the entire architecture of the nucleus [1]. Here, we studied the DNA damage response within the heterochromatin. The distinct delimitation of euchromatin and heterochromatin in embryonic mouse fibroblasts, forming heterochromatic chromocenters, enabled us to clearly distinguish between these two chromatin fractions. For an aimed irradiation of the chromocenters with single ions we made use of the unique submicrometer resolution of the GSI microprobe.