Fig 7. Results for cells exposed to 0.5 Gy radiation.

Panel A. The figure shows the radial radial distribution function for H4K20 antibodies representing heterochromatin (methylated histone variants) for non-irradiated and irradiated cells. Error bars represent the standard deviation of the mean value after averaging over the sample of cells. The value for g(r) at small distances goes up to around 20, indicating the high marker densities in regions where heterochromatin is located. The rapid drop off of the radial distribution function within a distance of less than 100 nm shows that heterochromatin forms small clustered areas that are spread throughout the cell nucleus. Upon exposure to 0.5 Gy γ-irradiation, a dramatic change in the correlation function can be observed in cells that were imaged after 30 min. The value at small radial distances drops to around 6, or around 70% smaller than in non-irradiated cells. This indicates that the density in the heterochromatic regions becomes on average much lower in irradiated cells. In cells measured 48 h after irradiation, the correlation function have recovered again and the value at small r is at around 14, only 30% less than in unirradiated cells. Panel B. The distribution of edge lengths in the Delaunay triangulation of the markers confirms these observations. A sharp peak in the distribution at around 30 nm can be seen in untreated cells. In 30 min post-irradiation cells the peak vanishes and a spread distribution can be seen. In 48 h post-irradiation cells however, the peak reappears again but less pronounced than in untreated cells.