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Ionizing radiation is omnipresent in our world either by natural radiation, due to medical treatments or cosmic radiation on a long haul flight. The main threat of radiation is DNA damage which can cause chromosomal aberrations, genetic instability and cancer. Therefore, many investigations con-centrate to understand how the cell recognizes those damages and how they are repaired. Over time one protein became the gold standard for indication of radiation induced DNA DSB, the yH2AX pro-tein. After radiation this protein acts as an initiator for the recruitment of repair proteins. One of these repair proteins is the MRE11 which is a member of the MRN complex and present during HR. Both proteins form foci at the DNA break and were the focus of many studies on radiation-induced DNA damage. For the analysis of these two proteins the fluorescence microscopy was applied over years. But with time new microscopy methods were developed, which are able to give a better in-sight to the protein distribution due to a higher resolution. One of these high resolution microscope methods is the SPDM, which was used in this thesis.
After the determination of the parameters which described the foci of the yH2AX or MRE11 foci at best, cells were seeded on coverslips and were irradiated with 2Gy x-ray and investigated from 10 min to 48h. During the analysis the foci were examined in the number of clusters in a focus plane, fo-ci diameter, signal distribution inside and outside a cluster and many more.
We were able to identify a maximum of clusters yH2AX foci after 120 min in MCF7 cells and after 180 min in fibroblasts. Whereas, MRE11 foci indicated two maximum peaks in both cell lines, the first af-ter 60 min which was identical in both cell lines but the second peak was in MCF7 after 180 min whilst fibroblasts showed it after 48h. Interestingly, the number of MRE11 clusters was higher com-pared to yH2AX clusters. It was also shown that the diameter of yH2AX foci in MCF7 cells was above 500nm early after radiation treatment. Furthermore, we identified an opening of the clusters for DNA repair which leads to a compression of material in the rest of the nucleus. Only through applica-tion of the high resolution microscopy method it was possible to identify the reorganization during DNA repair and allows new insights to the organization of chromosomal material in and outside of damage foci.