title: Sister chromatid exchanges occur in G2-irradiated cells.
abstract: DNA double-strand breaks (DSBs) are arguably the most important lesions induced by ionizing radiation (IR) since unrepaired or mis-repaired DSBs can lead to chromosomal aberrations and cell death. The two major pathways to repair IR-induced DSBs are non-homologous end-joining (NHEJ) and homologous recombination (HR). Perhaps surprisingly, NHEJ represents the predominant pathway in the G1 and G2 phases of the cell cycle, but HR also contributes and repairs a subset of IR-induced DSBs in G2. Following S-phase-dependent genotoxins, HR events give rise to sister chromatid exchanges (SCEs), which can be detected cytogenetically in mitosis. Here, we describe that HR occurring in G2-irradiated cells also generates SCEs in ~50% of HR events. Since HR of IR-induced DSBs in G2 is a slow process, SCE formation in G2-irradiated cells requires several hours. During this time, irradiated S-phase cells can also reach mitosis, which has contributed to the widely held belief that SCEs form only during S phase. We describe procedures to measure SCEs exclusively in G2-irradiated cells and provide evidence that following IR cells do not need to progress through S phase in order to form SCEs.
authors.affiliation: Darmstadt University of Technology, Radiation Biology and DNA Repair, Darmstadt, Germany.
authors.forename: Sandro
authors.initials: S
authors.lastname: Conrad
authors.forename: Julia
authors.initials: J
authors.lastname: Künzel
authors.forename: Markus
authors.initials: M
authors.lastname: Löbrich
mesh_terms: D045744:Cell Line, Tumor; D053903:DNA Breaks, Double-Stranded; D004260:DNA Repair; D016195:G2 Phase; D006657:Histones / Q000378:metabolism; D006801:Humans; D008938:Mitosis; D011839:Radiation, Ionizing; D011995:Recombination, Genetic; D016196:S Phase; D012854:Sister Chromatid Exchange / Q000502:physiology*
chemical_list: D006657:Histones