Sister chromatid exchanges occur in G2-irradiated cells

DNA double-strand breaks (DSBs) are arguably the most important lesions induced by ionizing radiation (IR) since unrepaired or misrepaired 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 G(1) and G(2) phases of the cell cycle, but HR also contributes and repairs a subset of IR-induced DSBs in G(2). 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 G(2)-irradiated cells also generates SCEs in similar to 50% of HR events. Since HR of IR-induced DSBs in G(2) is a slow process, SCE formation in G(2)-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 G(2)-irradiated cells and provide evidence that following IR cells do not need to progress through S phase in order to form SCEs.