FIGURE 1.

DNA damage response. DNA damage is caused by endogenous agent oxygen species (ROS) or exogenous agents such as UV light, ionizing radiation (IR) and chemotherapy agents. DNA damage response (DDR) is induced to deal with the lesions, including signal transduction, transcriptional regulation, cell-cycle checkpoints, induction of apoptosis, multiple DNA repair pathways as well as damage tolerance processes. DNA repair pathways include nuclear and mitochondrial DNA repair pathways. Direct repair, BER, MMR and recombinational repair (HR and NHEJ) are existence in both nuclear and mitochondrial repair systems. NER has been reported only appearance in nucleus, and the existence of TLS pathway in mitochondria is unknown. NDNA, nuclear DNA; MtDNA, mitochondrial DNA; BER, base excision repair; HR, homologous recombination repair; NHEJ, non-homologous end joining; MMR, mismatch repair; TLS, translesion synthesis; NER, nucleotide excision repair.