1.5 DNA Repair MechanismDNA double-strand breaks (DBS) and single-strand breaks (SSBs) occur every day in cells and are mainly caused by ionizing radiation, ultraviolet light , reactive oxygen species, errors during DNA replication, enzymes during meiosis. Repair of these DSBs and SSBs is essential for maintaining genomic fidelity and stability. To combat DBS and SSB, cells have evolved multiple distinct DNA repair mechanisms that detect damaged DNA, signal its presence, and promote damage repair (Jackson and Bartek, 2009). One such mechanism is base excision repair (BER). BER is a multistep process that corrects nonbulky base damage resulting from oxidation, methylation, deamination, or spontaneous loss of the DNA base itself. In BER, DNA glycosylase recognizes the damaged base and mediates base removal before proliferating cell nuclear antigen (PCNA), polymerase β, and DNA ligase I or DNA ligase III complete the repair process (Jackson and Bartek , 2009, David et al., 2007) . Nucleotide excision repair (NER) is perhaps the most flexible of the DNA repair pathways. NER recognizes and repairs lesions caused by helical distortion of the DNA duplex and pyrimidine dimers (cyclobutane-pyrimidine dimers and 6-4 photoproducts) caused by the UV component of sunlight. Other NER substrates include bulky chemical adducts, DNA intrastrand cross-links, and some forms of oxidative damage. There are two distinct NER pathways: transcription-coupled NER that focuses on transcripts that block lesions, and global genome NER that examines the entire genome for skewed damage ( Jackson and Bartek, 2009 , David et al., 2007 ). The DNA mismatch repair (MMR) pathway plays an essential role in the means of correcting paper proteins. The large protein kinase, consisting of 4128 amino acids (aa), has a molecular weight of 469 kDA (33, 85-87). DNA-PKcs associates with the Ku70/80 heterodimer and forms a catalytic active DNA-PK holoenzyme ( Falck et al., 2005 ). The kinase activity of DNA-PKcs is activated by interaction with free DNA. The protein can bind to DNA fragments in the absence of the Ku complex. However, its kinase activity appears to be much lower (Hammarsten and Chu, 1998). DNA-PKcs mediates the synapsis and ligation of the two DNA fragments (Block et al., 2004; Kysela et al., 2005). Autophosphorylation of DNA-PKc results in remodeling of DNA-Pk ( Block et al., 2004 ). Furthermore, DNA-PK phosphorylates histones H2AX and H1. This may indicate that DNA-PK modifies chromatin structure to facilitate access of other DNA repair complexes to DSBs (Kysela et al., 2005