DNA double-strand breaks (DSBs) are highly lethal lesions that jeopardize genome integrity. However, DSBs are also used to generate diversity during the physiological processes of meiosis or establishment of the immune repertoire. Therefore, DSB repair must be tightly controlled. Two main strategies are used to repair DSBs: homologous recombination (HR) and non-homologous end joining (NHEJ). HR is generally considered to be error-free, whereas NHEJ is considered to be error-prone. However, recent data challenge these assertions. Here, we present the molecular mechanisms involved in HR and NHEJ and the recently described alternative end-joining mechanism, which is exclusively mutagenic. Whereas NHEJ is not intrinsically error-prone but adaptable, HR has the intrinsic ability to modify the DNA sequence. Importantly, in both cases the initial structure of the DNA impacts the outcome. Finally, the consequences and applications of these repair mechanisms are discussed. Both HR and NHEJ are double-edged swords, essential for maintenance of genome stability and diversity but also able to generate genome instability.
Keywords: DNA repair; double-strand break repair; genome instability; genome rearrangements; homologous recombination; ionizing radiation; mutagenesis; non-homologous end joining; telomeres.