Dsb

Double-stranded break (DSB) refers to a damage event in which both strands of the DNA double helix are severed on the same location. DSBs are among the most severe forms of DNA damage because they can lead to chromosome rearrangements, gene loss, or cell death if not correctly repaired. They can be caused by ionizing radiation, certain chemicals, reactive oxygen species, replication stress, or programmed during meiosis and, in genome editing, by programmable nucleases.

DSBs trigger cellular responses that sense damage, halt cell cycle progression, and coordinate repair or, if damage is irreparable, cell death. If left unrepaired or misrepaired, they contribute to genome instability and have been implicated in aging and cancer development.

Repair of DSBs occurs mainly through two major pathways. Non-homologous end joining (NHEJ) ligates broken ends directly and is active throughout the cell cycle but can introduce small insertions or deletions. Homologous recombination (HR) uses a homologous template, typically the sister chromatid, to restore the original sequence with high fidelity and is restricted to S and G2 phases. Alternative end joining, including microhomology-mediated end joining (MMEJ), is more error-prone and can lead to deletions or complex rearrangements. The choice of pathway depends on cell cycle stage, chromatin context, and the presence of repair factors.

DSBs are central to cancer therapy and genome editing. Ionizing radiation and many chemotherapeutics induce DSBs to kill cancer cells, while defects in DSB repair genes (for example BRCA1/2) create therapeutic vulnerabilities. In genome editing, DSBs introduced by tools like CRISPR-Cas9 are repaired by NHEJ or HR to create targeted mutations or insertions. Detection methods include γ-H2AX foci, comet assays, and sequencing-based approaches.