strandbreak

Strand break is a term used in molecular biology to describe a disruption in the phosphodiester backbone of a DNA strand. Breaks can affect only a single strand (single-strand breaks, SSB) or both strands of the double helix (double-strand breaks, DSB). SSBs often arise from spontaneous hydrolysis or oxidative damage and are generally less immediately lethal than DSBs, but both types threaten genome integrity if not repaired promptly.

Causes include ionizing radiation, reactive oxygen and nitrogen species, certain chemotherapeutic agents, and replication-associated stress. Mechanical stress during chromatin condensation and exposure to alkylating chemicals can also produce strand breaks. In cells, replication forks encountering an SSB can convert it into a DSB, increasing the risk of genome instability.

Detection methods include the comet assay, which measures DNA migration under an electric field, and immunofluorescence of γ-H2AX or 53BP1 foci as markers of DSBs. In neutral comet assays, primarily DSBs are detected, while alkaline versions detect SSBs as well as DSBs.

Repair pathways differ by break type. SSBs are repaired mainly through base excision repair and single-strand break repair, involving enzymes such as PARP1, XRCC1, PNKP, and DNA ligase III. DSBs are repaired by non-homologous end joining, which operates throughout the cell cycle but can be error-prone, and homologous recombination, which uses a sister chromatid as a template for accurate repair.

Biological significance. Accumulation of strand breaks contributes to mutations, chromosomal rearrangements, and cell death. In medicine, intentional induction of DSBs underpins many cancer therapies, and measurement of strand breaks serves as a biomarker of DNA damage and treatment efficacy. In scientific literature, strand break is more common than strandbreak, though both forms appear.