STRAND BREAK REPAIR
- Single strand breaks are very rapidly repaired, by a process that is little more than ligation.
Figure Mammalian cells in culture were treated with H2O2 to induce DNA damage (mainly single strand breaks) (solid circles). The cells were incubated for two hours, and at intervals samples were taken for measurement of the strand breaks remaining, using the comet assay. The open squares represent control, untreated cells.
- Double strand breaks are potentially serious; one unrepaired DSB can lead to cell death, presumably because of the disruption of replication and mitosis that results. They are rejoined more slowly than SSBs, by homologous recombination or non-homologous end-rejoining. During homologous recombination, the homologous DNA sequence is used to guide the repair. They are rejoined more slowly than SSBs, by homologous recombination (HR), or by non-homologous end-joining (NHEJ). During HR, the homologous sequence is used as a guide, and repair is accurate.
NHEJ tends to be error-prone, as some sequence gets lost as the broken ends are brought together.
Figure Non-homologous end-joining
Ku protein facilitates sealing by keeping the broken DNA ends together.