Double stranded breaks are the most dangerous form of DNA damage occurring in the genome. Double strand breaks are caused by internal agents like reactive oxygen species and many external agents like radiation. Replication errors also cause double strand breaks. Living organisms have two methods for correcting this error: Homologous recombination(HR) found both in bacteria and eukaryotes and non-homologous end joining (NHEJ) found mostly in eukaryotes. HR is a method which corrects the double strand breaks by using the homologous chromosome as the reference (this is the reason for diploid nature of most eukaryotes). Except for gene conversion, it does not cause mutation to the genes in which the double strand break has occurred (usually broken ends have damaged bases which are removed and correct bases are added by HR using the homologous chromosome as the template). But NHEJ just joins the broken ends without replacing the damaged bases (as it does not have a template to refer) leading to mutation of the genes in which double strand breaks occurred.
