 Double @-@ strand breaks can be repaired through homologous recombination or through non @-@ homologous end joining ( NHEJ ) . NHEJ is a DNA repair mechanism which , unlike homologous recombination , does not require a long homologous sequence to guide repair . Whether homologous recombination or NHEJ is used to repair double @-@ strand breaks is largely determined by the phase of cell cycle . Homologous recombination repairs DNA before the cell enters mitosis ( M phase ) . It occurs during and shortly after DNA replication , in the S and G2 phases of the cell cycle , when sister chromatids are more easily available . Compared to homologous chromosomes , which are similar to another chromosome but often have different alleles , sister chromatids are an ideal template for homologous recombination because they are an identical copy of a given chromosome . In contrast to homologous recombination , NHEJ is predominant in the G1 phase of the cell cycle , when the cell is growing but not yet ready to divide . It occurs less frequently after the G1 phase , but maintains at least some activity throughout the cell cycle . The mechanisms that regulate homologous recombination and NHEJ throughout the cell cycle vary widely between species . 
