Models of homologous recombination. DSBs can be repaired using the homologous recombination machinery in a variety of ways. The DNA ends are first processed into 3' ssDNA tails. These tails invade a homologous template (red) priming new DNA synthesis (dashed line). Three possible outcomes from this invasion are shown. A) In canonical DSBR, both the initial invading strand and the captured second end anneal to the homologous template and prime new DNA synthesis, resulting in a double Holliday junction that can be resolved by nucleases into crossover or non-crossover products (non-crossover product shown). B) Alternatively, after the single ssDNA tail invades the homologous template, a round of DNA synthesis is primed from the 3' end (dashed red line). Synthesis-dependent strand annealing (SDSA) occurs when the invading strand, along with the newly synthesized segment, is unwound by a helicase and annealed with the other resected end. C) In break-induced replication (BIR), one end of the DSB is lost and the remaining end invades the homologous template priming DNA synthesis to the end of the chromosome.