Proposed recombination models based on a linear dsDNA substrate. (A) Strand invasion model. A linear DNA is resected on both ends by a 5' to 3' exonuclease (e.g. Redα, RecE, not shown) and the single stranded 3' tails are bound by a recombinase (Redβ or RecT; red circles) to generate nucleoprotein filaments on each end of the linear DNA, which promote a double-crossover joint molecule by strand invasion into target dsDNA. (B) DNA annealing model. Exonucleolytic resection and recombinase assembly is the same as A. However, the nucleoprotein filaments anneal directly to complementary single stranded 3' tails generated from a DSB in the target DNA molecule. (C) Chicken foot model. Exonucleolytic resection and recombinase assembly is the same as A. One end of the linear DNA anneals to a single-stranded region at the lagging strand of the replication fork. A 'chicken foot' intermediate is formed. Backtracking of the chicken foot intermediate by branch migration reestablishes the replication fork (adapted from ). D) Bridge model. The linear DNA is inserted in between two colliding replication forks. Each nucleoprotein filament at the ends of the cassette (shown in blue) anneals to the lagging strand of one or the other replication fork thereby inserting the cassette in the middle of the two replication forks (adapted from ). E) Schematic illustration of an alternative digestion product after 5' to 3' Redα exonuclease activity. Instead of degradation from both ends of the linear dsDNA, Redα starts only at one end and complete digestion leaves an intact single-strand behind.