English: Tyr-Recombinases: Details of the crossover step

Top: Classical view including strand-exchange followed by branch-migration (proofreading). The mechanism takes within a synaptic complex (1) including both DNA-sites in parallel orientation. While branch-migration provides a straightforward explanation regarding the specific homology requirements an the reversibility of the process, it is difficult to reconcile with the motions the recombinase subunits have to undergo in three dimensions

Bottom: Current view. Two simultaneous strand-swaps shown, each depending on the homology of three successive bases at the edge of the (6-8 bp long) spacer. Didactic complications arise from the fact that the model has to account for the fact that the synaptic complex (1) contains both substrates in anti-parallel orientation

Both representations illustrate only half of the respective pathways. Both parts are separated by a Holliday-junction/isomerization step, before the product (3) is released