sentences:  It is within this synaptic complex that the strand exchange takes place, as the DNA is cleaved and rejoined by controlled transesterification reactions.During strand exchange, each double-stranded DNA molecule is cut at a fixed point within the crossover region of the recognition site, releasing a deoxyribose hydroxyl group, while the recombinase enzyme forms a transient covalent bond to a DNA backbone phosphate.This phosphodiester bond between the hydroxyl group of the nucleophilic serine or tyrosine residue conserves the energy that was expended in cleaving the DNA.