Genetic recombination: the nature of a crossed strand-exchange between two homologous DNA molecules.

Abstract Molecular models have been used to demonstrate that a crossed strand-exchange between two homologous DNA molecules, a likely intermediate in genetic recombination, can be formed with all of the bases in the two double-helices remaining paired. There are two “outside” and two connecting, or “bridging”, strands in this structure. Since the positions of these two pairs of strands can be interchanged in the model by rotation, all four strands participating in the exchange must be regarded as equivalent. As a consequence, when pairing is terminated by strand-scissions, there is an equal probability of generating two kinds of DNA molecules: those with genes flanking the region of strand-exchange in parental configuration and those with flanking genes recombined.