Figure 5.

The polarity paradox. Circles indicate the location of RecA protein or one of its homologs. A spur at the end of a line designates a 3′ end. The heavy line in E represents new DNA synthesis. (A) Polarity, 5′ to 3′ with respect to the single strand on which the nucleoprotein filament had formed, would dissociate all joints. (B–E) Another view of the role of polarity, according to which the polarity intrinsic to the recombination filament is only manifested in the migration of paranemic joints. Before strand invasion (B and C), 5′ to 3′ polarity would serve to drive a paranemic joint toward the 3′ end of the single strand where strand invasion (D) can occur. As in the double-strand break model for recombination, new DNA synthesis would extend and stabilize the joint (E). The subsequent action of enzymes like RuvA, B, and C can extend the region of heteroduplex DNA and finally resolve the joints into recombined products. According to this hypothesis, the polarity intrinsic to the filament only plays a biological role before strand invasion, after which the actions of other enzymes supervene.