Strand Uptake -The displaced recipient strand forms a single stranded displacement loop (D loop). According to the commonly accepted view, recombination begins with a homologous dsDNA molecule taking up the free end of a displaced strand. This results in the formation of a triple-stranded segment, This concept of strand uptake has been applied to the initiation of nonreciprocal recombination in bacterial transformation and conjugation, and to initiation of reciprocal crossing-over in eukaryotes. In φX174 (RFI) the super coiled DNA spontaneously takes up homologous ssDNA fragments in vitro. The reaction produces a D loop in which the donor strand pairs with the homologous recipient strand to from a triple-stranded structure
