Model of homologuous recombination•R. Holliday(1964) (Fig. 25.18)•chromosome pairing (synapsis)•single strand nick on two paird chromosomes•strand invasion •ligation•form holliday junction •branch migration (strand exchange)•isomerization •cleavage strand at junction-patch recombinant heteroduplex-splice recombinant heteroduplex-----1. Strand displacement synthesis occurs;2. A displaced single-strand end of the nicked duplex, A, invades the homologous region of the unbroken duplex, B;3. The displaced loop on duplex B is cleaved and partially degraded;4. The displaced end from duplex A is ligated to B; 5. Isomerization then occurs, as in the Holliday model, with the originally unbroken strands crossed.