Fig. 3

The mechanism for homologous recombination proposed by Holliday (1964). (a) Two homologous chromosomes have different alleles at the A, B, and C loci. Lines represent individual DNA strands and arrowheads are the 3′-end. (b) Nicks are made in two strands and the DNA is unwound partially. (c) Heteroduplex DNA forms as DNA from each original duplex forms base pairs with complementary DNA in the other duplex (red with black), in a structure called a Holliday junction. Two isomeric forms of the Holliday junction are shown that differ by 180° rotation of the lower duplex relative to the upper duplex. (d) The Holliday junction is resolved by two nuclease cleavages followed by ligation of the DNA ends. The heteroduplex DNA (B paired with b in this example) can be repaired to B/B or b/b (not shown). The recombination products can have crossing over of the flanking markers A and C or not depending on how the nuclease cleavages are made