Genome rearrangement by homologous recombination between repetitive sequences. A circular bacterial genome in illustrated. The dashed line represents the replication origin-terminus axis about which bi-directional replication of the chromosome occurs. Red arrows indicate the positions and relative orientations of the repeat sequences, W, X, Y and Z. The lower-case letters a, b, c and d represent sequences bounded by some of these repeat sequences. (a) Recombination between non-allelic repeat sequences (Y and Z) present on sister chromosomes after replication, can lead to duplication of the Y - d - Z region. (b) Recombination between repeat sequences in the same orientation on the same chromosome (Y and Z) can lead to the excision of a DNA fragment (Z/Y - d) that can recombine at another repeat position on the chromosome (W), resulting in a translocation. (c) Recombination between repeat sequences in inverse orientations on the chromosome (X and Z) can lead to inversion of the intervening sequence.