Holliday-Whitehouse Model

During Meiosis I, (a) two homologous chromosomes line up. Each shows a cis configuration of the A & B loci, one is AB and other ab , which may be written AB // ab. (b) Single-stranded nicking of one of the two DNA strands in each chromosome provides an exposed 5' end on each strand [trace this out] that can (c) be displaced or 'jump' to the other DNA molecule. (d) Ligation of the strands produces a covalently-joined heteroduplex branch region, in which the bases are paired between opposite chromosomes. (e) As the chromosomes are pulled apart during Meiosis, the heteroduplex branch region moves towards the opposite end of the pairing. [Think of the centromeres as being out of sight, to the left, such that the blue molecule is being pulled up and the red molecule is being pulled down]. (f) When this is shown with the heteroduplex drawn as an "X", the heteroduplex region is clear, and when the "X" is rotated, (g) the pattern of base-pairing in the heteroduplex is still clearer. (h) Single-stranded nicks in the non-recombinant chromatids from step (b) separate the heteroduplex into two (i) recombinant molecules, which are then ligated.

The genetic effect of the molecular mechanism is to reverse the linkage relationships (phases) of the alleles at loci on either side of the recombination event, so that the original cis configuration AB // ab has been changed to a trans configuration Ab // aB. If the markers were originally in trans, they would be reversed to cis.