Holliday junction

A Holliday junction is a mobile junction between four strands of DNA, formed during recombination between the two-stranded DNA helices of two homologous chromatids.

In prophase I of meiosis, duplicated homologous chromosomes pair and align end-to-end. Crossover can occur between aligned chromatids, leading to exchange of homologous segments by recombination. In the original Holliday model for homologous recombination, single-strand breaks occur at the same point on one strand of each parental DNA. Free ends of each broken strand then migrate across to the other DNA helix, where the invading strands are joined to the free ends they encounter. The resulting crossover junction is the Holliday junction.

As each crossover strand reanneals to its original partner strand it displaces the original complementary strand ahead of it, causing the Holliday junction to migrate. This creates heteroduplex DNA segments.