Genetic evidence suggests that crossing over takes place after DNA synthesis

rt Two double-stranded DNA molecules from homologous chromosomes align.

^ Single-strand breaks occur in the same position on both DNA molecules.

|3| The free end of each broken strand migrates to the other DNA molecule.

^ Each invading strand joins to the broken end of the other DNA molecule, creating a Holliday junction, and begins to displace the original complementary strand.

Holliday junction

112.23 In the Holliday model, homologous recombination is accomplished through a single-strand break in each DNA duplex, strand displacement, branch migration, and resolution of a single Holliday junction.

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