pair. The inference from this is that base pairing is involved during the process. Damage to DNA stimu-
lates recombination, suggesting strongly that homologous recombination is initiated from broken DNA
strands. Presumably, under normal circumstances the cell creates such breaks enzymatically to promote
homologous recombination.
6.8.1.1.1 The Holliday Junction. Numerous models have been proposed to explain the mechanism
of homologous recombination. A key intermediate in almost all of these is the Holliday junction, named after
its proposer (Figure 6.36).^57 One of the key properties of this structure is its ability to migrate along the
DNA helices by a process called branch migration. A reversal of the Holliday junction formation gives a
strand swapping event. Finally, DNA replication fixes the new arrangement in the daughter chromosomes.
Genes and Genomes 239
Figure 6.34 Repair of double-stranded DNA breaks in eukaryotes
Figure 6.35 Homologous recombination involves the reciprocal exchange of DNA between chromosomes