Definition of Holliday Model

Holliday model is a model which elucidates the mechanism of the homologous recombination at a molecule level. Homologous recombination is the process in which the identical DNA segments are exchanged between the homologous chromosomes and the Holliday junctions which are cross-shaped structures, help in the pairing of segments of the DNA. The recombination is done in three steps which are: strand exchange, branch migration and resolution.


The model explains the events in such a way that the two homologous chromosomes form a pair with the identical sequences being in front of each other. Then an endonuclease comes which breaks the corresponding regions of the strands. Consequently, the dissociated ends of both the chromosomes invade one another.


The nicks are then ligated through DNA ligase and a structure known as Holliday intermediate is formed. The branches are migrated and the region of heteroduplex is being extended. Finally, the resolution is done by cleaving the strand in the duplex and the two separate DNA molecules are formed. 


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