The Holliday Model of DNA cossover

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 Robin Holliday proposed in 1964, a model for the exchange of genetic material. His model and junction is drawn from knowledge gained from his observation of yeast.

The Model

The Holliday Model of DNA crossover

A) Two homologous DNA duplexes line up (e.g.nonsister chromatids in meiosis).

B) Cuts in one strand of both DNAs at the same position.

C) Cut strands cross and join homologous strands, forming the Holliday structure or junction.

D) Heteroduplex forms by branch migration.

E) Resolution of the Holliday structure. Figure is a different view than (d). Strands may be cut and re-joined along the vertical (strands not previously cut) or horizontal line (same strands as initially).

F) Vertical cut results in crossover.

G) The horizontal cut does not lead to crossover.

In both cases a heteroduplex region is produced which will eventually be corrected by mismatch repair - this may lead to gene conversion.

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