|MoBio||The Holliday Model of DNA Crossover||Chapter 8|
The widely accepted model for DNA crossover was first proposed by Robin Holliday in 1964. It involves several steps as illustrated in the following figure.
The detailed mechanism of homologous recombination was mainly obtained from the study of E. coli. Although bacteria do no undergo meiosis, homologous recombination could occur during or immediately after DNA replication. It may also occur in a mating process called conjugation.
In E. coli, the recombination is initiated by the enzyme RecBCD, consisting of three subunits: RecB, RecC and RecD. This enzyme has both helicase and nuclease activities. The enzyme first uses its helicase activity to unwind DNA. When it hits the Chi site (with sequence GCTGGTGG), one of the exposed strand will be cut by its nuclease activity. The reason why this special site is called the "Chi site" is because the Greek letter χ (chi) looks like a crossover point. The Chi site is the position of the Holliday junction and also the position of chiasma.
After DNA strands are cut by RecBCD, the strand invasion is catalyzed by RecA proteins, which can wrap around single stranded DNA and direct it to form the Holliday structure.
Finally, the branch migration is catalyzed by RuvA and RuvB. The Holliday structure is resolved by the protein RuvC.