Holliday intermediate

In this double-strand break repair model for recombination, the 3 ends are used to initiate the genetic exchange. Once paired with the complementary strand on the intact homolog, a region of hybrid DNA is created that contains complementary strands from two different parent DNAs (the product of step (2)in Fig. 25-31a). Each of the 3 ends can then act as a primer for DNA replication. The structures thus formed, Holliday intermediates (Fig. 25 31b), are a feature of homologous genetic recombination pathways in all organisms. 

The invading 3 end is extended by DNA polymerase plus branch migration, eventually generating a DNA molecule with two crossovers called Holliday intermediates. 

S) Cleavage of the Holliday intermediates by specialized nucleases generates either of the two recombination products. In product set 2, the DNA on either side of the region undergoing repair is recombined. 

Once a Holliday intermediate has formed, a host of enzymes—topoisomerases, the RuvAB branch migration protein, a resolvase, other nucleases, DNA polymerase... 

I or III, and DNA ligase—are required to complete recombination. The RuvC protein (Mr 20,000) of E. coli cleaves Holliday intermediates to generate full-length, unbranched chromosome products. 

To complete the reaction, the process must be repeated at a second point within each of the two recombination sites (steps (3) and (4)). In some systems, both strands of each recombination site are cut concurrently and rejoined to new partners without the Holliday intermediate. The exchange is always reciprocal and precise, regenerating the recombination sites when the reaction is complete. We can view a recombinase as a site-specific endonuclease and ligase in one package. 

FIGURE 25-41 DNA deletion to undo a deleterious effect of re-combinational DNA repair. The resolution of a Holliday intermediate during recombinational DNA repair (if cut at the points indicated by red arrows) can generate a contiguous dimeric chromosome. A specialized site-specific recombinase in E. coli, XerCD, converts the dimer to monomers, allowing chromosome segregation and cell division to proceed. 

Site-specific recombination occurs only at specific target sequences, and this process can also involve a Holliday intermediate. Recombinases cleave the DNA at specific points and ligate the strands to new partners. This type of recombination is found in virtually all cells, and its many functions include DNA integration and regulation of gene expression. 

DNA transposition 978 meiosis 979 branch migration 980 double-strand break repair model 980 Holliday intermediate 980... 

Holliday intermediates in homologous genetic recombination differ from their formation in site-specific recombination ... 

That this really occurs is shown by the fact that the Int protein cleaves synthetic Holliday intermediates derived from the att core and reseals the strands to give the expected products.581 Cleavage of the Holliday intermediate at points a (Fig. 

Holliday intermediate. An intermediate in genetic recombination in which two double-stranded DNA molecules are joined by virtue of a reciprocal crossover involving one strand of each molecule. 

The two nicked strand segments crossover, thus forming a Holliday intermediate. 

Homologous recombination can vary in many details from one species to another, but most of the steps outlined above are generally present in some form. There are two ways to cleave, or resolve, the Holliday intermediate so that the two recombinant products carry genes in the same linear order as in the substrates—the original, unrecombined chromosomes (step of Fig. 

Homologous genetic recombination can take place between any two DNA molecules that share sequence homology. In meiosis (in eukaryotes), this type of recombination helps to ensure accurate chromosomal segregation and create genetic diversity. In both bacteria and eukaryotes it serves in the repair of stalled replication forks. A Holliday intermediate forms during homologous recombination. 

Processing the Holliday junction. Completion of the recombination process requires "resolution" of the Holliday intermediate by endonuclease action followed by ligation and perhaps by gap repair. The major recombination pathway in E. coli employs a binding protein, a nuclease, and a helicase encoded by genes RuvA, B, and RuvA is a DNA binding... 

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