Homologous recombination is a type of genetic recombination, a process of physical rearrangement occurring between two strands of DNA. Homologous recombination involves the alignment of similar sequences, formation of a Holliday junction, and breaking and repair, known as resolution, of the DNA to produce an exchange of material between the strands. The process of homologous recombination naturally occurs in organisms and is also utilized as a molecular biology technique for introducing genetic changes into an organism.

Homologous Recombination in Prokaryotic Organisms

Because of its standing as a model organism, homologous recombination in Escherichia coli is the most well-known version of the pathway, known as the RecBCD pathway. It is used in DNA repair, to restart replication forks that have been stalled or damaged, and to regulate gene expression (as in the function of transposons). Additionally, due to recognition of recombination enzymes of specialized sites within the bacterial chromosome, foreign DNA can be degraded and, thus, destroyed, protecting the E. coli cell.

In the Double-Strand Break (DSB) Repair Pathway, Homologous recombination is mediated by RecA and RecBCD, along with RuvABC. These enzymes are attracted to double strand breaks, search for homology between the duplex strands and catalyze formation of the Holliday junction, branch migration, and resolution. RecBCD assembles at the (DSB), then, by exonuclease activity, chews off the DNA until it reaches a chi site. Once this occurs, RecD is inactivated or lost, and the enzyme continues to cut the DNA strand, leaving a 3' tail.

RecA binds to the single stranded DNA, forming a nucleoenzyme filament. RecA protects the DNA from single strand binding proteins by coating the 3' tail, as it facilitates hybridization between the single strand region and the double stranded DNA, known as strand invasion, needing only about 15 base pairs of homologous region. Where the strands cross is known as the Holliday junction.

RuvA binds to the Holliday junction and recruits RuvB. Branch migration, movement of the Holliday junction down the DNA strand, is catalyzed by RuvB, a hexameric ATPase. RuvC is an endonuclease that cuts with slight specificity, allowing some degree of branch migration before resolving the junction.

There are two types of products resulting from recombination, stemming from resolution of the Holliday junction: splice and patch. Splice products are crossover products, where there is reassortment of genes, and patch resolution yields non-crossover products.

Homologous Recombination in Eukaryotic Organisms

The process of homologous recombination occurs during chromosomal crossover, a process occurring during meiosis in eukaryotic organisms that results in a shuffling of genetic material. Homologous recombination is also involved in DNA repair, as organisms repair a damaged region using the material from a partner chromosome as a template.^[1] In bacteria, homologous recombination introduces DNA into a bacterium through conjugation, transduction, or natural transformation.

Homologous Recombination in eukaryotes has the additional role of protecting organisms from cancer. In organisms that possess mutations in the pathway, there is a tendency to develop certain cancers.

Homologous Recombination in Other Biologically Important Molecules

RecA homologs have been found in other bacteria, archaea, eukarya and bacteriophages.

Artificial homologous recombination

Many methods for introducing DNA sequences into organisms to create recombinant DNA and genetically modified organisms use the process of homologous recombination.^[2] Also called "gene targeting", the method is especially common in yeast and mouse genetics. The gene targeting method in the mouse model utilizes mouse embryonic stem cells to deliver artificial genetic material (mostly of therapeutic interest) which represses the target gene of the mouse by the principle of homologous recombination. The mouse thereby acts as a working model to understand the effects of a specific mammalian gene (More on knockout mice). This work yielded Mario Capecchi, Martin Evans and Oliver Smithies the 2007 Nobel Prize for Physiology or Medicine.^[3]

References

1. ↑ 12.5. Recombination between Homologous DNA Sites: Double-Strand Breaks in DNA Initiate Recombination
2. ↑ 8.5. Gene Replacement and Transgenic Animals: DNA Is Transferred into Eukaryotic Cells in Various Ways
3. ↑

See also

• Genetic recombination
• Chromosomal crossover
• DNA repair
• Gene targeting

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