Homologous Recombination is a recombination that occurs between or within a sister chromatid or between DNA molecules containing homologous sequences on the same chromosome. Homologous recombination requires a series of protein catalysis, such as RecA, RecBCD, RecF, RecO, Rad51, Mre11-Rad50, etc. Homologous recombination reactions are usually divided into three stages according to the formation and resolution of the cross-molecule or Holliday Structure (Holliday Juncture Structure), namely the pre-association phase, the association formation and the resolution of the Holliday structure.
Homologous recombination reactions are strictly dependent on homology between DNA molecules. Recombination between 100% recombinant DNA molecules is common in homologous recombination between non-sister chromosomes, called Homologous Recombination, and less than 100% homology. Recombination between or within DNA molecules is called Hemologus Recombination. The latter can be "edited" by proteins responsible for base mismatching, such as MutS in prokaryotic cells or proteins such as MSH2-3 in eukaryotic cells. Homologous recombination can exchange DNA molecules in both directions, and can also transfer DNA molecules in one direction. The latter is also called Gene Conversion. Since homologous recombination is strictly dependent on homology between molecules, homologous recombination of prokaryotes usually occurs during DNA replication, while homologous recombination of eukaryotes is common after the S phase of the cell cycle.
Figure 1. Homologous Recombination (HR).
In genetic engineering, homologous recombination is used as a gene targeting in which engineered mutations are introduced into specific genes as a means of studying gene function. In this way, exogenous DNA having a sequence similar to the target gene but flanked upstream and downstream of the target gene position is introduced into the cell. The cell recognizes the same flanking sequence as a homologue, resulting in the exchange of the target gene DNA with the foreign DNA sequence during replication. Exchange inactivates or "knocks out" the target gene. In mice, this method is used to target specific alleles in embryonic stem cells, thereby enabling the production of knockout mice. Artificial genetic material similar to the target gene is introduced into the nucleus of the embryonic stem cell, which inhibits the target gene by a homologous recombination process. By inactivating the target gene, scientists can infer and study its biological function in mice. With the help of gene targeting, many mouse genes have been eliminated, resulting in hundreds of mouse models of human disease, including cancer, diabetes, cardiovascular disease and neurological diseases.
Geneknockout is a gene whose structure is known but whose function is unknown. Design experiments at the molecular level, remove the gene, or replace it with other similar genes, and then observe the experimental animal from the whole, and speculate the corresponding gene. Features. This is similar to the trilogy of observing the common-predictive function commonly used in early physiology studies. In addition to halting the expression of a gene, gene knockout includes the introduction of new genes and the introduction of site-directed mutagenesis. It is possible to knock out the corresponding normal gene with a mutant gene or other gene, or to knock out the corresponding mutant gene with a normal gene.
According to the difference in amino acid sequence similarity and catalytic mechanism, site-specific recombinases are mainly divided into two major families, namely, integrase and dissociation/invertase, which are far apart. The mechanism by which the integrase family catalyzes recombination is tyrosine-mediated chain exchange. This family contains recombinase systems that have been studied and widely used, such as Cre/loxP and FLP/FRT. The catalytic mechanism of the dissociation enzyme/invertase family is mediated by serine. When a phosphorus-containing serine linkage is formed between the enzyme and the DNA, the four strands of the DNA at the junction undergo coordinated staggered cleavage, and then recombined to complete the homology. Reorganization. Members of this family include φC31 integrase from Streptomyces phage, TP901-1 integrase from lactococcal phage, R4 integrase from actinomycete phage, and the like. The C31 integrase (φC31-int) can effectively mediate the recombination reaction between the attP site in the phage genome and the attB site on the chromosome of the bacterial host, and integrate the foreign gene into the genome to enable continuous and efficient expression of the transgene. Non-viral vector transfer systems have opened up new avenues for the application of gene therapy for genetic diseases.
1. Jasin M.; et al. Repair of strand breaks by homologous recombination. Cold Spring Harbor Perspectives in Biology.2013,5 (11): a012740.