Homologous Recombination

Homologous recombination is defined as the exchange of segments which are homologous and from two different DNA molecules. Studies done both genetically and cytologically have indicated that the cross-over process occurs during meiosis of higher organisms.1

Homologous recombination can be used to eliminate DNA double strand breaks, restart stalled replication forks and provide a means for the maintenance of telomere length in cells where telomerase is lacking.

Homologous recombination is used when an unwanted (or sometimes mutated) allele is to be replaced by a more desirable one, this process is also known as knock out genes or gene silencing and is most commonly seen in "knock out mice". These mice are a type of genetically modified organism in which both alleles of specific gene are replaced by an inactive version of the allele.2 the required homologous recombination is usually followed by two or more generations of selective breeding after which breeding pair is isolated that have both alleles or the target gene knocked out or replaced. This allows the researcher to easily determine the role a certain gene plays by observing the phenotype of individuals who are completely lacking the particular gene.2

In order to perform homologous recombination the sequence of the DNA that is to be replaced must be known and with this knowledge it is possible to replace the allele with various DNA constructs.3 Once the DNA sequence is known the next step is to design and fabricate the DNA construct which is to replace the unwanted allele.2 This could be a defective or broken gene that needs to be replaced. Once the DNA construct has been fabricated it needs to be integrated into the organisms existing DNA.

The prototypical model for homologous recombination was proposed in 964 by Robin Holliday. The model requires the corresponding strands of homologous DNA to be nicked or cut. It is at these nicks that crossing over takes place after which the nicks are sealed to form a four-way junction known as a Holliday junction (see figure 1 below).1

Figure 1.2

All the bases in the strands form normal Watson-Crick base pairs without any apparent strain. The Holliday junction can be resolved into two different duplex DNAs. The first duplex is when cleavage occurs at the strands which did not cross over leading to the exchange of the ends of the original