Types of Genetic Recombination

The recombination mechanisms can be classified into the following three categories:

    1. General
    2. Site Specific
    3. Recombination of Non-homologous DNA

General Recombination:

General recombination involves a reciprocal exchange of homologous parts of DNA in non-sister chromatids. The exchange occurs between faithfully aligned alleles or homologous DNA sequences. As a result equal portions of DNA are reciprocally exchanged. It occurs in eukaryotes during sexual reproduction at the time of meiosis.

The General recombination events are dependent on the product of rec A gene. A mutants don’t exhibit crossing over. Howard Flanders (1984) provided molecular explanation for the need of rec A gene product for general recombination. West and Stasiak provided model of a spiraling rec A protein that interacts with DNA during recombination.

Site Specific Recombination:

Site specific recombination involves insertion of a stretch of alien of DNA into a specific locus or at specific site in the chromosomal DNA. Integration of phage lambda DNA in the chromosome of E.coil at a site between gal and bio operons is an example of site specific recombination.

Mechanism of Site Specific Recombination:

Phage lambda DNA has a linear double stranded DNA. On being injected into E.coil it becomes circular due to complementary base pairing of ‘sticky ends’. Both lambda DNA and E.coil DNA have a common 15 bp stretch of DNA in their genomes. Its base sequence is GCTTTTTTATACTAA… In lambda, it is called att or attachment locus. Lambda DNA also possesses genes int (integration gene) and xis (excision gene). These produce necessary enzymes for excision in the both the DNAs and for their integration.

Recombination of Non-homologous DNA (Transpositions):

This recombination occurs between unrelated and non-homologous stretches of DNA. When these were first discovered, these were called illegitimate recombination. Such a recombination is mediated by transposable elements or transposons and is now called transposition.

Transposition differs from reciprocal, non-reciprocal and even site specific recombination because these recombination occur between basically related homologous chromosome or between homologous segments of unrelated chromosomes, but in transpositions the segments are totally unrelated. There are evidence that transposition is an ubiquitous process and occurs regularly.