General Genetics/Molecular Evolution

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Recombination[edit]

Site-Specific Recombination[edit]

When recombination is site-specific, it can only occur at specific regions of homology.

Illegitimate Recombination[edit]

Originally called "bastard recombination," illegitimate recombinations can occur even when there is little to no homology. Transposons are capable of illegitimate recombination.

Homologous or Generalized Recombination[edit]

Holliday model of recombination[edit]

After homologous chromosomes align, a single strand of each DNA breaks and invades the other DNA. Enzymes such as ligase seal the crossed strands, and the two double-helices swap

The Holliday intermediate will be resolved in one of two ways: creation of a patched duplex or creation of a spliced duplex.

Meselson and Radding model of recombination[edit]

After homologous chromosomes align, a single strand of DNA is nicked and replicated. The replicated DNA displaces the original strand, which now dangles from the double-helix and is known as a whisker. The whisker invades the homologous double-helix and displaces that DNA, which is excised. The whisker is ligated to the new double-helix. Branch migration ensues and, from there, this model is identical to the Holliday model.

This model is supported by the discovery of Rec A and Rec BCD. Rec BCD is an E. coli protein with DNA helicase activity capable of making single-stranded nicks. This would allow it to form the "whiskers" characteristic of the Meselson-Radding model. Rec A is capable of coating exposed ssDNA and integrating it into dsDNA, which would occur during whisker invasion of the homolog in the Meselson-Radding model. E. coli is not recombination when the genes that encode Rec A and Rec BCD are knocked out.

Other proteins with potential roles in recombination[edit]

PRDM9 is a zinc finger protein. methylates lysine 4 of histone 3. In the process, it recruits enzymes that creates double-stranded DNA breaks. Since PRDM9 is polymorphic, it can promote DNA cleavage at different sites. A zinc finger is a common protein structural motif characterized by stabilization of its folds by zinc ions. Most DNA-binding proteins have the zinc finger motif.

Ruv (repair UV radiation) AB promotes branch migration. Ruv C binds to the Holliday junction and cleaves the crossover strands.