Recombination not requiring extensive homology comprises site-specific recombination and illegitimate recombination. Site-specific recombination takes place at specific sites on one or both chromosomes, e. g. interaction of viral genomes and insertion sequences in prokaryotes and eukaryotes. Pairing of the two broken parental strands can take place at sites of very limited homology and is recA-independent. In the lambda phage both sites of crossover, i.e. in the phage DNA and in receptor DNA, are specific. In the transposable elements and insertion, sequences, the crossover site is specific, in the unit, but more or less random the receptor DNA. Illegitimate recombination requires neither specific homology nor specific sites. It occurs at a low frequency in the bacterial chromosome, and results in gene duplication and some types of deletions. The mechanism of recombination has been explained by three general groups of theories, breakage and reunion breakage and copying and complete copy choice.
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