ABSTRACT
Recombination is any process generating new combinations of preexisting genetic material. Intermolecular or interchromosomal recombination generates new combinations by mixing discrete chromosomes (independent assortment of eukaryotic chromosomes at meiosis, and reassortment of segmented viral genomes), whereas intramolecular or intrachromosomal recombination is an enzyme-dependent process where new combinations of genetic material arise by cutting and joining DNA. There are five types of intramolecular recombination (Table 25.1), although the molecular reactions — cleaving DNA, exchange of strands between duplexes, DNA repair and resolution — are similar in each case. Different types of intramolecular recombination Homologous recombination Requires homology between the recombining partners. The proteins mediating this process (e.g. RecA in E. coli) are not sequence-specific but are homology-dependent. Long regions of homology are often involved (e.g. during meiosis) Site-specific recombination Does not require homology between recombining partners. The proteins mediating this process (site-specific recombinases) recognize short, specific DNA sequences in the donor and recipient molecules, and interaction between the proteins facilitates recombination. Homology often exists between the donor and recipient sites because the same recombinase protein binds to both recognition sites Transposition Does not require homology between recombining partners. The proteins mediating this process (transposases, integrases) recognize short, specific DNA sequences in one of the recombining partners only, which is a transposable element (the site of recognition is usually at the junction between the transposable element and the host DNA). The recipient site is usually relatively nonspecific in sequence, and recombination integrates the transposable element into the host DNA (see Mobile Genetic Elements) Illegitimate recombination Requires little or no homology between recombining partners and results from aberrant cellular processes. Includes illegitimate end-joining, and strand-slipping or looping during replication. Unequal crossing over (q.v.) is often described as illegitimate, although the mechanism is normal (albeit misplaced) homologous recombination Artificial recombination Recombination carried out by DNA ligation (q.v.) in vitro using purified enzymes and substrates (see Recombinant DNA)
