ABSTRACT
https://www.niso.org/standards/z39-96/ns/oasis-exchange/table"> Homologous recombination This is sometimes called generalized recombination and involves the exchange of DNA between two regions of homology (i.e. similar but not necessarily identical DNA sequence) on two different DNA molecules. RecBCD digests double-stranded DNA until it reaches a chi sequence, where the activity of RecBCD then changes to a single strand-specific 5–3’ exonuclease, so that a single DNA strand remains as a long 3’ overhang. The free single-stranded DNA becomes coated with RecA protein, and can migrate into the other DNA molecule in a process of strand invasion. Once the nicks in the DNA strands have been joined, the two DNA molecules adopt a cross-shaped formation known as the Holliday junction. The binding of RuvA to the Holliday junction leads to the binding of RuvB to adjacent double-stranded DNA. RuvB is a translocase, and promotes the movement of the Holliday junction along the DNA molecules (branch migration). The single-stranded parts of the junction are cleaved (strand exchange) and the two molecules resolve when RuvC binds to the RuvAB-DNA complex. Site-specific recombination Recombination between regions of DNA homology of only a few base pairs can occur via site-specific recombination mediated byXerC and XerD. A similar method is used to integrate bacteriophage X DNA into its host’s genome. X Integrase makes cuts on both strands about 15 bp apart at a specific site. Integration host factor (IHF) catalyzes recombination between the homologous sites on the phage and host genome, resulting in the X genome insertion into the E. coli DNA. The reverse of the process is catalyzed by the phage-encoded excisionase, stimulated by host IHF and the protein Xis. Transposition Transposition is the insertion of short DNA fragments into any position in the genome. This is sometimes called illegitimate recombination as it appears to require no homology between sequences. Insertion sequences (IS) use this mechanism to move between sites and can be detected in many chromosomes and plasmids. Transposable elements could be looked on as very basic viruses without the means to encode a protein coat for protection outside the cell. The IS encoded transposase makes cuts at another site in the host chromosome and is excised to be integrated into the new site. The IS is flanked by inverted terminal repeats, which aid in the transposition process. As the transposase cuts are staggered, the bases either side of the repeats are duplicated after integration. The transposable viruses do not excise themselves during transposition, but place a copy else where on the geno instead (replicative transposition). Transposon mutagenesis can offer a convenient method of generating mutants via Tn5 derivatives. Related topics
(F4) Transcription
(F10) Bacteriophages
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