Four-stranded DNA : an intermediate of homologous recombination and transposition

Stem-and-loop structures found in native duplex DNA from Pennisetum typhoides mitochondria suggest that four-straned DNA mediates homologous recombination and transposition. A space-filling model of DNA has been constructed for the stem-and-loop structure by using atomic coordinates of Watson-Crick DNA B Ⅰ model but with reversed chain directions(Hopkins’ B Ⅱ model). The stem forms four-stranded DNA by interduplex base pairing between inverted repeat sequences. A molecular pathway of homologous recombination involving four-stranded DNA formation has been proposed and tested with the space-filling model. Biological significance of the four-stranded DNA and repetitive sequences is discussed.