A main point of this paper is to develop the idea that synapsis of DNA duplexes might take place by Watson-Crick base pairing between essentially intact duplex structures to form a four stranded intermediate. This intermediate is the same regular and compact four strand structure already discussed (e.g., McGavin, 1971a, J. molec. Biol., 55, 293-298; 1979, J. theor. Biol., 77, 83-99) and which has been used in several related recombination models (see, for example, McGavin, 1977, Heredity, 39, 15-25; 1984, J. theor. Biol., 107, 37-56; Wilson, 1979, Proc. natn. Acad. Sci. U.S.A., 76, 3641-3645; Nash et al., 1980, Cold Spring Harbor Symposium of Quantitative Biology, 45, 417-428; Nash, 1981, A. Rev. Genetics, 15, 143-167). These models can also be related to one recently suggested by Hopkins (1986, J. theor. Biol. 120, 215-222). An immediate stimulus to the development of the idea was the recent work of Griffith & Nash (1985, Proc. natn. Acad. Sci. U.S.A., 82, 3124-3128), on the site specific recombination system of the lambda bacteriophage. They observed trefoil knots with exclusively positive nodes among the products of the interaction of two relatively inverted attachment sites within circular molecules. The model discussed here may though be of interest in itself. The paper also compares several closely related models for recombination which involve formation of either identical or closely related four strand secondary structures. © 1989 Academic Press Limited.
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