Construction and use of models designed to demonstrate some topological properties of multistranded nucleic acid structure and its possible bearing on recombination.

A description is given of the construction and use of a simple model designed to show the consequences of association between duplexes to form some kind of four strand structure in recombination. The model was first devised to help in consideration of the work of Nash et al. (1980). Conclusions drawn from it may however be of wider interest. The model was designed with a detailed molecular model in mind (McGavin, 1971a, 1979). It could however apply to any close juxtaposition of duplexes. A close juxtaposition of duplexes is a feature of several models for recombination, although this is not often referred to explicitly as four strand structure.