in tetrahedral form
When strand exchange occurs between two DNA duplexes, they become
linked by a structure known as a Holliday junction.
The animation on this page shows the symmetry of Holliday structures. Another round of strand exchange can resolve the junction into two distinct duplexes again, but the result can be either a recombinant or non-recombinant pair, depending on which strands are exchanged. This animation, made by your webmaster (BE -- who has far better things he should have been doing), shows the equivalence of these two kinds of exchange.
Another interesting property of Holliday junctions is that they can "slide" along the length of the two double helixes creating heteroduplex DNA in their paths, as illustrated in animations elsewhere on this web site. A more realistic animation of a migrating Holliday junction along with representations of some of the proteins that control it can be found at: http://www.sdsc.edu/journals/mbb/ruva.html