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(a) The Topology of Borromean Rings. Note that cleaving any ring leaves the other two rings unlinked; the key is that the nodes on the inside are of a different sign from those on the outside. (b) Forming Borromean Rings from DNA. A 3-arm DNA branched junction made of B-DNA (top) is linked to a 3-arm junction made of Z-DNA (bottom) through hairpins at the equator of the product molecule.
