Figure 2.

The hooked juxtaposition hypothesis. The hypothesis put forth by Buck and Zechiedrich stipulates that type-2 topoisomerases unknot and decatenate by selective segment passages at hooked juxtapositions but not at free juxtapositions. Schematized here, as an example, is the hypothesized decatenating mechanism by type-2 topoisomerases of two daughter chromosomes, one red and one blue. Shown is the perspective of the small topoisomerase and the global linkage is hard to ascertain globally. Type-2 topoisomerase (schematically represented by a green circle) catalyzes segment passage specifically at a hooked juxtaposition (top row), which is more likely to occur when the two chromosomes are linked globally. The type-2 enzyme will not act at a free juxtaposition (bottom row), which is more likely to occur when two chromosomes are not linked globally (71). Although depicted here for decatenation, this model is the same for knot-generated juxtapositions as well.