Figure 7.

Plasmids and catenanes. The plasmid (on the left) contains a single recognition site (indicated with a hatchmark) within a specified segment of the circular DNA (indicated in blue). The remainder of the DNA (in red) contains only non-specific sequences. A protein (green sphere) bound to the non-specific (red) DNA can reach the recognition site by 1-D diffusion around the circle or by 3-D transfer (not shown). The plasmid is converted by a recombinase into a catenane (on the right) with two interlinked circles of DNA: a small circle (in blue) that carries the recognition sequence; a large circle (in red) with only non-specific sequences. A protein bound to the non-specific DNA in the large (red) circle cannot reach the recognition site in the small circle by 1-D diffusion but must instead dissociate from the large ring before re-associating with the small ring: since both rings are covalently continuous, the dissociation cannot occur from an end of a chain.