Fig. 1.

(a) Due to the Watson-Crick base pair mechanism and neglecting the difference of excluded volumes between loops and stems, single filaments of RNA have a branched secondary structure (here, visualized by the FORNA³¹ software). These structures have been shown^6–8 to play a crucial functional role in the RNA behavior inside viral capsids. (b) The increase of the level of supercoiling in circular polymers (rings) gives rise to the formation of plectonemes: by neglecting plectonemes torsional properties, rings assume the shape of branched structures⁹ which reproduce the experimental behaviour of circular DNA inside the bacterial nucleoid¹⁰. According to the total amount and distribution of supercoiling the ring may continuously switch between different branched structures. (c) By neglecting the excluded volume interactions between double-folded strands, a single ring polymer constrained in a matrix of fixed obstacles^21–23 resembles a branched polymer with annealed connectivity: in this situation, an “elk” may change into a “camel” and viceversa.