EN
The pa per pres ents some as pects cor re lat ing ther mal sta bil ity of RNA fold ing and the oc cur rence of struc tural mo tifs in nat u ral ri bo nu cleic ac ids. Par tic u larly, the ther- mody namic sta bil ity of 2 -5' and 3'-5' linked RNA and the contribution of unpaired ter mi nal nu cleo tides (dan gling ends) in sec ond ary (2D) and ter tiary (3D) struc tures of RNA are dis cussed. Both ex am ples sug gest that dur ing evo lu tion na ture se lected sequences and struc tures of RNA which are the most ther mally sta ble and ef fi cient for their bi o log i cal func tion.