Figure 4. Schematic presentation of ribozyme activities that might have played a role in the RNA world.

a) self-modification, e.g. alkylation; b) 5'-terminal modification by ribozyme-supported addition of an activated building block; c) internal modification by ribozyme-supported fragment exchange; d) ribozyme-supported 5' –triphosphorylation with trimetaphosphate; e) ribozyme-supported RNA polymerization with nucleoside-2',3'-cyclic phosphates (in 3'→5'-direction) or nucleoside-5'-triphosphates (in 5'→3'-direction) as activated building blocks.