A molecular model for transcription in the RNA world based on the ribosome large subunit.

Examination of the tRNA binding sites in the ribosome suggests that it might once have been able to catalyse the polymerisation of RNA. Based on a viral RNA-dependant RNA polymerase, the geometry of a potential active site for this ribopolymerase was constructed. From examination of the active site geometry, along with other arguments, it is suggested that the ribopolymerase synthesised a parallel complementary strand. When combined into a dimeric polymerase, this strategy minimises the exposure of single-stranded RNA and prevents self-hybridisation: both previously difficult problems for the RNA world hypothesis.