3 Protocells: Genetic Polymers Inside Membrane Vesicles

Abstract

The reality of the “RNA WORLD,” an RNA-dominated stage in the early evolution of life prior to the evolution of coded protein synthesis, has been firmly established by recent structural studies of the ribosome. These studies show that the catalytic peptidyl-transferase center of the ribosome is composed solely of RNA (Steitz and Moore 2003). The fact that the ribosome is a ribozyme implies that protein synthesis evolved during an early stage of cellular life in which biochemical functions were carried out largely by RNA molecules. This is a delightful confirmation of the logical deduction of the existence of the RNA World (Woese 1967; Crick 1968; Orgel 1968) as a way of resolving the apparent paradox that proteins are required to synthesize nucleic acids, whereas nucleic acids are required to synthesize proteins. The idea of RNA-catalyzed RNA replication resolves this paradox by providing a simple model for the origin of replicating, evolving biochemical systems; it also provides an explanation for the organization of modern biochemistry through the later evolution of molecules specialized for information storage (DNA) and for structural and catalytic activities (proteins). With the reality of the RNA World so strongly supported, the questions being asked about this early stage of life now focus on the transition from prebiotic chemistry to the RNA World, and then from the RNA World to the world of modern biochemistry that had evolved by the time of the last common ancestor of all extant life.

Most discussions of the biochemistry of the RNA World...