A minimal living system and the origin of a protocell.

Based on our current knowledge of molecular biology a living entity is a negentropic system, made of specific catalytic and informational macromolecules, which has the ability to reproduce itself, selectively interact with the environment, increase complexity, undergo mutation and evolve by natural selection. At the cellular level the essential molecular attributes of a minimal living system are considered to be the following: (a) Linear informational molecules, protoRNA (protoDNA) at least 10 mononucleotides long, capable to code for at least 2 amino acids; (b) Code-translating molecules, aminoacyl-proto tRNAs at least 5 mononucleotides long; (c) Protenzymes, oligopeptides of at least 2 amino acids, with measurable catalytic activity for phosphodiester, pyrophosphate and peptide bond formation; (d) Protoribosomes, RNA-peptide complexes which facilitate the interaction of the above three types of molecules, and (e) Protomembranes, liposomes made of amphiphilic lipids and peptides which can provide the semipermeability and the proton gradient necessary for the synthesis of pyrophosphate, ATP and other biochemical compounds. The transition from subcellular Lamarckian evolution to cellular Darwinian evolution required the cooperative interaction, within an internal microenvironment, of the above five types of molecular species.