Fig. 2.

The transcription–translation autocatalytic network. In this schematic diagram, we coarsely show how the transcription–translation machinery self-replicates via three main coupled autocatalytic cycles: 1) The ribosome autocatalytic cycle, which comprises two reactions: one for ribosomes that synthesize ribosomal proteins and one for RNA polymerase that synthesizes rRNA. The ribosomal proteins and the rRNAs merge in a self-assembly reaction to form new ribosomes. 2) The RNA polymerase autocatalytic cycle, in which RNA polymerases transcribe the mRNAs that catalyze the production of the Rpo protein subunits, which, in turn, self-assemble to form new RNA polymerases. 3) The tRNA-charging reaction where, e.g., aa-tRNA-synt. catalyze the charging of tRNA with amino acids, which, in turn, transfer the amino acids to ribosomes that translate mRNAs, including the mRNAs of aa-tRNA-synt. Any substrate that is not consumed by the reaction can be considered as a catalyst; for example, mRNA can be viewed as a catalyst for protein synthesis, but its catalysis rate is $n_i$ times higher than that of a single ribosome, where $n_i$ is the average number of ribosomes cotranslating this mRNA. Importantly, in the absence of any type of material inputs—either catalysts like tRNAs, mRNAs, ribosomes, RNA polymerases, aa-tRNA-synt., or substrates like amino acids or rRNAs—will bring the autocatalysis of the entire network to a halt.