In this first lesson, we discussed the various arguments underlying the hypothesis, now widely accepted, of a primitive RNA world, in which the activities of supporting genetic information and catalysis are carried out exclusively by RNA molecules. If the capacity of RNAs to store and transmit information is obvious, the second, admittedly limited, activity is also real: there are ribozymes, enzymes based exclusively on RNA, which catalyze biological reactions, such as ribunuclease P or the ribosome. This discovery earned T. Cech the Nobel Prize in 1989.
The second indication that RNA precedes DNA lies in the observation that in all living organisms, deoxyribonucleotides, the precursors of DNA, are synthesized from the corresponding ribonucleotides. The next step is the transition from the RNA world to the more robust "RNA-protein" world, thanks to the fantastic catalytic capacities of proteins. Such a transition was only possible with the "invention" of the ribosome, the machinery that translates RNA into protein. The following facts are well in line with the hypothesis of an initial RNA world transforming into an AR/protein world, of which molecular relics remain in living organisms today: (i) RNAs are necessary for protein synthesis; (ii) certain amino acids, such as histidine, are synthesized from ribonucleotides; (iii) a very large number of enzymatic cofactors, such as s-adenosylmethionine, NADH, ATP, coenzyme a, are ribonucleotides; (iv) certain key systems such as the ribosome are RNA/protein complexes.
