The RNA world hypothesis is a theory that proposes the existence of a RNA cell (ribocyte) before the modern cells were evolved. The properties of the RNA to act as a genetic information storage molecule and as a catalyst, like proteins (enzyme) make the idea of RNA world conceptually possible. The discovery of reactions such as the 5′ nucleolytic processing of tRNA precursors and the self splicing of the Tetrahynema large ribosomal RNA by ribonuclease P, and linking of these phospoester transfer reactions to the self splicing group II introns and other natural hydrolytic catalysts such as hammerhead, hairpin ribozyme, hepatitis delta motif may give the impression that RNA may have accelerated a diverse set of vital reactions during the RNA world. Furthermore, RNA can be transcribed into DNA (reverse transcription), the reverse of the normal phenomenon of DNA replication, amino acid activation, aminoacyl-RNA synthesis, peptide synthesis and coding, making us to consider that the RNA world could be the origin of the modern DNA world.
However, the synthesis of the nucleotides and achieving the replication of RNA under prebiotic conditions is proved to be challenging and this gives us a second thought that probably, RNA was not the first self replicative molecule and there should have been existed a pre-RNA world. There is a method called selection amplification (SELEX) that is employed to study the ribocyte possibility which involves the cyclical, repititive purifications from randomized sequences, and observing the ability of nucleic acids to replicate after each selection or the possible reaction catalysis. The discovery of group II introns to be capable of releasing pyrophosphate from a nucleoside tryphosphate had made it clear that the modern RNA replicase catalysis could use the same reactants and leaving group. The following is an equation that was proposed in a study that is based on the bayes’ theorem, which is a fundamental theorem of the probability calculus.
P(RNA world/Experiments) / P(RNA world) = P(Experiments/RNA world) / P(Experiments)
Where P= probabilities; left hand side(LHS)= ratio of the probability of RNA world given certain results; RHS= probability of the experimental results given that RNA world did exist.
According to this equation, if we obtain the right hand side numerator value of 1, after the substitution of experimental values in the equation (when n experiments are carried) indicates the probability that there has been a RNA world,
Possible reasons for evolutionary change to DNA world:
Today’s DNA/Protein world is proposed to have evolved from the RNA world. DNA, because of its greater chemical stability might have taken over the role of data storage function of the RNA and the proteins because of their higher flexibility to catalyze through a great variety of amino acids might have substituted the catalytic properties of the RNA. The mRNA which is an intermediate molecule between the DNA and the proteins stands as an evolutionary remnant of the RNA world.
