Pre-RNA world

The hypothesis that our biological world built on the DNA-RNA-protein central dogma was preceded by an RNA world in which RNA molecules carried both the genetic information and executed the gene functions (through ribozyme activity) is now widely accepted [130]. However, it is also well recognized that RNA due to its vulnerability to hydrolysis - especially as a result of catalysis by divalent metal ions - would not have been able to evolve in a harsh pre-biotic environment Also the formation of RNA under presumed pre-biotic conditions is extremely inefficient It is not so far-fetched to propose that a peptide nucleic acid-like molecule may have been able to function as a form of pre-biotic genetic material since it 

This work has been supported by The Danish Cancer Society, The Lundbeck Foundation and the European Commission (QLKl-CT-2000-01658 and QLRT-2000-00634). 

Nielsen P.E., Egholm M., Berg R.H., Buchardt O. Sequence-selective recognition of DNA by strand displacement with a thymine-substituted polyamide. Science 1991, 254 1497-1500. 

Egholm M., Buchardt O., Christensen L., Behrens C., Ereier S.M., Driver D.A., Berg, R. H., Kim, S.K., Nor-DEN B., Nielsen, P.E. PNA hybridizes to complementary oligonucleotides obeying the Watson—Crick hydrogen-bonding rules. Nature 1993, 365 566-568. 

Nielsen P.E., Haaima G. Peptide nucleic acid (PNA). A DNA mimic with a pseudopeptide backbone. Chem. Soc. Rev. 

---

All this makes it clear that the discussions on pyrimidine synthesis are by no means over. The question is whether there was an RNA world or not. Recently, attempts have been made to bypass this problem by postulating a phase of development before the RNA world a pre-RNA world (see Sect. 6.7). But this is also hypothetical ... 

Other studies have supported the reports of considerable stabilisation of carbohydrates by borates the thermal stability of ribose increases under acidic conditions, while glucose is favoured under basic conditions. Thus, the pre-RNA world chemistry of furanosyl borate diesters of ribose could have taken place at high temperatures and low pH. A pre-metabolism world at high temperatures and in the basic pH range would have involved glucose borates and anion borates (Scorei and Cimpoia u, 2006). 

Although many questions are still open, peptide nucleic acids are easier to synthesize via simple reaction routes than is natural RNA. The PNAs have another important advantage they are achiral and uncharged, i.e., they contain no chiral centres in the polymeric backbone (see Sect. 9.4). Unfortunately, however, they do not fulfil all the necessary conditions for molecular information storage and transfer. Thus, the search for other possible candidates for a pre-RNA world continues. 

Although the pre-RNA world is now much more the centre of scientific attention in prebiotic chemistry, there have been several attempts in recent years to understand the synthesis of oligonucleotides from the normal nucleotides by using simulation experiments (Ferris, 1998). In condensation reactions in aqueous media, there is always competition between synthesis and hydrolysis synthesis is generally only successful when supported by catalysts. 

Whether the polymers shown schematically in Fig. 6.21 (or similar forms) were ever of importance on the Earth as information carriers is an open question. Leslie Orgel considers the alanyl nucleotides synthesized by Diederichsen to be possible and suitable candidates for a pre-RNA world (Orgel, 2003). 

The idea of takeover many biogeneticists are of the opinion that the RNA world must have had a simpler precursor pre-RNA world . 

It is assumed that tholins also played an important role in the PAH world hypothesis. The PAH world could have existed as a pre-RNA world on the primeval Earth. It is suggested that PAHs can undergo stacking, thus forming structures to which nucleobases are chemically bound (to OH functions which are formed by photochemical derivatisation of the PAHs). The PAH stacks replace the more complex phosphate-D-ribose-phosphate chains in the nucleic acids (Ehrenfreund et al., 2006). 

Was there a pre-RNA world before the hypothetical RNA world or Did other molecular species act as information carriers ... 

To some extent, this message has already been accepted in the literature, as the quest for the pristine RNA world has been shifted into a pre-RNA world. In a recent paper, Orgel (2003) appears to accept the view that the action of amino acids might have possibily formed the basis of this pre-RNA world. 

Lazcano, A. MiUer, S.L. (1996) The origin and early evolution of life prebiotic chemistry, the pre-RNA world, and time. Cell 85, 793-798. 

Brief review of developments in studies of the origin of life primitive atmospheres, submarine vents, autotrophic versus heterotrophic origin, the RNA and pre-RNA worlds, and the time required for life to arise. 

Another possibility is that the RNA world has been preceded by another information storage/catalytic function system (pre-RNA world, Fig. 1) that could for instance be derived from peptide analogues of nucleic acids called Peptide Nucleic Acids (PNAs) [15,18-20]. However, there is no remaining evidence of a switch from this early information storage system into that based on nucleic acids. 

It seems unlikely that a protein world only existed in the early phases, however, in a pre-RNA world containing prebiotic soups with all the products of the Oro, Butlerov, Strecker, and Miller syntheses likely took place. 

Again, one has to make plausible assumptions. It is likely that in a pre-RNA world peptides interacted in an environment that allowed plugged pores to concentrate the required molecules. Since the plugged pores likely developed in niches of hydrothermal vents and submarine geysers, one tends to add a further combination to it. 

Knight, Robin D., and Laura F. Landweber. The Early Evolution of the Genetic Code. Cell 101 (2000) 569-72. This very accessible paper describes the requirements of a prebiotic, pre-RNA world and potential protein nucleic acid (PNA) molecules. 

---