The prebiotic RNA world

Scheme 2.2 Origin of life, in the popularized version of the prebiotic RNA world.

There is of course still the possibility that some brilliant chemist will soon discover a prebiotic scenario for making RNA sequences - in a way we all hope that this will be the case, it would indeed be a good day for those studying the origin of life. However, for the time being, the prebiotic RNA world is grounded on the above-mentioned dream, and not on solid science. 

Why then is there this popularity of the prebiotic RNA world There are three reasons that come to mind. One is the already mentioned great success of the RNA world at large, which, by inference, gives confidence in the power of RNA. Another reason is that from self-rephcating and mutating ribozymes, one can conceive in paper a route to DNA and proteins - and then one has the whole story. A third reason is the lack of a good competitive model - namely the fact that there is no alternative mechanism that is supported experimentally. 

Figure 2.3 The relation between the compartmentalistic approach and the prebiotic RNA world and the missing link (items 1 and 2).

It seems that there is a missing link between the scenarios of the prebiotic RNA-world and the compartmentalishc approach and the missing link is how to make RNA by a prebiotic sequence or network of internalized reactions. 

I have brought into this chapter another line of criticism against the naive version of the prebiotic RNA world, in particular about the assumption that selfreplication and the corresponding molecular evolution processes may be sustained by one single molecule. Clearly, self-replication in a prebiotic scenario, in order to be chemically important, has to respect realistic concentrations and rate constants. It may be different in a fully fledged cell, once specialized enzymes and biochemical matrices have evolved - but this is a point of arrival and not of origination. 

The RNA cells eventually have to evolve into protein/DNA cells. And this is a long and certainly not easy pathway. However, this is the beauty of the prebiotic RNA world that at least on paper, a possible pathway leading to DNA and proteins can be conceived. One ideal pathway showing the transition from the RNA to the DNA cell is illustrated in the Figure 11.4. 

I believe that the bottom-up approach to the origin of life will enjoy a considerable boost, when conditions are found for the prebiotic synthesis of many identical copies of long (>30) co-oligopeptide or co-oligonucleotide sequences. This would at least show that the prebiotic synthesis of enzymes and/or RNA is in principle possible. This remains the main problem with the prebiotic RNA world. 

Shapiro (1995) studied the prebiotic role of adenine, i.e., the question as to whether or not this nucleobase could have been involved in the hypothetical RNA world (see Chap. 6). He lists a series of problems ... 

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. 

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. 

Pentoses are of prime importance for contemporary organisms as structural components of nucleic acids. The existence of the primordial RNA world, in which RNA is suggested to act as a catalyst as well as an informational macromolecule, assumes a large prebiotic source of ribose. Alternatively, the possible existence of pre-RNA molecules with backbones different from ribose phosphate has been considered [3]. 

In the early RNA world, the increasing populations of replicating RNA mol -ecules would have consumed the building blocks of RNA that had been generated over long periods of time by prebiotic reactions. A shortage of these compounds would have favored the evolution of alternative mechanisms... 

Miller, Stanley L. From the Primitive Atmosphere to the Prebiotic Soup to the Pre-RNA World. Washington, D.C. National Aeronautics and Space Administration, 1996. 

Srivatsan, S.G. (2004) Modeling prebiotic catalysis with nucleic arid-like polymers and its implication for the proposed RNA world. Pure Appl. Chem., 76 (12), 2085-2099. 

There are a number of possible ways to stabilize sugars the most interesting one is to attach the sugar to a purine or pyrimidine, i.e., by converting the carbohydrate to a glycoside, but the synthesis of nucleosides is difficult under plausible prebiotic conditions. It has therefore been suggested that ribonucleotides could not have been the first components of prebiotic informational macromolecules (59). This has led to propositions of a number of possible substitutes for ribose in nucleic acid analogues, in what has been dubbed the "pie-RNA World" (60). 

The RNA World, Second Edition The Nature of Modern RNA Suggests a Prebiotic RNA World. Gesteland, R. Atkins, J. F., Eds. Cold Spring Harbor Laboratory Press Cold Spring Harbor, NY, 1999 p 709. 

Gesteland RF, Cech TR, Atkins JF (1999) The RNA World The nature of modern RNA suggests a Prebiotic RNA, 2nd ed., Cold Spring Harbor Laboratory Press, New York. 

It remains an open question as to whether this newly identified species of amino acid was important in the construction of peptide nucleic acids (see Sect. 6.7) in the prebiotic chemistry of the RNA world phase of biogenesis (Meierheinrich et al 2004). 

The RNA world hypothesis caused prebiotic phosphate chemistry to become an attractive research area again unfortunately, no clear evidence for a realistic nucleotide synthesis under the simplified conditions of a primitive Earth has yet appeared. Important work on nucleoside phosphorylation has, however, been done. It is important to distinguish between ... 

The RNA world was the first biological world. If this is the case, we can learn or predict but little about the prebiotic chemistry of the RNA world from the biochemistry of today perhaps only the fact that the formation and polymerisation of nucleotides were once prebiotic processes. Thus Orgel is not of the... 

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