RNA-world

There was a sense however that the sequence that later came to be called RNA World was the most reasonable one RNA World denotes an early period in the development of self replicating systems... 

The discovery of nbozymes (Section 28 11) in the late 1970s and early 1980s by Sidney Altman of Yale University and Thomas Cech of the University of Colorado placed the RNA World idea on a more solid footing Altman and Cech independently discovered that RNA can catalyze the formation and cleavage of phosphodiester bonds—exactly the kinds of bonds that unite individual ribonucleotides in RNA That plus the recent discovery that ribosomal RNA cat alyzes the addition of ammo acids to the growing peptide chain in protein biosynthesis takes care of the most serious deficiencies in the RNA World model by providing precedents for the catalysis of biologi cal processes by RNA... 

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... 

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. 

This definition was previously used by Horowitz and Miller (1962). An undefined external energy source was included in this definition. The growing influence of the RNA world can be seen in the second NASA definition ... 

Fig. 1.5 Schematic representation of the evolution of life from its precursors, on the basis of the definition of life given by the authors. If bioenergetic mechanisms have developed via autonomous systems, the thermodynamic basis for the beginning of the archiving of information, and thus for a one-polymer world such as the RNA world , has been set up. Several models for this transition have been discussed. This phase of development is possibly the starting point for the process of Darwinian evolution (with reproduction, variation and heredity), but still without any separation between genotype and phenotype. According to the authors definition, life begins in exactly that moment when the genetic code comes into play, i.e., in the transition from a one-polymer world to a two-polymer world . The last phase, open-ended evolution, then follows. After Ruiz-Mirazo et al. (2004)...

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). 

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 ... 

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 ... 

Ribose is relatively unstable, which leads all the critics of an RNA world to look for other models. The half-life of ribose is only 73 minutes at 373 K and pH 7, but increases at 273 K to 44 years. 

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). 

The synthesis of dipeptides under the conditions found on primeval Earth appears highly likely. The discovery that these small molecules can act as catalysts makes it possible to discuss their being involved in basic synthetic reactions occurring in an (as yet hypothetical) RNA world (Weber and Pizzarello, 2006). 

However, there are also biogenesis models which do not require phosphate, such as the inorganic hypothesis of the origin of life proposed by Cairns-Smith (see Sect. 7.1), the thioester world proposed by de Duve (see Sect. 7.4) or the sulphur-iron world suggested by Wachtershauser (see Sect. 7.3). The RNA world (see Chap. 6), however, cannot exist without phosphate. 

The RNA world hypothesis then rekindled interest in the still open question of the origin of the phosphorus. How could acceptable synthetic routes to the nucleic acids be developed, if it was not even clear which phosphate derivatives were available on the primeval Earth An RNA world is not possible without reactive phosphates or similarly efficient phosphorus compounds Thus, the search for effective phosphorus or phosphate sources continued unabated. 

In today s discussion of the origin of life, the RNA World (Chapter 6) is seen as much more important, and is much better publicized, than the protein world . However, nucleic acids and proteins are of equal importance for the vital metabolic functions in today s life forms. Peptides and proteins are constructed from the same building blocks (monomers), the aminocarboxylic acids (generally known simply as amino acids). The way in which the monomers are linked, the peptide bond, is the same in peptides and proteins. While peptides consist of only a few amino acids (or to be more exact, amino acid residues), proteins can contain many hundreds. The term protein (after the Greek proteuein, to be the first) was coined by Berzelius in 1838. 

Even its proponents do not always discuss the RNA world in the same terms. However, there are three basic assumptions on which there is consensus (Joyce and Orgel, 1999) ... 

During the development phase of the RNA world, there were no functioning processes which were influenced by protein catalysts. 

The differences between the different versions of the RNA world mainly concern the questions ... 

Which complex mechanisms were active in the RNA world ... 

In spite of all the enthusiasm for the RNA world, critical observers point out (as do Joyce and Orgel in 1989) that the RNA world is only a hypothesis It has, however, led to the development of a series of interesting biogenesis models. Some points have been verified by means of successful experiments, but the hypothesis poses questions to which we have no answers at present (see Sect. 6.6). 

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 ... 

In many enzymes, the value of kc-JK lies between 108 and 109 M s. The value for L 19 RNA is 103 M 1 s1, i.e., five orders of magnitude lower than for protein enzymes with high catalytic activity. However, L 19 RNA does compare in its efficiency to the enzyme ribonuclease A. The capabilities of ribozymes referred to above dealt solely with interactions of RNA (i.e., ribozymes) with RNA molecules. In a (hypothetical) RNA world, they would, however, need to be capable of doing much more, e.g., carrying out reactions at the carbon skeletons of biomolecules. 

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