Origin of life scenario

Once the broad outlines of DNA replication and protein biosynthesis were established scien tists speculated about how these outlines af fected various origins of life scenarios A key question concerned the fact that proteins are re quired for the synthesis of DNA yet the synthesis of these proteins is coded for by DNA Which came first DNA or proteins How could DNA store genetic infor mation if there were no enzymes to catalyze the polymerization of its nucleotide components How could there be proteins if there were no DNA to code for them ... 

This question is linked to that asked in the previous chapter, relative to the onset of products under kinetic control, such as ATP and, at another level, specihc macromolecular sequences. In fact, the critical point in the origin-of-life scenario is the emergence of kinetic control in chemical reactions. Can this property emerge spontaneously from a scenario of reactions under thermodynamic control Or is it too much to expect from emergence ... 

The matrix effect is thus a way to reproduce a vesicle population of a given size distribution. In an origin of life scenario, the constancy of size during selfreproduction is probably important, as it would have ensured a constancy of physicochemical and biological properties over various generations. 

Darwinians view the origin of life as a lucky strike.1,2 Chance events, however, are irreducible and irreproducible so that comparison between the old and the new model becomes possible only from the moment when life had been established. Of course, every one invokes chemistry when it comes to the origin of life but for chemistry the single is out of character. Nonetheless, the consequences of any origin of life scenario should be expressed in the fossil record and this is the part where comparison of models becomes possible. In as much as the Darwinists deal with the topic of this chapter with one word (chance), the Genomic Potential Hypothesis is alone in its effort to build a conceptual basis for biogenesis. 

Remarkably, HCN-polymers are efficiently obtained at low temperatures suggesting a plausible cold origin-of-life scenario [63-65]. This latter hypothesis provides a mechanism to account for the concentration of HCN and a more stable environment for the newly formed nucleic acid components. Moreover, HCN is easily concentrated at its eutectic temperature (- 23.4 °C) [66,67] and the production of DAMN was found to be accelerated by lowering the temperature of the reaction medium. A large panel of purine derivatives, adenine 1, hypoxanthine 2, diaminopurine 3, xanthine 4 and guanine 5 was obtained after acidic and alkaline hydrolysis at high temperatures of the HCN-polymer produced in a frozen ammonium cyanide solution at - 78 °C for 27 years (Scheme 6 yields of products are reported... 

To its credit, the Voet and Voet text contains a disclaimer at the beginning of the standard discussion of a Stanley Miller-like origin-of-life scenario, which states that there are valid scientific objections to this scenarios... 

The origin-of-life scenario was instrumental for the new theory of the cell, because it led to the the conclusion that the ribotype had an evolutionary priority over genotype and phenotype. More precisely, the scenario described a precellular ribotype world (not to be confused with the RNA world) where some ribosoids could act as templates (ribogenotype), others as enzymes (ribophenotype), and others as polymerising ribosoids (ribotype) that were responsible for the growth and the quasi-replication of the ribonucleoprotein systems. 

This final paper is an extended consideration of the minimal requirements for true self-replication, divided into three parts. Part 1 considers the abstract design required to allow self-replication. It analyzes what sorts of processes, components, and information is needed for any self-replication to occur. Part 2 analyzes the potential physical implementation possibilities and the various design considerations when choosing implementation materials. Part 3 compares the minimal artificial self-replicator to the self-replicators found in nature—namely cell biology. This part examines possible origin-of-life scenarios based on the analysis of the design requirements of self-replication. 

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. 

Ribo also suggests that this might be relevant in a prebiotic scenario, at the early stages of the origin of life, since there may well have been vortices determining permanent sign directions in primordial times. 

A large number of successful experimental studies which tried to work out plausible chemical scenarios for the origin of life have been conducted in the past (Mason, 1991). A sketch of a possible sequence of events in prebiotic evolution is shown in Figure 3. Most of the building blocks of present day biomolecules are available from different prebiotic sources, from extraterrestrial origins as well as from processes taking place in the primordial atmosphere or near hot vents in deep oceans. Condensation reactions and polymerization reactions formed non-in-structed polymers, for example random oligopeptides of the protenoid type (Fox... 

The sudden appearance on Earth of a system capable of both metabolism and replication is too unlikely to be taken seriously. All reasonable theories on the origin of life assume therefore that chemical evolution started from systems that could perform only one of those functions. Hence the great schism between metabolism-first theories (Oparin s paradigm) and replication-first scenarios... 

First, Barbieri presents a theory about the origin of life. Extant organisms possess both genotype, in the form of DNA molecules, and phenotype, in the form of proteins, cells, and other products of epigenesis. Previous scenarios treated proteins or DNA as coming... 

Finally, we think that the action of chiral forces is a very intriguing, open scenario which deserves the attention of researchers owing to the possible applications and suggestions on the origin of life. 

Some problems tend to be hidden, but deserve to be highlighted. A general problem with all scenarios for the origin of life appears to be that of local concentrations of the initial molecular precursors. Tlie image of the warm little pond by the side of the sea, concentrated by evaporation and replenished by wave splashes, had been proposed by Darwin (1859) and has been espoused by Miller and Orgel (1974)... 

The question then is how to produce the first self-replicating species. Several text books and researchers in the field accept the notion that such pristine self-replicating species was a family of RNA which by self-replication and mutation gave rise to ribozymes which then originated proteins and DNA. This is the so-called RNA world scenario of the origin of life. 

Flores JJ, Bonner WA, Massey GA (1977) Asymmetric photolysis of (RS)-leucine with circularly polarized ultraviolet light. J Am Chem Soc 99 3622-3626 Fliigel RM (2010) The precellular scenario of genovirions. Virus Genes 40 151-154 Follmann H, Brownson C (2009) Darwin s warm little pond revisited from molecules to the origin of life. Naturwissenschaften 96 1265-1292... 

Whatever the scenario, metabolism first or genetics first, the origin of life required more than the replication of chemicals. For a chemical system to be alive, it must display evolution by the process of natural selection. 

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