Metabolism, self-replicating system

What does your intuition say did macromolecular self-replication systems come first in the origin of life or should they be seen as the product of a mature cellular or proto-cellular metabolism ... 

Two common processes - variation and selection - provide a powerful mechanism for self-replicating systems to evolve. For a system to evolve it must first display a range of variations. Natural systems display random variations through mutations, which are undirected changes in the chemical makeup of key biomolecules. Most variations are neutral or they harm the organism and are doomed to failure. Once in a while, however, a random mutation leads to an improved trait - a more efficient metabolism, better camouflage, swifter locomotion, or greater tolerance for extreme environmental conditions. Such beneficial variations are more likely to survive in the competitive natural world such variations fuel the process of natural selection. 

The opposition between the autotrophic and heterotrophic approaches to metabolism has resulted in different emphases on research avenues. The autotrophic school is enthralled with geochemical cycles and their biochemical implications, whereas the heterotrophic school concentrates more on the emergence of self-replicating systems. In effect, the heterotrophy versus autotrophy question becomes engulfed in the fundamental quarrel between the proponents of replication-first versus metabolism-first processes in the emergence of life on the early Earth. Measured aspects of this confrontation are found in Chaps. 8 and 9. It should also be said that there are attempts to bridge the gap between these two viewpoints by devising a metabolism-driven replication scheme [17]. 

The RNA world requires a system capable of self-replication as a precondition for the beginnings of life. In contrast, the surface metabolism theory proposed by Wachtershauser postulates that the initial step is metabolism, from which complex replication systems can evolve later. This metabolism would have occurred at the... 

The minimal cell, as the simplest system which has all the required properties of life (metabolism, self-reproduction and the ability to evolve), is presently studied as part of a new research discipline synthetic biology. This includes subjects such as synthesis in branches of biological systems, for example, of new RNA species, new peptides and new nucleic acid analogues, as well as the synthesis of peptide nucleic acids. One example is the work of M. R. Ghadiri and G. von Kiedrowski on self-replication of oligonucleotides and oligopeptides (Luisi, 2006b). 

It is in principle possible for a free enzyme to promote reaction in a geochemical system, but enzyme kinetics are invoked in geochemical modeling most commonly to describe the effect of microbial metabolism. Microbes are sometimes described from a geochemical perspective as self-replicating enzymes. This is of course a considerable simplification of reality, as we will discuss in the following chapter (Chapter 18), since even the simplest metabolic pathway involves a series of enzymes. 

At the point where amphiphiles were recruited to provide the precursors to cell membranes, stable lipid vesicles could have evolved [141] to enclose autocatalytic chiral hypercycles. Credible models for the subsequent evolution of vesicles containing self-replicating chiral molecules have appeared in the literature. [193,194] These vesicles could then emerge from the feldspar spaces [134,192] as micron-sized self-reproducing, energy-metabolizing vesicular systems protobacteria ready to face the hydrothermal world on their own terms. 

A self-replicating polymer would quickly use up available supplies of precursors provided by the relatively slow processes of prebiotic chemistry. Thus, from an early stage in evolution, metabolic pathways would be required to generate precursors efficiently, with the synthesis of precursors presumably catalyzed by ri-bozymes. The extant ribozymes found in nature have a limited repertoire of catalytic functions, and of the ribozymes that may once have existed, no trace is left. To explore the RNA world hypothesis more deeply, we need to know whether RNA has the potential to catalyze the many different reactions needed in a primitive system of metabolic pathways. 

Oparin s [3a] and Haldane s [3b] heirs. Eigen [3d, e] and Kuhn [3f], gave these events a time perspective, and vith their information also described the vector of the Grand Process . Self-replication, mutation, and metabolism (as prerequisites for selection) made up the list of criteria through these, information and its origin, evaluation, processing, and optimization had governed the evolutionary history of prebiotic and biotic systems [2f, 3]. 

Compartmentalized protein synthesis, as noticed above, combines cell-free expression with liposome technology. Several important systems have been designed and experimentally done, as shown by the several reports collected in Table 17.1. There are, however, goals not yet achieved, snch as the simnltaneous protein synthesis (inside the vesicle) and vesicle self-reprodnction, as done in the case of Qj8 replicase experiment. Moreover, these two processes shonld occur not only simultaneonsly, bnt mnst be fnnc-tionally coupled, i.e. the first process should affect the second, as tentatively done in the case of lipid-synthesizing liposomes. However, this would not snffice. In order to make an autopoietic minimal cell, the self-reproduction (or self-replication) of all internalized metabolic components is reqnired, so that the issue of death by dilution is avoided. 

Such models can be developed for the computational design of catalytic sterns that self organize and adapt themselves for optimum catalytic performance. Adaptation occurs in the reproduction process with mutation of the self-replicating molecules coupled to the metabolic system. The metabolic system acts as the bio-immune molecular recognition and response system. The conditions for the emergence of such a stem are far from equilibrium in the complex regime. The behavior as a function of time is unpredictable, similar to the class 4 system proposed by Wolfram. 

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