Replication first theory

Alternative polar solvents offer potential solutions to paradoxes presented by replicator-first theories of the origin of life. That is particularly true for formamide. Formamide has been shown to be a suitable precursor of... 

The concept of postchemical evolution, in conclusion, allows us to realise that there is another important dichotomy in the origin of life field. In addition to the distinction between metabolism-first and replication-first theories, it is necessary to distinguish between theories of chemical evolution and theories of postchemical evolution. 

The environmental conditions of the primitive Earth were surely different from those of Spiegelman s and Eigen s test-tubes, but this can be regarded as a secondary complication, and in a first approximation it can be ignored. What we cannot ignore, however, is the fact that any replication process is inevitably affected by errors, and it is therefore imperative to understand the consequences that such errors have for the very survival of a replicating system. This is a crucial problem for all replication-first theories, because it has been proved that any self-replicating system can tolerate replication errors only below a critical threshold. Above such a threshold, the system is overwhelmed by a runaway error catastrophe, and is inexorably condemned to collapse. This is a fundamental problem, and in order to address it we need first to quantify the critical threshold. 

The wide gap between the two opposing theories, replication first and metabolism first , was analysed by Pross from the Ben Gurion University of the Negev (Israel). Pross concludes that replication came first He is convinced that a causality between the two theories can only be established if it is assumed that the replication-first thesis is correct. His analysis also shows that more of the experimental results and theoretical rationales favour the replication thesis. The author finds his assumption justified that life processes are strongly kinetically controlled and that the development of metabolic pathways can only be understood if life is considered as a manifestation of replicative chemistry (Pross, 2004). 

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... 

The first theory is that life started as a self replicating RNA single strand, that lined up free nucleotides and ligated sequentially in proper 3 -5 phosophodiester bonds, then the two strands would separate and cycle. This theory, obviously driven by the symmetry of double stranded DNA and RNA helices, has not yet worked (7). 

DNA has two broad functions replication and expression. First, DNA must be able to replicate itself so that the information coded into its primary structure is transmitted faithfully to progeny cells. Second, this information must be expressed in some useful way. The method for this expression is through RNA intermediaries, which in turn act as templates for the synthesis of every protein in the body. The relationships of DNA to RNA and to protein are often expressed in a graphic syllogism called the central dogma. The concept was proposed by Crick in 1958 and was revised in 1970 to accommodate the discovery of the RNA-dependent DNA polymerase. Crick s original theory suggested that the flow of information was always from RNA to protein and could not be reversed, yet it allowed for the possibility of DNA synthesis from RNA. 

The results obtained appeared quite promising, but the real sensation was the detection of pyruvate, the salt of 2-oxopropanoic acid (pyruvic acid), which is one of the most important substances in contemporary metabolism. Pyruvic acid was first obtained in 1835 by Berzelius from dry distillation of tartaric acid. The labile pyruvate was detected in a reaction mixture containing pure FeS, 1-nonanethiol and formic acid, using simulated hydrothermal conditions (523 K, 200 MPa). The pyruvate yield, 0.7%, was certainly not overwhelming, but still remarkable under the extreme conditions used, and its formation supports Wachtershauser s theory. Cody concludes from these results that life first evolved in a metabolic system prior to the development of replication processes. 

Eigen s theory concerns itself with phase two, the focal point of the genesis of the replication process. Apart from the first, each phase requires the previous one as its precondition. 

The theory that life began in an RNA World suggests that the first self-replicating system was a set of RNA molecules. The catalytic and informationtransferring properties of RNA indicate a possible scenario ... 

Why did we introduce this purely experimental material into a chapter that emphasizes theoretical considerations It is because the ability to replicate Tafel s law is the first requirement of any theory in electrode kinetics. It represents a filter that may be used to discard models of electron transfer which predict current-potential relations that are not observed, i.e., do not predict Tafel s law as the behavior of the current overpotential reaction free of control by transport in solution. 

The end of a linear chromosome is called a telomere. Telomeres require a special mechanism, because the ends of a linear chromosome can t be replicated by the standard DNA polymerases. Replication requires both a template and a primer at whose 3 end synthesis begins. The primer can t be copied by the polymerase it primes. What copies the DNA complementary to the primer In a circular chromosome, the primer site is to the 3 direction of another polymerase, but in a linear chromosome, no place exists for that polymerase to bind. As a result, unless a special mechanism for copying the ends of chromosomes is used, there will be a progressive loss of information from the end of the linear chromosome. Two characteristics about telomeres help avoid this situation. First, they consist of a short sequence—for example, AGGGTT—repeated many times at the end of each chromosome. Telomeres, therefore, are part of the highly repetitive DNA complement of a eukaryotic cell. Secondly, a specific enzyme, telomerase, carries out the synthesis of this reiterated DNA. Telomerase contains a small RNA subunit that provides the template for the sequence of the telomeric DNA. Eukaryotic somatic cells have a lifespan of only about 50 doublings, unless they are cancerous. One theory holds that a lack of telomerase in cells outside the germ line causes this limitation. 

That view of the origin of life has commonly been called metabolism first the absence of a genetic polymer has been equated with the lack of any mechanism for heredity. As we have seen, replicator theories center on the spontaneous formation of large, information-bearing organic polymers endowed with the ability to copy themselves. The hereditary information carried in the sequence of such a polymer is called a genome. 

The first scientific theories on the origin of life were proposed by Alexander Oparin in 1924 and by J.B.S. Haldane in 1929. Oparin discovered that a solution of proteins can spontaneously produce microscopic aggregates - which he called coacervates - that are capable of a weak metabolism, and proposed that the first cells came into being by the evolution of primitive metabolic coacervates. Haldane, on the other hand, was highly impressed by the replication properties of viruses, and attributed the origin of life to the evolution of viruslike molecular replicators. 

Up until the origin of RNA molecules, Dyson describes the logical consequences of the initial hypotheses, and his scheme is therefore a coherent theory of chemical evolution. But the mathematical model does not say anything about the subsequent integration of RNAs and hosts, and on this point Dyson resorts to a supplementary conjecture. He proposes that primitive RNAs invaded their metabolic hosts, and used them for their own replication, like viruses do, which is exactly Haldane s hypothesis. Dyson concludes therefore that, after Oparin s metabolism stage, came Haldane s replication stage, and his final scheme becomes metabolism first, replication second . That RNAs... 

Steinberg and Bursztyn (1998) developed additional theory to support the conclusions of their earlier article. They also compared the power to detect a dispersion effect in several types of robust design experiments. There are two important conclusions from their article. First, they found that explicit inclusion of noise factors in an experiment significantly increases the power to detect a dispersion effect relative to experiments with simple replication. Second, they found that the increase in power is obtained only via the response model analysis. The performance measure analysis does not enjoy the same gains in power. 

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