Replication-first

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). 

Kuhn s biogenesis models were developed further (Kuhn and Kuhn, 2003). The basic principle remains unchanged replication first As before, no exact single steps are elaborated, but only the main aspects of the biogenesis process are dealt with. 

Five steps can be distinguished in virus replication. First the organism has to penetrate the host... 

From the complementary duplex structure of DNA described in chapter 25, it is a short intuitive hop to a model for replication that satisfies the requirement for one round of DNA duplication for every cell division. In chapter 26, DNA Replication, Repair, and Recombination, key experiments demonstrating the semiconservative mode of replication in vivo are presented. This is followed by a detailed examination of the enzymology of replication, first for how it occurs in bacteria and then for how it occurs in animal cells. Also included in this chapter are select aspects of the metabolism of DNA repair and recombination. The novel process of DNA synthesis using RNA-directed DNA polymerases is also considered. First discovered as part of the mechanisms for the replication of nucleic acids in certain RNA viruses, this mode of DNA synthesis is now recognized as occurring in the cell for certain movable genetic segments and as the means whereby the ends of linear chromosomes in eukaryotes are synthesized. 

Pross, A. 2004. Causation and the origin of life Metabolism or replication first Origins Life Evol. Biosph. 34 307-321. 

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... 

We can say therefore that the metabolism-firsti ta. (the metabolism paradigm) goes back to Oparin, while the replication-first concept (the replication paradigm) goes back to Haldane. And since metabolism is based on proteins, and replication on nucleic acids, Oparin s paradigm is equivalent to saying that proteins (the hardware) came first, whereas Haldane s paradigm maintains that it was nucleic acids (the software) that had priority. 

The first mixed systems came from the evolution of one or more systems made of nucleic acids where eventually proteins also appeared (solution replication first). 

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 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 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 Zn world scenario shows how the metabolism first and replication first concepts can be reconciled and combined. The scenario comprises a mechanism of the continuous abiogenic photosynthesis of metabolites and their further conversion by ZnS-confined replicating entities. 

Pross A Causation and the origin of life. Metabolism or replication first Orig Life Evol Biosph 2004, 34(3) 307-321. 

The problem of origin of life on Earth (abiogenesis) remains one of the central and most intractable problems of modern biology. The current hypotheses cluster either around the replication first paradigm or the metabolism first concept, see [1-10] for consideration of the controversy between the two concepts. The... 

To appreciate the rationale for the dideoxy method, we must first understand certain aspects of the biochemistry of DNA replication. First, DNA replication takes place when cells divide. During replication, the sequence of nucleotides in one strand is copied as a complementary strand to form the second strand of a double-stranded DNA molecule. Synthesis of the complementary strand is catalyzed by the enzyme DNA polymerase. As shown in the following equation, the DNA chain grows by adding each new unit to the free 3 -OH group of the chain ... 

The road for the replication first camp gets much easier after this point. Once nucleotides are present in large amounts, phosphate chemistry makes the chains assemble in the right orientation by themselves. Even if they re not all perfectly oriented, the chains can still pair up to copy information. Several labs have made nucleotide chains that compete and even evolve, although the smallest of these is still pretty long to come together by itself (about 100 nucleotides long). 

As the years go by, these clues should converge. This may already be happening. Metal sulfides could produce metabolites at white smokers ( 3) like they do at black smokers ( 4). Nucleotide synthesis ( 6) may succeed under the hot springs conditions ( 5) that also have a chemical profile that matches life s biochemical profile. Rock compartments ( 2) show up in clue after clue. If the clues continue to overlap, they may eventually converge. Even the metabolism first and replication first camps may converge if conditions are found that can make RNA strands at white smokers, black smokers, or hot springs. 

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]. 

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