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1.
The Path from the RNA World 总被引:1,自引:0,他引:1
We describe a sequential (step by step) Darwinian model for the evolution of life from the late stages of the RNA world through
to the emergence of eukaryotes and prokaryotes. The starting point is our model, derived from current RNA activity, of the
RNA world just prior to the advent of genetically-encoded protein synthesis. By focusing on the function of the protoribosome
we develop a plausible model for the evolution of a protein-synthesizing ribosome from a high-fidelity RNA polymerase that
incorporated triplets of oligonucleotides. With the standard assumption that during the evolution of enzymatic activity, catalysis
is transferred from RNA → RNP → protein, the first proteins in the ``breakthrough organism' (the first to have encoded protein
synthesis) would be nonspecific chaperone-like proteins rather than catalytic. Moreover, because some RNA molecules that pre-date
protein synthesis under this model now occur as introns in some of the very earliest proteins, the model predicts these particular
introns are older than the exons surrounding them, the ``introns-first' theory. Many features of the model for the genome
organization in the final RNA world ribo-organism are more prevalent in the eukaryotic genome and we suggest that the prokaryotic
genome organization (a single, circular genome with one center of replication) was derived from a ``eukaryotic-like' genome
organization (a fragmented linear genome with multiple centers of replication). The steps from the proposed ribo-organism
RNA genome → eukaryotic-like DNA genome → prokaryotic-like DNA genome are all relatively straightforward, whereas the transition
prokaryotic-like genome → eukaryotic-like genome appears impossible under a Darwinian mechanism of evolution, given the assumption
of the transition RNA → RNP → protein. A likely molecular mechanism, ``plasmid transfer,' is available for the origin of
prokaryotic-type genomes from an eukaryotic-like architecture. Under this model prokaryotes are considered specialized and
derived with reduced dependence on ssRNA biochemistry. A functional explanation is that prokaryote ancestors underwent selection
for thermophily (high temperature) and/or for rapid reproduction (r selection) at least once in their history.
Received: 14 January 1997 / Accepted: 19 May 1997 相似文献
2.
Relics from the RNA World 总被引:1,自引:0,他引:1
An RNA world is widely accepted as a probable stage in the early evolution of life. Two implications are that proteins have
gradually replaced RNA as the main biological catalysts and that RNA has not taken on any major de novo catalytic function
after the evolution of protein synthesis, that is, there is an essentially irreversible series of steps RNA → RNP → protein.
This transition, as expected from a consideration of catalytic perfection, is essentially complete for reactions when the
substrates are small molecules. Based on these principles we derive criteria for identifying RNAs in modern organisms that
are relics from the RNA world and then examine the function and phylogenetic distribution of RNA for such remnants of the
RNA world. This allows an estimate of the minimum complexity of the last ribo-organism—the stage just preceding the advent
of genetically encoded protein synthesis. Despite the constraints placed on its size by a low fidelity of replication (the
Eigen limit), we conclude that the genome of this organism reached a considerable level of complexity that included several
RNA-processing steps. It would include a large protoribosome with many smaller RNAs involved in its assembly, pre-tRNAs and
tRNA processing, an ability for recombination of RNA, some RNA editing, an ability to copy to the end of each RNA strand,
and some transport functions. It is harder to recognize specific metabolic reactions that must have existed but synthetic
and bio-energetic functions would be necessary. Overall, this requires that such an organism maintained a multiple copy, double-stranded
linear RNA genome capable of recombination and splicing. The genome was most likely fragmented, allowing each ``chromosome'
to be replicated with minimum error, that is, within the Eigen limit. The model as developed serves as an outgroup to root
the tree of life and is an alternative to using sequence data for inferring properties of the earliest cells.
Received: 14 January 1997 / Accepted: 19 May 1997 相似文献
3.
4.
József Garay 《Bio Systems》2011,103(1):1-12
I propose a hypothesis on the origin of chiral homogeneity of bio-molecules based on chiral catalysis. The first chiral active centre may have formed on the surface of complexes comprising metal ions, amino acids, other coenzymes and oligomers (short RNAs). The complexes must have been dominated by short RNAs capable of self-reproduction with ligation. Most of the first complexes may have catalysed the production of nucleotides. A basic assumption is that such complexes can be assembled from their components almost freely, in a huge variety of combinations. This assumption implies that “a few” components can constitute “a huge” number of active centre types. Moreover, an experiment is proposed to test the performance of such complexes in vitro.If the complexes were built up freely from their elements, then Darwinian evolution would operate on the assembly mechanism of complexes. For the production of complexes, first their parts had to appear by forming a proper three-dimensional structure. Three possible re-building mechanisms of the proper geometric structure of complexes are proposed. First, the integration of RNA parts of complexes was assisted presumably by a pre-intron. Second, the binding of RNA parts of a complex may give rise to a “polluted” RNA world. Third, the pairing of short RNA parts and their geometric conformation may have been supported by a pre-genetic code.Finally, an evolutionary step-by-step scenario of the origin of homochirality and a “polluted” RNA world is also introduced based on the proposed combinatorial complex chemistry. Homochirality is evolved by Darwinian selection whenever the efficiency of the reflexive autocatalysis of a dynamical combinatorial library increases with the homochirality of the active centres of reactions cascades and the homochirality of the elements of the dynamical combinatorial library. Moreover, the potential importance of phospholipid membrane is also discussed. 相似文献
5.
Moulton V Gardner PP Pointon RF Creamer LK Jameson GB Penny D 《Journal of molecular evolution》2000,51(4):416-421
Opinion is strongly divided on whether life arose on earth under hot or cold conditions, the hot-start and cold-start scenarios,
respectively. The origin of life close to deep thermal vents appears as the majority opinion among biologists, but there is
considerable biochemical evidence that high temperatures are incompatible with an RNA world. To be functional, RNA has to
fold into a three-dimensional structure. We report both theoretical and experimental results on RNA folding and show that
(as expected) hot conditions strongly reduce RNA folding. The theoretical results come from energy-minimization calculations
of the average extent of folding of RNA, mainly from 0–90°C, for both random sequences and tRNA sequences. The experimental
results are from circular-dichroism measurements of tRNA over a similar range of temperatures. The quantitative agreement
between calculations and experiment is remarkable, even to the shape of the curves indicating the cooperative nature of RNA
folding and unfolding. These results provide additional evidence for a lower temperature stage being necessary in the origin
of life.
Received: 1 March 2000 / Accepted: 14 June 2000 相似文献
6.
Many of the biosynthetic pathways, especially those leading to the coenzymes, must have originated very early, perhaps before
enzymes were available to catalyze their synthesis. While a number of enzymatic reactions in metabolism are known to proceed
nonenzymatically, there are no examples of entire metabolic sequences that can be achieved in this manner. The most primitive
pathway for nicotinic acid biosynthesis is the reaction of aspartic acid with dihydroxyacetone phosphate. We report here that
nicotinic acid (NAc) and its metabolic precursor, quinolinic acid (QA), are produced in yields as high as 7% in a six-step
nonenzymatic sequence from aspartic acid and dihydroxyacetone phosphate (DHAP). The biosynthesis of ribose phosphate could
have produced DHAP and other three carbon compounds. Aspartic acid could have been available from prebiotic synthesis or from
the ribozyme synthesis of pyrimidines. These results suggest that NAD could have originated in the RNA world and that the
nonenzymatic biosynthesis of the cofactor nicotinamide could have been an inevitable consequence of life based on carbohydrates
and amino acids. The enzymes of the modern pathway were later added in any order.
Received: 22 May 2000 / Accepted: 7 August 2000 相似文献
7.
Davide De Lucrezia Fabrizio Anella Cristiano Chiarabelli 《Origins of life and evolution of the biosphere》2007,37(4-5):379-385
The discovery of catalytic RNA has revolutionised modern molecular biology and bears important implications for the origin
of Life research. Catalytic RNA, in particular self-replicating RNA, prompted the hypothesis of an early “RNA world” where
RNA molecules played all major roles such information storage and catalysis. The actual role of RNA as primary actor in the
origin of life has been under debate for a long time, with a particular emphasis on possible pathways to the prebiotic synthesis
of mononucleotides; their polymerization and the possibility of spontaneous emergence of catalytic RNAs synthesised under
plausible prebiotic conditions. However, little emphasis has been put on the chemical reality of an RNA world; in particular
concerning the chemical constrains that such scenario should have met to be feasible. This paper intends to address those
concerns with regard to the achievement of high local RNA molecules concentration and the aetiology of unique sequence under
plausible prebiotic conditions.
Presented at: International School of Complexity – 4th Course: Basic Questions on the Origins of Life; “Ettore Majorana” Foundation and Centre for Scientific Culture, Erice, Italy, 1–6 October 2006. 相似文献
8.
Arthur L. Weber 《Journal of molecular evolution》1997,44(4):354-360
To identify the energy source that drives the biosynthesis of amino acids, lipids, and nucleotides from glucose, we calculated
the free energy change due to redox disproportionation of the substrate carbon of (1) 26-carbon fermentation reactions and
(2) the biosynthesis of amino acids and lipids of E. coli from glucose. The free energy (cal/mmol of carbon) of these reactions was plotted as a function of the degree of redox disproportionation
of carbon (disproportionative electron transfers (mmol)/mmol of carbon). The zero intercept and proportionality between energy
yield and degree of redox disporportionation exhibited by this plot demonstrate that redox disproportionation is the principal
energy source of these redox reactions (slope of linear fit =−10.4 cal/mmol of disproportionative electron transfers). The
energy and disproportionation values of E. coli amino acid and lipid biosynthesis from glucose lie near this linear curve fit with redox disproportionation accounting for
84% and 96% (and ATP only 6% and 1%) of the total energy of amino acid and lipid biosynthesis, respectively. These observations
establish that redox disproportionation of carbon, and not ATP, is the primary energy source driving amino acid and lipid
biosynthesis from glucose. In contrast, we found that nucleotide biosynthesis involves very little redox disproportionation,
and consequently depends almost entirely on ATP for energy. The function of sugar redox disproportionation as the major source
of free energy for the biosynthesis of amino acids and lipids suggests that sugar disproportionation played a central role
in the origin of metabolism, and probably the origin of life.
Received: 18 April 1996 / Accepted: 31 October 1996 相似文献
9.
Modified purines are found in all organisms in the tRNA, rRNA, and even DNA, raising the possibility of an early role for
these compounds in the evolution of life. These include N
6-methyladenine, 1-methyladenine, N
6,N
6-dimethyladenine, 1-methylhypoxanthine, 1-methylguanine, and N
2-methylguanine. We find that these bases as well as a number of nonbiological modified purines can be synthesized from adenine
and guanine by the simple reaction of an amine or an amino group with adenine and guanine under the concentrated conditions
of the drying-lagoon or drying-beach model of prebiotic synthesis with yields as high as 50%. These compounds are therefore
as prebiotic as adenine and guanine and could have played an important role in the RNA world by providing additional functional
groups in ribozymes, especially for the construction of hydrophobic binding pockets.
Received: 7 August 1998 / Accepted: 31 December 1998 相似文献
10.
Benoit Cousineau Fabrice Leclerc Robert Cedergren 《Journal of molecular evolution》1997,45(6):661-670
Sequence similarity has given rise to the proposal that IF-2, EF-G, and EF-Tu are related through a common ancestor. We evaluate
this proposition and whether the relationship can be extended to other factors of protein synthesis. Analysis of amino acid
sequence similarity gives statistical support for an evolutionary affiliation among IF-1, IF-2, IF-3, EF-Tu, EF-Ts, and EF-G
and suggests further that this association is a result of gene duplication/fusion events. In support of this mechanism, the
three-dimensional structures of IF-3, EF-Tu, and EF-G display a predictable domain structure and overall conformational similarity.
The model that we propose consists of three consecutives duplication/fusion events which would have taken place before the
divergence of the three superkingdoms: eubacteria, archaea, and eukaryotes. The root of this protein superfamily tree would
be an ancestor of the modern IF-1 gene sequence. The repeated fundamental motif of this protein superfamily is a small RNA
binding domain composed of two α-helices packed along side of an antiparallel β-sheet.
Received: 17 October 1996 / Accepted: 10 June 1997 相似文献
11.
Thomas Cavalier-Smith 《Journal of molecular evolution》2001,53(4-5):555-595
I attempt to sketch a unified picture of the origin of living organisms in their genetic, bioenergetic, and structural aspects.
Only selection at a higher level than for individual selfish genes could power the cooperative macromolecular coevolution
required for evolving the genetic code. The protein synthesis machinery is too complex to have evolved before membranes. Therefore
a symbiosis of membranes, replicators, and catalysts probably mediated the origin of the code and the transition from a nucleic
acid world of independent molecular replicators to a nucleic acid/protein/lipid world of reproducing organisms. Membranes
initially functioned as supramolecular structures to which different replicators attached and were selected as a higher-level
reproductive unit: the proto-organism. I discuss the roles of stereochemistry, gene divergence, codon capture, and selection
in the code's origin. I argue that proteins were primarily structural not enzymatic and that the first biological membranes
consisted of amphipathic peptidyl-tRNAs and prebiotic mixed lipids. The peptidyl-tRNAs functioned as genetically-specified
lipid analogues with hydrophobic tails (ancestral signal peptides) and hydrophilic polynucleotide heads. Protoribosomes arose
from two cooperating RNAs: peptidyl transferase (large subunit) and mRNA-binder (small subunit). Early proteins had a second
key role: coupling energy flow to the phosphorylation of gene and peptide precursors, probably by lithophosphorylation by
membrane-anchored kinases scavenging geothermal polyphosphate stocks. These key evolutionary steps probably occurred on the
outer surface of an `inside out-cell' or obcell, which evolved an unambiguous hydrophobic code with four prebiotic amino acids
and proline, and initiation by isoleucine anticodon CAU; early proteins and nucleozymes were all membrane-attached. To improve
replication, translation, and lithophosphorylation, hydrophilic substrate-binding and catalytic domains were later added to
signal peptides, yielding a ten-acid doublet code. A primitive proto-ecology of molecular scavenging, parasitism, and predation
evolved among obcells. I propose a new theory for the origin of the first cell: fusion of two cup-shaped obcells, or hemicells,
to make a protocell with double envelope, internal genome and ribosomes, protocytosol, and periplasm. Only then did water-soluble
enzymes, amino acid biosynthesis, and intermediary metabolism evolve in a concentrated autocatalytic internal cytosolic soup,
causing 12 new amino acid assignments, termination, and rapid freezing of the 22-acid code. Anticodons were recruited sequentially:
GNN, CNN, INN, and *UNN. CO2 fixation, photoreduction, and lipid synthesis probably evolved in the protocell before photophosphorylation. Signal recognition
particles, chaperones, compartmented proteases, and peptidoglycan arose prior to the last common ancestor of life, a complex
autotrophic, anaerobic green bacterium.
Received: 19 February 2001 / Accepted: 9 April 2001 相似文献
12.
One of the most debated issues concerning the origin of life, is how enzymes which are essential for existence of any living
organism, evolved. It is clear that, regardless of the exact mechanism, the process should have been specific and reproducible,
involving interactions between different molecules. We propose that substrate templating played a crucial role in maintaining
reproducible and specific formation of prebiotic catalysts. This work demonstrates experimentally, for the first time, substrate-directed
formation of an oligopeptide that possesses a specific catalytic activity toward the substrate on which it was formed. In
our experiments we used the substrate o-nitrophenol-β-d-galactopyranoside (ONPG) as a molecular template for the synthesis of a specific catalyst that is capable of cleaving the
same substrate. This was achieved by incubation of the substrate with free amino acids and a condensing agent (dicyandiamide)
at elevated temperatures. A linear increase with time of the reaction rate (d[product]/d2t), pointed to an acceleration regime, where the substrate generates the formation of the catalyst. The purified catalyst,
produced by a substrate-directed mechanism, was analyzed, and identified as Cys2-Fe+2. The mechanism of substrate-directed formation of prebiotic catalysts provides a solution to both the specificity and the
reproducibility requirements from any prebiotic system which should evolve into the biological world.
Received: 26 January 1996 / Accepted: 22 April 1997 相似文献
13.
The Origin of Chlorarachniophyte Plastids, as Inferred from Phylogenetic Comparisons of Amino Acid Sequences of EF-Tu 总被引:4,自引:0,他引:4
Ken-ichiro Ishida Ying Cao Masami Hasegawa Norihiro Okada Yoshiaki Hara 《Journal of molecular evolution》1997,45(6):682-687
A molecular phylogenetic analysis of elongation factor Tu (EF-Tu) proteins from plastids was performed in an attempt to identify
the origin of chlorarachniophyte plastids, which are considered to have evolved from the endosymbiont of a photosynthetic
eukaryote. Partial sequences of the genes for plastid EF-Tu proteins (1,080–1,089 bp) were determined for three algae that
contain chlorophyll b, namely, Gymnochlora stellata (Chlorarachniophyceae), Bryopsis maxima (Ulvophyceae), and Pyramimonas disomata (Prasinophyceae). The deduced amino acid sequences were used to construct phylogenetic trees of the plastid and bacterial
EF-Tu proteins by the maximum likelihood, the maximum parsimony, and the neighbor joining methods.
The trees obtained in the present analysis suggest that all plastids that contain chlorophyll b are monophyletic and that the chlorarachniophyte plastids are closely related to those of the Ulvophyceae. The phylogenetic
trees also suggest that euglenophyte plastids are closely related to prasinophycean plastids. The results indicate that the
chlorarachniophyte plastids evolved from a green algal endosymbiont that was closely related to the Ulvophyceae and that at
least two secondary endosymbiotic events have occurred in the lineage of algae with plastids that contain chlorophyll b.
Received: 10 March 1997 / Accepted: 28 July 1997 相似文献
14.
An experimental system for determining the potential ability of sequences resembling 5S ribosomal RNA (rRNA) to perform as
functional 5S rRNAs in vivo in the Escherichia coli cellular environment was devised previously. Presumably, the only 5S rRNA sequences that would have been fixed by ancestral
populations are ones that were functionally valid, and hence the actual historical paths taken through RNA sequence space
during 5S rRNA evolution would have most likely utilized valid sequences. Herein, we examine the potential validity of all
sequence intermediates along alternative equally parsimonious trajectories through RNA sequence space which connect two pairs
of sequences that had previously been shown to behave as valid 5S rRNAs in E. coli. The first trajectory requires a total of four changes. The 14 sequence intermediates provide 24 apparently equally parsimonious
paths by which the transition could occur. The second trajectory involves three changes, six intermediate sequences, and six
potentially equally parsimonious paths. In total, only eight of the 20 sequence intermediates were found to be clearly invalid.
As a consequence of the position of these invalid intermediates in the sequence space, seven of the 30 possible paths consisted
of exclusively valid sequences. In several cases, the apparent validity/invalidity of the intermediate sequences could not
be anticipated on the basis of current knowledge of the 5S rRNA structure. This suggests that the interdependencies in RNA
sequence space may be more complex than currently appreciated. If ancestral sequences predicted by parsimony are to be regarded
as actual historical sequences, then the present results would suggest that they should also satisfy a validity requirement
and that, in at least limited cases, this conjecture can be tested experimentally.
Received: 27 August 1996 / Accepted: 14 April 1997 相似文献
15.
Two forces are in general, hypothesized to have influenced the origin of the organization of the genetic code: the physicochemical
properties of amino acids and their biosynthetic relationships. In view of this, we have considered a model incorporating
these two forces. In particular, we have studied the optimization level of the physicochemical properties of amino acids in
the set of amino acid permutation codes that respects the biosynthetic relationships between amino acids. Where the properties
of amino acids are represented by polarity and molecular volume we obtain indetermination percentages in the organization
of the genetic code of approximately 40%. This indicates that the contingent factor played a significant role in structuring
the genetic code. Furthermore, this result is in agreement with the genetic code coevolution hypothesis, which attributes
a merely ancillary role to the properties of amino acids while it suggests that it was their biosynthetic relationships that
organized the code. Furthermore, this result does not favor the stereochemical models proposed to explain the origin of the
genetic code. On the other hand, where the properties of amino acids are represented by polarity alone, we obtain an indetermination
percentage of at least 21.5%. This might suggest that the polarity distances played an important role and would therefore
provide evidence in favor of the physicochemical hypothesis of genetic code origin. Although, overall, the analysis might
have given stronger support to the latter hypothesis, this did not actually occur. The results are therefore discussed in
the context of the different theories proposed to explain the origin of the genetic code.
Received: 10 September 1996 / Accepted: 3 March 1997 相似文献
16.
Song-Kun Shyue Stéphane Boissinot Horacio Schneider Iracilda Sampaio Maria Paula Schneider C.R. Abee Lawrence Williams David Hewett-Emmett Harry G. Sperling Jill A. Cowing Kanwaljit S. Dulai David M. Hunt Wen-Hsiung Li 《Journal of molecular evolution》1998,46(6):697-702
Although most New World monkeys have only one X-linked photopigment locus, many species have three polymorphic alleles at
the locus. The three alleles in the squirrel monkey and capuchin have spectral peaks near 562, 550, and 535 nm, respectively,
and the three alleles in the marmoset and tamarin have spectral peaks near 562, 556, and 543 nm, respectively. To determine
the amino acids responsible for the spectral sensitivity differences among these pigment variants, we sequenced all exons
of the three alleles in each of these four species. From the deduced amino acid sequences and the spectral peak information
and from previous studies of the spectral tuning of X-linked pigments in humans and New World monkeys, we estimated that the
Ala → Ser, Ile → Phe, Gly → Ser, Phe → Tyr, and Ala → Tyr substitutions at residue positions 180, 229, 233, 277, and 285,
respectively, cause spectral shifts of about 5, −2, −1, 8, and 15 nm. On the other hand, the substitutions His → Tyr, Met
→ Val or Leu, and Ala → Tyr at positions 116, 275, and 276, respectively, have no discernible spectral tuning effect, though
residues 275 and 276 are inside the transmembrane domains. Many substitutions between Val and Ile or between Val and Ala have
occurred in the transmembrane domains among the New World monkey pigment variants but apparently have no effect on spectral
tuning. Our study suggests that, in addition to amino acid changes involving a hydroxyl group, large changes in residue size
can also cause a spectral shift in a visual pigment.
Received: 17 July 1997 / Accepted: 7 December 1997 相似文献
17.
We have assumed that the coevolution theory of genetic code origin (Wong JT, Proc Natl Acad Sci USA 72:1909–1912, 1975) is
essentially correct. This theory makes it possible to identify at least 10 evolutionary stages through which genetic code
organization might have passed prior to reaching its current form. The calculation of the minimization level of all these
evolutionary stages leads to the following conclusions. (1) The minimization percentages increased linearly with the number
of amino acids codified in the codes of the various evolutionary stages when only the sense changes are considered in the
analysis. This seems to favor the physicochemical theory of genetic code origin even if, as discussed in the paper, this observation
is also compatible with the coevolution theory. (2) For the first seven evolutionary stages of the genetic code, this trend
is less clear and indeed is inverted when we consider the global optimisation of the codes due to both sense changes and synonymous
changes. This inverse correlation between minimization percentages and the number of amino acids codified in the codes of
the intermediate stages seems to favor neither the physicochemical nor the stereochemical theories of genetic code origin,
as it is in the early and intermediate stages of code development that these theories would expect minimization to have played
a crucial role, and this does not seem to be the case. However, these results are in agreement with the coevolution theory,
which attributes a role to the physicochemical properties of amino acids that, while important, is nevertheless subordinate
to the mechanism which concedes codons from the precursor amino acids to the product amino acids as the primary factor determining
the evolutionary structuring of the genetic code. The results are therefore discussed in the context of the various theories
proposed to explain genetic code origin.
Received: 25 October 1998 / Accepted: 19 February 1999 相似文献
18.
Michael Yarus 《Journal of molecular evolution》1998,47(1):109-117
Numerous RNA binding sites for specific amino acids are now known, coming predominantly from selection-amplification experiments.
These sites are chemically discriminating despite being predominantly small, simple RNA structures: internal and bulge loops.
Recent studies of sites for hydrophobic side chains suggest that there are other generalizable structural features which recur
in hydrophobic RNA sites. Further, sites for hydrophobic side chains can contain codons for the bound amino acid, as has also
long been known for the polar amino acid arginine. Such findings are comprehensively reviewed, and the implications for the
origin of coded peptide synthesis are considered. An origins hypothesis which accommodates all the data, DRT (direct RNA templating),
is formulated.
Received: 22 December 1997 / Accepted: 13 February 1998 相似文献
19.
Radu Popa 《Journal of molecular evolution》1997,44(2):121-127
A sequential model is proposed regarding the origin of biological chirality. Three major stages are presumed: a symmetry
breaking (prebiotic chiral disruption in enantiomeric mixtures of monomers), a chiral amplification (prebiotic increase of
the chiral character of the monomers affected first by the symmetry breaking), and a chiral expansion (proto biological increase
of the chiral character and spread of the chirality to molecules which were less affected by prebiotic chiralizations). As
a symmetry-breaking mechanism, the model proposed by Deutsch (1991) is used, which involves a dissymmetric exposure of amino
acids (AA) to ultraviolet circularly polarized light (UV-CPL) on evaporative seashores. It is presumed that the chiral amplification,
up to a protobiologic significance, was influenced by a periodic overlapping of two abiotic events, a synchronization between
tidal-based hydrous–anhydrous cycles, and littoral asymmetric photolysis cycles. This long-term astronomic asymmetry acted
around 3.8–4.2 billion years ago and was unique to the Earth in our solar system. It is also presumed that the abiotic symmetry
breaking is heterogenous, that only a few l-AAs were used in the beginning, and that the chirality expanded later to all 20 AAs based on a coevolutionary strategy of
the genetic code and on a physiological relationship between AAs. In this scenario the d-chirality of pentoses in polynucleotides was attributed to both d-pentose/l-AA relationships and to a structural evolution.
Received: 10 May 1996 / Accepted: 13 August 1996 相似文献
20.
Anastassia Kanavarioti 《Journal of molecular evolution》1998,46(6):622-632
Phosphoimidazolide-activated ribomononucleotides (*pN; see Scheme I) are useful substrates for the nonenzymatic synthesis
of oligonucleotides. In the presence of metal ions dilute neutral aqueous solutions of *pN (0.01 M) typically yield only small amounts of dimers and traces of oligomers; most of *pN hydrolyzes to yield nucleoside 5′-monophosphate
(5′NMP). An earlier investigation of *pN reactions in highly concentrated aqueous solutions (up to 1.4 M) showed, as expected, that the percentage yield of the condensation products increases and the yield of the hydrolysis product
correspondingly decreases with *pN concentration (Kanavarioti 1997). Here we report product distributions in reactions with
one, two, or three reactive components at the same total nucleotide concentration. *pN used as substrates were the nucleoside 5′-phosphate 2-methylimidazolides, 2-MeImpN, with N= cytidine (C), uridine (U), or guanosine (G). Reactions were conducted as self-condensations, i.e., one nucleotide only,
with two components in the three binary U,C, U,G, and C,G mixtures, and with three components in the ternary U,C,G mixture.
The products are 5′NMP, 5′,5′-pyrophosphate-, 2′,5′-, 3′,5′-linked dimers, cyclic dimers, and a small percentage of longer
oligomers. The surprising finding was that, under identical conditions, including the same total monomer concentration, the
product distribution differs substantially from one reaction to another, most likely due to changing intermolecular interactions
depending on the constituents. Even more unexpected was the observed trend according to which reactions of the U,C,G mixture
produce the highest yield of internucleotide-linked dimers, whereas the self-condensations produce the least and the reactions
with the binary mixtures produce yields that fall in between. What is remarkable is that the approximately two-fold increase
in the percentage yield of internucleotide-linked dimers is not due to a concentration effect or a catalyst, but to the increased
complexity of the system from a single to two and three components. These observations, perhaps, provide an example of how
increased complexity in relatively simple chemical systems leads to organization of the material and consequently to chemical
evolution. A possible link between prebiotic chemistry and the postulated RNA world is discussed.
Received: 12 September 1997 / Accepted: 24 November 1997 相似文献