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2541 KiB

Open AccessArticle

Better than Membranes at the Origin of Life?

by Helen Greenwood Hansma

Life 2017, 7(2), 28; https://doi.org/10.3390/life7020028 - 20 Jun 2017

Cited by 26 | Viewed by 6838

Organelles without membranes are found in all types of cells and typically contain RNA and protein. RNA and protein are the constituents of ribosomes, one of the most ancient cellular structures. It is reasonable to propose that organelles without membranes preceded protocells and [...] Read more.

Organelles without membranes are found in all types of cells and typically contain RNA and protein. RNA and protein are the constituents of ribosomes, one of the most ancient cellular structures. It is reasonable to propose that organelles without membranes preceded protocells and other membrane-bound structures at the origins of life. Such membraneless organelles would be well sheltered in the spaces between mica sheets, which have many advantages as a site for the origins of life. Full article

(This article belongs to the Special Issue The Landscape of the Emergence of Life)

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236 KiB

Open AccessFeature PaperConference Report

The Landscape of the Emergence of Life

by Sohan Jheeta

Life 2017, 7(2), 27; https://doi.org/10.3390/life7020027 - 16 Jun 2017

Cited by 18 | Viewed by 8347

Abstract

This paper reports on the various nuances of the origins of life on Earth and highlights the latest findings in that arena as reported at the Network of Researchers on Horizontal Gene Transfer and the Last Universal Common Ancestor (NoR HGT and LUCA) [...] Read more.

This paper reports on the various nuances of the origins of life on Earth and highlights the latest findings in that arena as reported at the Network of Researchers on Horizontal Gene Transfer and the Last Universal Common Ancestor (NoR HGT and LUCA) which was held from the 3–4th November 2016 at the Open University, UK. Although the answers to the question of the origin of life on Earth will not be fathomable anytime soon, a wide variety of subject matter was able to be covered, ranging from examining what constitutes a LUCA, looking at viral connections and “from RNA to DNA”, i.e., could DNA have been formed simultaneously with RNA, rather than RNA first and then describing the emergence of DNA from RNA. Also discussed are proteins and the origins of genomes as well as various ideas that purport to explain the origin of life here on Earth and potentially further afield elsewhere on other planets. Full article

(This article belongs to the Special Issue The Landscape of the Emergence of Life)

2696 KiB

Open AccessReview

DNA Protection Protein, a Novel Mechanism of Radiation Tolerance: Lessons from Tardigrades

by Takuma Hashimoto and Takekazu Kunieda

Life 2017, 7(2), 26; https://doi.org/10.3390/life7020026 - 15 Jun 2017

Cited by 45 | Viewed by 20965

Abstract

Genomic DNA stores all genetic information and is indispensable for maintenance of normal cellular activity and propagation. Radiation causes severe DNA lesions, including double-strand breaks, and leads to genome instability and even lethality. Regardless of the toxicity of radiation, some organisms exhibit extraordinary [...] Read more.

Genomic DNA stores all genetic information and is indispensable for maintenance of normal cellular activity and propagation. Radiation causes severe DNA lesions, including double-strand breaks, and leads to genome instability and even lethality. Regardless of the toxicity of radiation, some organisms exhibit extraordinary tolerance against radiation. These organisms are supposed to possess special mechanisms to mitigate radiation-induced DNA damages. Extensive study using radiotolerant bacteria suggested that effective protection of proteins and enhanced DNA repair system play important roles in tolerability against high-dose radiation. Recent studies using an extremotolerant animal, the tardigrade, provides new evidence that a tardigrade-unique DNA-associating protein, termed Dsup, suppresses the occurrence of DNA breaks by radiation in human-cultured cells. In this review, we provide a brief summary of the current knowledge on extremely radiotolerant animals, and present novel insights from the tardigrade research, which expand our understanding on molecular mechanism of exceptional radio-tolerability. Full article

(This article belongs to the Special Issue Extremophiles and the Origin of Life)

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1188 KiB

Open AccessReview

Cryosphere and Psychrophiles: Insights into a Cold Origin of Life?

by Georges Feller

Life 2017, 7(2), 25; https://doi.org/10.3390/life7020025 - 11 Jun 2017

Cited by 36 | Viewed by 7970

Abstract

Psychrophiles thrive permanently in the various cold environments on Earth. Their unsuspected ability to remain metabolically active in the most extreme low temperature conditions provides insights into a possible cold step in the origin of life. More specifically, metabolically active psychrophilic bacteria have [...] Read more.

Psychrophiles thrive permanently in the various cold environments on Earth. Their unsuspected ability to remain metabolically active in the most extreme low temperature conditions provides insights into a possible cold step in the origin of life. More specifically, metabolically active psychrophilic bacteria have been observed at −20 °C in the ice eutectic phase (i.e., the liquid veins between sea ice crystals). In the context of the RNA world hypothesis, this ice eutectic phase would have provided stability to the RNA molecules and confinement of the molecules in order to react and replicate. This aspect has been convincingly tested by laboratory experiments. Full article

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575 KiB

Open AccessHypothesis

Has Inositol Played Any Role in the Origin of Life?

by Adolfo Saiardi

Life 2017, 7(2), 24; https://doi.org/10.3390/life7020024 - 5 Jun 2017

Cited by 9 | Viewed by 7213

Abstract

Phosphorus, as phosphate, plays a paramount role in biology. Since phosphate transfer reactions are an integral part of contemporary life, phosphate may have been incorporated into the initial molecules at the very beginning. To facilitate the studies into early phosphate utilization, we should [...] Read more.

Phosphorus, as phosphate, plays a paramount role in biology. Since phosphate transfer reactions are an integral part of contemporary life, phosphate may have been incorporated into the initial molecules at the very beginning. To facilitate the studies into early phosphate utilization, we should look retrospectively to phosphate-rich molecules present in today’s cells. Overlooked by origin of life studies until now, inositol and the inositol phosphates, of which some species possess more phosphate groups that carbon atoms, represent ideal molecules to consider in this context. The current sophisticated association of inositol with phosphate, and the roles that some inositol phosphates play in regulating cellular phosphate homeostasis, intriguingly suggest that inositol might have played some role in the prebiotic process of phosphate exploitation. Inositol can be synthesized abiotically and, unlike glucose or ribose, is chemically stable. This stability makes inositol the ideal candidate for the earliest organophosphate molecules, as primitive inositol phosphates. I also present arguments suggesting roles for some inositol phosphates in early chemical evolution events. Finally, the possible prebiotic synthesis of inositol pyrophosphates could have generated high-energy molecules to be utilized in primitive trans-phosphorylating processes. Full article

(This article belongs to the Special Issue Phosphorus (P) and the Origins of Life)

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3025 KiB

Open AccessArticle

Flexible Proteins at the Origin of Life

by Andrew Pohorille, Michael A. Wilson and Gareth Shannon

Life 2017, 7(2), 23; https://doi.org/10.3390/life7020023 - 5 Jun 2017

Cited by 17 | Viewed by 6964

Abstract

Almost all modern proteins possess well-defined, relatively rigid scaffolds that provide structural preorganization for desired functions. Such scaffolds require the sufficient length of a polypeptide chain and extensive evolutionary optimization. How ancestral proteins attained functionality, even though they were most likely markedly smaller [...] Read more.

Almost all modern proteins possess well-defined, relatively rigid scaffolds that provide structural preorganization for desired functions. Such scaffolds require the sufficient length of a polypeptide chain and extensive evolutionary optimization. How ancestral proteins attained functionality, even though they were most likely markedly smaller than their contemporary descendants, remains a major, unresolved question in the origin of life. On the basis of evidence from experiments and computer simulations, we argue that at least some of the earliest water-soluble and membrane proteins were markedly more flexible than their modern counterparts. As an example, we consider a small, evolved in vitro ligase, based on a novel architecture that may be the archetype of primordial enzymes. The protein does not contain a hydrophobic core or conventional elements of the secondary structure characteristic of modern water-soluble proteins, but instead is built of a flexible, catalytic loop supported by a small hydrophilic core containing zinc atoms. It appears that disorder in the polypeptide chain imparts robustness to mutations in the protein core. Simple ion channels, likely the earliest membrane protein assemblies, could also be quite flexible, but still retain their functionality, again in contrast to their modern descendants. This is demonstrated in the example of antiamoebin, which can serve as a useful model of small peptides forming ancestral ion channels. Common features of the earliest, functional protein architectures discussed here include not only their flexibility, but also a low level of evolutionary optimization and heterogeneity in amino acid composition and, possibly, the type of peptide bonds in the protein backbone. Full article

(This article belongs to the Special Issue Origin of Cellular Life)

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1005 KiB

Open AccessConcept Paper

Frozen Accident Pushing 50: Stereochemistry, Expansion, and Chance in the Evolution of the Genetic Code

by Eugene V. Koonin

Life 2017, 7(2), 22; https://doi.org/10.3390/life7020022 - 23 May 2017

Cited by 59 | Viewed by 11607

Abstract

Nearly 50 years ago, Francis Crick propounded the frozen accident scenario for the evolution of the genetic code along with the hypothesis that the early translation system consisted primarily of RNA. Under the frozen accident perspective, the code is universal among modern life [...] Read more.

Nearly 50 years ago, Francis Crick propounded the frozen accident scenario for the evolution of the genetic code along with the hypothesis that the early translation system consisted primarily of RNA. Under the frozen accident perspective, the code is universal among modern life forms because any change in codon assignment would be highly deleterious. The frozen accident can be considered the default theory of code evolution because it does not imply any specific interactions between amino acids and the cognate codons or anticodons, or any particular properties of the code. The subsequent 49 years of code studies have elucidated notable features of the standard code, such as high robustness to errors, but failed to develop a compelling explanation for codon assignments. In particular, stereochemical affinity between amino acids and the cognate codons or anticodons does not seem to account for the origin and evolution of the code. Here, I expand Crick’s hypothesis on RNA-only translation system by presenting evidence that this early translation already attained high fidelity that allowed protein evolution. I outline an experimentally testable scenario for the evolution of the code that combines a distinct version of the stereochemical hypothesis, in which amino acids are recognized via unique sites in the tertiary structure of proto-tRNAs, rather than by anticodons, expansion of the code via proto-tRNA duplication, and the frozen accident. Full article

(This article belongs to the Special Issue The Origin and Evolution of the Genetic Code: 100th Anniversary Year of the Birth of Francis Crick)

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2681 KiB

Open AccessArticle

Peptidyl Transferase Center and the Emergence of the Translation System

by Savio Torres de Farias, Thais Gaudêncio Rêgo and Marco V. José

Life 2017, 7(2), 21; https://doi.org/10.3390/life7020021 - 25 Apr 2017

Cited by 15 | Viewed by 6231

Abstract

In this work, the three-dimensional (3D) structure of the ancestral Peptidyl Transferase Center (PTC) built by concatamers of ancestral sequences of tRNAs was reconstructed, and its possible interactions with tRNAs molecules were analyzed. The 3D structure of the ancestral PTC was also compared [...] Read more.

In this work, the three-dimensional (3D) structure of the ancestral Peptidyl Transferase Center (PTC) built by concatamers of ancestral sequences of tRNAs was reconstructed, and its possible interactions with tRNAs molecules were analyzed. The 3D structure of the ancestral PTC was also compared with the current PTC of T. thermophilus. Docking experiments between the ancestral PTC and tRNAs suggest that in the origin of the translation system, the PTC functioned as an adhesion center for tRNA molecules. The approximation of tRNAs charged with amino acids to the PTC permitted peptide synthesis without the need of a genetic code. Full article

(This article belongs to the Special Issue The Landscape of the Emergence of Life)

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543 KiB

Open AccessArticle

The Maximal C³ Self-Complementary Trinucleotide Circular Code X in Genes of Bacteria, Archaea, Eukaryotes, Plasmids and Viruses

by Christian J. Michel

Life 2017, 7(2), 20; https://doi.org/10.3390/life7020020 - 18 Apr 2017

Cited by 44 | Viewed by 4604

Abstract

In 1996, a set

X

of 20 trinucleotides was identified in genes of both prokaryotes and eukaryotes which has on average the highest occurrence in reading frame compared to its two shifted frames. Furthermore, this set

X

has an interesting mathematical property as [...] Read more.

In 1996, a set

X

of 20 trinucleotides was identified in genes of both prokaryotes and eukaryotes which has on average the highest occurrence in reading frame compared to its two shifted frames. Furthermore, this set

X

has an interesting mathematical property as

X

is a maximal

C^{3}

self-complementary trinucleotide circular code. In 2015, by quantifying the inspection approach used in 1996, the circular code

X

was confirmed in the genes of bacteria and eukaryotes and was also identified in the genes of plasmids and viruses. The method was based on the preferential occurrence of trinucleotides among the three frames at the gene population level. We extend here this definition at the gene level. This new statistical approach considers all the genes, i.e., of large and small lengths, with the same weight for searching the circular code

X

. As a consequence, the concept of circular code, in particular the reading frame retrieval, is directly associated to each gene. At the gene level, the circular code

X

is strengthened in the genes of bacteria, eukaryotes, plasmids, and viruses, and is now also identified in the genes of archaea. The genes of mitochondria and chloroplasts contain a subset of the circular code

X

. Finally, by studying viral genes, the circular code

X

was found in DNA genomes, RNA genomes, double-stranded genomes, and single-stranded genomes. Full article

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1853 KiB

Open AccessReview

Small and Random Peptides: An Unexplored Reservoir of Potentially Functional Primitive Organocatalysts. The Case of Seryl-Histidine

by Rafal Wieczorek, Katarzyna Adamala, Tecla Gasperi, Fabio Polticelli and Pasquale Stano

Life 2017, 7(2), 19; https://doi.org/10.3390/life7020019 - 9 Apr 2017

Cited by 38 | Viewed by 12006

Abstract

Catalysis is an essential feature of living systems biochemistry, and probably, it played a key role in primordial times, helping to produce more complex molecules from simple ones. However, enzymes, the biocatalysts par excellence, were not available in such an ancient context, and [...] Read more.

Catalysis is an essential feature of living systems biochemistry, and probably, it played a key role in primordial times, helping to produce more complex molecules from simple ones. However, enzymes, the biocatalysts par excellence, were not available in such an ancient context, and so, instead, small molecule catalysis (organocatalysis) may have occurred. The best candidates for the role of primitive organocatalysts are amino acids and short random peptides, which are believed to have been available in an early period on Earth. In this review, we discuss the occurrence of primordial organocatalysts in the form of peptides, in particular commenting on reports about seryl-histidine dipeptide, which have recently been investigated. Starting from this specific case, we also mention a peptide fragment condensation scenario, as well as other potential roles of peptides in primordial times. The review actually aims to stimulate further investigation on an unexplored field of research, namely one that specifically looks at the catalytic activity of small random peptides with respect to reactions relevant to prebiotic chemistry and early chemical evolution. Full article

(This article belongs to the Special Issue The Emergence of Life: From Chemical Origins to Synthetic Biology)

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1960 KiB

Open AccessArticle

Evo-SETI: A Mathematical Tool for Cladistics, Evolution, and SETI

by Claudio Maccone

Life 2017, 7(2), 18; https://doi.org/10.3390/life7020018 - 6 Apr 2017

Viewed by 4822

Abstract

The discovery of new exoplanets makes us wonder where each new exoplanet stands along its way to develop life as we know it on Earth. Our Evo-SETI Theory is a mathematical way to face this problem. We describe cladistics and evolution by virtue [...] Read more.

The discovery of new exoplanets makes us wonder where each new exoplanet stands along its way to develop life as we know it on Earth. Our Evo-SETI Theory is a mathematical way to face this problem. We describe cladistics and evolution by virtue of a few statistical equations based on lognormal probability density functions (pdf) in the time. We call b-lognormal a lognormal pdf starting at instant b (birth). Then, the lifetime of any living being becomes a suitable b-lognormal in the time. Next, our “Peak-Locus Theorem” translates cladistics: each species created by evolution is a b-lognormal whose peak lies on the exponentially growing number of living species. This exponential is the mean value of a stochastic process called “Geometric Brownian Motion” (GBM). Past mass extinctions were all-lows of this GBM. In addition, the Shannon Entropy (with a reversed sign) of each b-lognormal is the measure of how evolved that species is, and we call it EvoEntropy. The “molecular clock” is re-interpreted as the EvoEntropy straight line in the time whenever the mean value is exactly the GBM exponential. We were also able to extend the Peak-Locus Theorem to any mean value other than the exponential. For example, we derive in this paper for the first time the EvoEntropy corresponding to the Markov-Korotayev (2007) “cubic” evolution: a curve of logarithmic increase. Full article

(This article belongs to the Special Issue The Landscape of the Emergence of Life)

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2855 KiB

Open AccessArticle

Prebiotic Factors Influencing the Activity of a Ligase Ribozyme

by Fabrizio Anella and Christophe Danelon

Life 2017, 7(2), 17; https://doi.org/10.3390/life7020017 - 6 Apr 2017

Cited by 5 | Viewed by 6015

Abstract

An RNA-lipid origin of life scenario provides a plausible route for compartmentalized replication of an informational polymer and subsequent division of the container. However, a full narrative to form such RNA protocells implies that catalytic RNA molecules, called ribozymes, can operate in the [...] Read more.

An RNA-lipid origin of life scenario provides a plausible route for compartmentalized replication of an informational polymer and subsequent division of the container. However, a full narrative to form such RNA protocells implies that catalytic RNA molecules, called ribozymes, can operate in the presence of self-assembled vesicles composed of prebiotically relevant constituents, such as fatty acids. Hereby, we subjected a newly engineered truncated variant of the L1 ligase ribozyme, named tL1, to various environmental conditions that may have prevailed on the early Earth with the objective to find a set of control parameters enabling both tL1-catalyzed ligation and formation of stable myristoleic acid (MA) vesicles. The separate and concurrent effects of temperature, concentrations of Mg²⁺, MA, polyethylene glycol and various solutes were investigated. The most favorable condition tested consists of 100 mM NaCl, 1 mM Mg²⁺, 5 mM MA, and 4 °C temperature, whereas the addition of Mg²⁺-chelating solutes, such as citrate, tRNAs, aspartic acid, and nucleoside triphosphates severely inhibits the reaction. These results further solidify the RNA-lipid world hypothesis and stress the importance of using a systems chemistry approach whereby a wide range of prebiotic factors interfacing with ribozymes are considered. Full article

(This article belongs to the Special Issue Origin of Cellular Life)

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8716 KiB

Open AccessReview

Self-Referential Encoding on Modules of Anticodon Pairs—Roots of the Biological Flow System

by Romeu Cardoso Guimarães

Life 2017, 7(2), 16; https://doi.org/10.3390/life7020016 - 6 Apr 2017

Cited by 24 | Viewed by 7796

Abstract

The proposal that the genetic code was formed on the basis of (proto)tRNA Dimer-Directed Protein Synthesis is reviewed and updated. The tRNAs paired through the anticodon loops are an indication on the process. Dimers are considered mimics of the ribosomes—structures that hold tRNAs [...] Read more.

The proposal that the genetic code was formed on the basis of (proto)tRNA Dimer-Directed Protein Synthesis is reviewed and updated. The tRNAs paired through the anticodon loops are an indication on the process. Dimers are considered mimics of the ribosomes—structures that hold tRNAs together and facilitate the transferase reaction, and of the translation process—anticodons are at the same time codons for each other. The primitive protein synthesis system gets stabilized when the product peptides are stable and apt to bind the producers therewith establishing a self-stimulating production cycle. The chronology of amino acid encoding starts with Glycine and Serine, indicating the metabolic support of the Glycine-Serine C1-assimilation pathway, which is also consistent with evidence on origins of bioenergetics mechanisms. Since it is not possible to reach for substrates simpler than C1 and compounds in the identified pathway are apt for generating the other central metabolic routes, it is considered that protein synthesis is the beginning and center of a succession of sink-effective mechanisms that drive the formation and evolution of the metabolic flow system. Plasticity and diversification of proteins construct the cellular system following the orientation given by the flow and implementing it. Nucleic acid monomers participate in bioenergetics and the polymers are conservative memory systems for the synthesis of proteins. Protoplasmic fission is the final sink-effective mechanism, part of cell reproduction, guaranteeing that proteins don’t accumulate to saturation, which would trigger inhibition. Full article

(This article belongs to the Special Issue The Origin and Evolution of the Genetic Code: 100th Anniversary Year of the Birth of Francis Crick)

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379 KiB

Open AccessConcept Paper

What Froze the Genetic Code?

by Lluís Ribas de Pouplana, Adrian Gabriel Torres and Àlbert Rafels-Ybern

Life 2017, 7(2), 14; https://doi.org/10.3390/life7020014 - 5 Apr 2017

Cited by 17 | Viewed by 5901

Abstract

The frozen accident theory of the Genetic Code was a proposal by Francis Crick that attempted to explain the universal nature of the Genetic Code and the fact that it only contains information for twenty amino acids. Fifty years later, it is clear [...] Read more.

The frozen accident theory of the Genetic Code was a proposal by Francis Crick that attempted to explain the universal nature of the Genetic Code and the fact that it only contains information for twenty amino acids. Fifty years later, it is clear that variations to the universal Genetic Code exist in nature and that translation is not limited to twenty amino acids. However, given the astonishing diversity of life on earth, and the extended evolutionary time that has taken place since the emergence of the extant Genetic Code, the idea that the translation apparatus is for the most part immobile remains true. Here, we will offer a potential explanation to the reason why the code has remained mostly stable for over three billion years, and discuss some of the mechanisms that allow species to overcome the intrinsic functional limitations of the protein synthesis machinery. Full article

(This article belongs to the Special Issue The Origin and Evolution of the Genetic Code: 100th Anniversary Year of the Birth of Francis Crick)

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8716 KiB

Open AccessArticle

Thermal Condensation of Glycine and Alanine on Metal Ferrite Surface: Primitive Peptide Bond Formation Scenario

by Md. Asif Iqubal, Rachana Sharma, Sohan Jheeta and Kamaluddin

Life 2017, 7(2), 15; https://doi.org/10.3390/life7020015 - 27 Mar 2017

Cited by 27 | Viewed by 9446

Abstract

The amino acid condensation reaction on a heterogeneous mineral surface has been regarded as one of the important pathways for peptide bond formation. Keeping this in view, we have studied the oligomerization of the simple amino acids, glycine and alanine, on nickel ferrite [...] Read more.

The amino acid condensation reaction on a heterogeneous mineral surface has been regarded as one of the important pathways for peptide bond formation. Keeping this in view, we have studied the oligomerization of the simple amino acids, glycine and alanine, on nickel ferrite (NiFe₂O₄), cobalt ferrite (CoFe₂O₄), copper ferrite (CuFe₂O₄), zinc ferrite (ZnFe₂O₄), and manganese ferrite (MnFe₂O₄) nanoparticles surfaces, in the temperature range from 50–120 °C for 1–35 days, without applying any wetting/drying cycles. Among the metal ferrites tested for their catalytic activity, NiFe₂O₄ produced the highest yield of products by oligomerizing glycine to the trimer level and alanine to the dimer level, whereas MnFe₂O₄ was the least efficient catalyst, producing the lowest yield of products, as well as shorter oligomers of amino acids under the same set of experimental conditions. It produced primarily diketopiperazine (Ala) with a trace amount of alanine dimer from alanine condensation, while glycine was oligomerized to the dimer level. The trend in product formation is in accordance with the surface area of the minerals used. A temperature as low as 50 °C can even favor peptide bond formation in the present study, which is important in the sense that the condensation process is highly feasible without any sort of localized heat that may originate from volcanoes or hydrothermal vents. However, at a high temperature of 120 °C, anhydrides of glycine and alanine formation are favored, while the optimum temperature for the highest yield of product formation was found to be 90 °C. Full article

(This article belongs to the Special Issue The Landscape of the Emergence of Life)

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2350 KiB

Open AccessReview

The Genetic Code and RNA-Amino Acid Affinities

by Michael Yarus

Life 2017, 7(2), 13; https://doi.org/10.3390/life7020013 - 23 Mar 2017

Cited by 61 | Viewed by 14060

Abstract

A significant part of the genetic code likely originated via a chemical interaction, which should be experimentally verifiable. One possible verification relates bound amino acids (or perhaps their activated congeners) and ribonucleotide sequences within cognate RNA binding sites. To introduce this interaction, I [...] Read more.

A significant part of the genetic code likely originated via a chemical interaction, which should be experimentally verifiable. One possible verification relates bound amino acids (or perhaps their activated congeners) and ribonucleotide sequences within cognate RNA binding sites. To introduce this interaction, I first summarize how amino acids function as targets for RNA binding. Then the experimental method for selecting relevant RNA binding sites is characterized. The selection method’s characteristics are related to the investigation of the RNA binding site model treated at the outset. Finally, real binding sites from selection and also from extant natural RNAs (for example, the Sulfobacillus guanidinium riboswitch) are connected to the genetic code, and by extension, to the evolutionary progression that produced the code. During this process, peptides may have been produced directly on an instructive amino acid binding RNA (a DRT; Direct RNA Template). Combination of observed stereochemical selectivity with adaptation and co-evolutionary refinement is logically required, and also potentially sufficient, to create the striking order conserved throughout the present coding table. Full article

(This article belongs to the Special Issue The Origin and Evolution of the Genetic Code: 100th Anniversary Year of the Birth of Francis Crick)

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