Abstract
Biological RNA is a uniform polymer in three senses: it uses nucleotides of a single chirality; it uses only ribose sugars and four nucleobases rather than a mixture of other sugars and bases; and it uses only 3′-5′ bonds rather than a mixture of different bond types. We suppose that prebiotic chemistry would generate a diverse mixture of potential monomers, and that random polymerization would generate non-uniform strands of mixed chirality, monomer composition, and bond type. We ask what factors lead to the emergence of RNA from this mixture. We show that template-directed replication can lead to the emergence of all the uniform properties of RNA by the same mechanism. We study a computational model in which nucleotides react via polymerization, hydrolysis, and template-directed ligation. Uniform strands act as templates for ligation of shorter oligomers of the same type, whereas mixed strands do not act as templates. The three uniform properties emerge naturally when the ligation rate is high. If there is an exact symmetry, as with the chase of chirality, the uniform property arises via a symmetry-breaking phase transition. If there is no exact symmetry, as with monomer selection and backbone regioselectivity, the uniform property emerges gradually as the rate of template-directed ligation is increased.
Highlights
An important feature of life is that it makes frequent use of a well-defined set of molecules, but it does not use many other kinds of molecules that have rather similar chemical properties. This has been called the ‘lego principle’ [1], and it has been proposed that this feature could be used as a signature of life on other planets. This principle applies to amino acids, where the set of possible amino acids is much larger than the 20 used in biological proteins [2], and to nucleotides, where there is a huge diversity of sugars [3] and nucleobases [4,5] that could potentially form polymers similar to RNA and DNA
The aim of this paper is to present a simple computational model that is able to treat the problems of chirality, monomer selection, and backbone regioselectivity in the same way
Introduction, reviewed the experimental evidence that oligomers with chirality, uniform monomer composition, or bond type are effective templates for the growth of complementary strands chirality, monomer composition, or bond type are effective templates for the growth of with the same chirality, monomer or bond type, and that oligomers mixed in any complementary strands with thecomposition, same chirality, monomer composition, or that bondaretype, and that of these properties less effective templates than uniform
Summary
An important feature of life is that it makes frequent use of a well-defined set of molecules, but it does not use many other kinds of molecules that have rather similar chemical properties. This has been called the ‘lego principle’ [1], and it has been proposed that this feature could be used as a signature of life on other planets. Non-living chemistry is governed by thermodynamics and reaction kinetics. A subset of molecules is able to catalyze formation of more of the same set of molecules that it requires for growth and reproduction
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