Sequence Space and Quasispecies Distribution

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Abstract
Self-reproduction, the basis of evolutionary adaptation, actually serves two purposes: first, it ensures conservation of information, despite the steady chemical decomposition of its structural carrier. 1 , 2 Second, it provides by its autocatalytic nature a mechanism of competitive growth and selection. Although proteins, owing to their more straightforward chemical synthesis mechanisms, might have appeared first on our planet, 3 , 4 functional adaptation and optimization had to await the arrival of a self-replicative system for information storage. 2 , 5 There are several strong indications that it was RNA rather than DNA which provided the first information carriers in early evolution: Ribose forms more easily under potential prebiotic conditions than deoxyribose does (aldol condensation). 4 The nucleophilicity of the 3′-OH of deoxyribose is weaker than the vicinal 2′-3′-hydroxyls of ribose. The efficiency of phosphodiester formation for any nonenzymic polynucleotide synthesis is thus higher for ribonucleotides. 4 The biosynthesis of deoxyribonucleotides proceeds via reduction of ribonucleoside diphosphates. Replication of DNA requires RNA primers 6 or — as Sol Spiegelman once phrased it — “DNA up to the present day hasn’t yet learned to reproduce itself without the help of RNA.” The ribo base-pairs are more stable than their deoxyribo analogues. 7 Internal folding is further stabilized by additional hydrogen bonds, often involving the 2′-hydroxyl. The larger tendency of RNA to undergo internal folding processes stabilizes singlestranded forms, while DNA prefers the double-helical form of the complementary strands. In the single-stranded forms, RNA offers a richer repertoire of tertiary structures than an analogue DNA strand would provide. As far is known today, all functional nucleic acids are of the ribo-type (cf. ribosomal RNA, tRNA, “ribozymes”). 8

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