= Solution
The <RNA world hypothesis> proposes an early stage in which <RNA>-like molecules carried heritable sequence information and also performed important catalytic functions, before the present division of labour among <DNA>, <RNA> and <proteins>. Complementary <base pairing> supplies a route to template-directed information copying, while sequence-dependent <RNA> folding can create catalytic structures. This helps address the apparent circularity of requiring <protein> <enzymes> to copy <genes> whose expression already requires an elaborate <protein>-synthesis apparatus.
Modern biology supplies evidence for the necessary capacities. <Ribozymes> catalyse reactions, and <self-splicing introns> demonstrate <RNA>-dependent processing without a dedicated <protein> <enzyme> for that chemistry. The <ribosome>'s peptidyl-transferase centre is built principally from <RNA>, with catalytic geometry located in its <RNA> core; structural work provides direct evidence that this central <protein>-synthesis chemistry is <RNA> based.<The structural basis of ribosome activity in peptide bond synthesis by Poul Nissen and others>[Structures and substrate-analogue complexes identify an RNA-based peptidyl-transferase active site]{ref} <RNA> also remains central as messenger, adaptor and regulatory material. These observations make <RNA> <catalysis> and information storage physically plausible rather than assuming that all <catalysts> must be <proteins>.
They do not prove a complete historical sequence. A viable early system still needs plausible <nucleotide> supply and activation, polymerization, template copying with sufficient fidelity, product-strand separation, compatibility of catalytic and copying conditions, an <energy> source and some means of maintaining useful combinations. <RNA>'s chemical lability and the challenge of separating paired products are constraints, not details supplied automatically by <base pairing>. The first hereditary chemistry might also have involved related polymers or cooperative <RNA>–peptide systems rather than an exclusively <RNA> organism.
The later advantages of <DNA> as a relatively stable storage medium and <proteins> as <catalysts> with chemically varied side chains help explain the present division of labour. The hypothesis is therefore a testable family of evolutionary scenarios supported by molecular capabilities and biological relics, not a direct observation of prebiotic life or proof that an isolated modern <RNA> already self-replicates autonomously.
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