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Origins of life
Understanding the prebiotic origins of the nucleic acids is a long-standing challenge. The latest experiments support the idea that the first nucleic acid encoded information using a mixed 'alphabet' of RNA and DNA subunits.
The genetic polymers RNA and DNA are central to information storage in all biological systems, and as such form the core of most hypotheses about the origin of life. The most prominent of these theories is the 'RNA world' hypothesis, which posits that RNA was once both the central information-carrier and the catalyst for biochemical reactions on Earth before the emergence of life1. However, studies in the past few years (see ref. 2, for example) have suggested that the first genetic systems might have been based on nucleic-acid molecules that contain both RNA and DNA nucleotides, which then gradually self-separated into today's RNA and DNA. On page 60, Xu et at.3 offer fascinating experimental support for a mixed RNA-DNA world.
Primordial geochemical processes are thought to have led to the formation of the building blocks of nucleic acids - nucleotides and nucleosides (nucleotides that lack a phosphate group). Under suitable conditions, these building blocks polymerized and the resulting strands eventually replicated, without assistance from modern protein enzymes.
Workers from the same research group as Xu et at. had previously identified4 a network of reactions promoted by ultraviolet light that resulted in the synthesis of two of the standard nucleosides found in RNA: uridine (U) and cytidine (C), which are collectively known as pyrimidines (Fig. 1). These reactions started from hydrogen cyanide (HCN) and derivatives thereof, simple molecules thought to have been readily available on early Earth. Further studies and development of this reaction network raised the intriguing possibility that protein and lipid precursors could have arisen simultaneously alongside nucleosides5 - thereby providing three of the main types of molecule needed to make cells. However, a...