The classically understood flow of information in present day organisms flows from DNA to RNA to protein. However, RNA can both store heritable information and carry out biochemical activities, and is thus capable of the functions principally served by DNA and protein. The RNA world hypothesis proposes that life based on RNA predated the current tripartite arrangement. However, the RNA world is not exclusively confined to the distant past. Recent studies of non-coding RNAs show that the roles of RNA have continued to evolve and expand alongside DNA and protein.
It’s become increasingly clear that genomes contain quite a bit of DNA which does not code for protein, but is nevertheless transcribed into RNA, yielding an abundance of RNA transcripts– and that this tendency gets more pronounced in more complex organisms. In humans, approximately 10- to 20-fold more genomic sequence is transcribed to non-coding RNA than to protein-coding RNA. One interesting non-coding RNA category is the micro-RNA (miRNAs), a class of short RNAs which can dock onto and affect other RNA transcripts. A very recent essay in Cell magazine proposes that these miRNAs form a sort of exchange currency in the cell, in that longer RNA species (including non-coding forms) regulate one another’s behavior by controlling levels of the miRNAs. Increased abundance in any “target” RNA will remove miRNAs from circulation, with consequences for other targeted RNAs.
One exciting feature of this hypothesis is the central role posited for mi-RNAs– very broadly affecting both coding and noncoding RNAs while remaining essentially intrinsic to the RNA economy. (the miRNA life cycle in present day life depends on proteins at several crucial steps, but target recognition itself is thought to be protein independent.) This is of particular interest since mi-RNAs may be relics of the RNA world. mi-RNAs have been indentified in all living realms and in viruses, and their recognition sequences are remarkably conserved, suggesting they derive from ancient common ancestors (or from co-evolving groups). Along with transfer RNAs and ribosomal RNAs, mi-RNAs may be a fundamental legacy of early terrestrial life.
Primordial Soup Image from the laboratory of James Brown, http://www.mbio.ncsu.edu/JWB/soup.html