The RNA world theory suggests that modern life arose from molecular ancestors in which RNA molecules both stored genetic information and catalyzed chemical reactions. This theory is largely supported by the importance of RNA molecules in essential contemporary cellular processes: RNA is involved in the decoding of genetic information, DNA replication, and chromosome-end maintenance; it mediates interference to defend cells against molecular parasites, and an increasing number of non-coding RNAs have recently been discovered that are involved in various biological processes. In addition, individual ribonucleotides and their coenzyme derivatives are highly involved in central metabolism. Moreover, the discovery of catalytic RNAs strongly reinforced the RNA world hypothesis. Seven naturally occuring classes of ribozymes have been identified yet, including the four classes of small self-cleaving RNAs. The latter are found on plant pathogenics RNA genomes, and are essentially involved in the pathogenicity by mediating the viral replication. Here, we will succinctly present the arguments in favor of the ancient RNA world. Then, we will describe the world of non-coding RNAs and natural ribozymes, and finally focus on small self-cleaving RNAs by presenting their structural and functional characteristics.
