Any RNA molecule that can catalyse changes to its own molecular structure. Self-splicing introns, found in some ribosomal RNA and protein-coding genes in organelle genomes, are examples of ribozymes (see RNA processing). RNA in the large subunit of ribosomes also has ribozyme activity, in catalysing the formation of a peptide bond between the incoming amino acid and the end of the growing polypeptide chain—the peptidyltransferase reaction (see translation). The spliceosome, responsible for removing introns and splicing exons during RNA processing, contains small nuclear RNA molecules that catalyse the splicing reactions. Ribozymes have properties very similar to viroids and virusoids. They can also copy themselves to some extent, and it is proposed that the earliest life forms relied on RNA both to store genetic information and to catalyse chemical reactions—the so-called ‘RNA world’ hypothesis.