Methyl-RNA: an evolutionary bridge between RNA and DNA?

Given the apparent limitation of double-stranded RNA (dsRNA) genomes to abo ut 30 kb, together with the complexity of DNA synthesis, it appears difficu lt for a dsRNA genome to encode all the information required before the tra nsition from an RNA to a DNA genome. Ribonucleotide reductase itself, which synthesises deoxyribonucleotides from ribonucleotides, requires complex pr otein radical chemistry, and RNA world genomes may have reached their limit s of coding capacity well before such complex enzymes had evolved. The tran sition from RNA to DNA thus appears to require intermediate steps, and we s uggest that the naturally occurring 2'-O-methylated RNA, with chemical prop erties intermediate between RNA and DNA, is a suitable candidate.