It would also take adding entire metabolic pathways so that the cells would produce the different PNAs. DNA and RNA differ by a single change of a hydrogen and a hydroxyl group at the #2 carbon of the ribose sugar and that small change is enough to specify the ability of enzymes to bind to the slightly different macromolecules. So, another complication is that any protein that has DNA-binding capability would not be able to recognize the PNA.