The RNA Metabolism of Nucleoli and Mitochondria in Mammalian Cells

Several years ago our understanding of the RNA metabolism of eucaryotic cells seemed relatively complete. The major types of RNA found initially in bacteria had been identified in higher organisms. These included ribosomal RNA, transfer RNA, and what was at that time commonly assumed to be messenger RNA. More recently, as methods of analysis have become more refined, it has become clear that the true picture of RNA metabolism is extremely complex and many species of RNA have been discovered whose function at this time are completely obscure.