Cells consist of a multitude of molecular structures, some of them exhibiting a staggering complexity. Ribosomes, the protein factories of the cell, belong to the biggest and most sophisticated complexes and are made up of RNA as well as a large number of proteins. They exist in every living being and are considered as one of the cellular machines that has changed the least through all stages of the evolution. But there are exceptions: In mitochondria, cellular organelles that serve as power plants, ribosomes look considerably different.