Eukaryotic cells contain a plurality of organelles distinguished by their specific membranes and contents. Their biogenesis occurs by growth and division of preexisting structures rather than de novo. Mitochondria and chloroplasts, which appear to be descended from prokaryotic ancestors, have retained some DNA and the biosynthetic capability for its expression. They synthesize, however, only a few of their proteins themselves. Most of their proteins are synthesized on free ribosomes in the cytoplasm and are only assembled in the correct membrane after synthesis is complete. The biogenesis of...