Chapter 5: Plastid & Mitochondrial Biogenesis

Plastid & Mitochondrial Biogenesis begins by examining organelle replication, detailing how these structures arise from preexisting organelles through fission, a process regulated by environmental signals and developmental needs. The text highlights pivotal autoradiographic experiments, such as those conducted on Neurospora, which demonstrated that mitochondrial membrane lipids are distributed evenly during division. A major focus is placed on the non-Mendelian, maternal inheritance patterns observed in these organelles, illustrated by variegation in plants and metabolic mutants in fungi, which result from the exclusion of male cytoplasm during fertilization. The chapter further explores the intricate cooperation between the nucleus and organelles in protein synthesis, distinguishing between proteins synthesized within the organelle and those imported from the cytoplasm. Critical experimental methods using specific inhibitors like chloramphenicol and cycloheximide are described to differentiate the sites of protein synthesis. The narrative then shifts to the complex mechanisms of protein targeting and import, elucidating the role of N-terminal signal sequences, cytosolic chaperones that maintain precursor proteins in an unfolded state, and specific receptors on organelle membranes. It details the energy requirements for translocation, including ATP hydrolysis and membrane potential, and describes the routing pathways that direct proteins to specific compartments such as the mitochondrial matrix, intermembrane space, or chloroplast thylakoids. The unique molecular genetics of organelles are also covered, including the circular nature of their DNA, variations in the genetic code—such as the mitochondrial reading of the UGA codon as tryptophan—and the diverse organization of mitochondrial genomes across species. Finally, the chapter concludes with the Serial Endosymbiosis Theory, providing compelling evidence from sequence homologies and extant symbiotic relationships to argue that mitochondria and plastids originated from ancient prokaryotic endosymbionts that established a permanent residence within ancestral eukaryotic cells.