Diagram of the Biological Functions of Mitochondrial Dynamics. A) The mitochondrial life cycle starts with growth and division of pre-existing organelles (biogenesis) and ends with degradation of impaired or surplus organelles by mitophagy (turnover). In between, mitochondria undergo frequent cycles of fusion and fission that allow the cell to generate multiple heterogeneous mitochondria or interconnected mitochondrial networks, depending on the physiological conditions. B) Fusion and fission of mitochondria are important for many biological functions. Division is required for inheritance and partitioning of organelles during cell division, for the release of pro-apoptotic factors from the intermembrane space, for intracellular distribution by cytoskeleton-mediated transport and for turnover of damaged organelles by mitophagy. Fused mitochondrial networks are important for the dissipation of metabolic energy through transmission of membrane potential along mitochondrial filaments and for the complementation of mitochondrial DNA (mtDNA) gene products in heteroplasmic cells to counteract decline of respiratory functions in ageing (X and Y depict alleles of different mitochondrial genes).