ALT Spatial organization of mitochondrial nucleoids. (A) Human mitochondrial DNA (mtDNA) is a covalently closed, circular DNA molecule composed of heavy (outer) and light (inner) DNA strands. Each strand has its own origin of replication, referred to as OH and OL, respectively. mtDNA contains 16,569 nucleotides and encodes 37 genes, including two rRNAs and 22 tRNAs (designated by letters). These genes are vital for protein synthesis within mitochondria. (B) Left, live-cell microscopy image of MCF7 breast cancer cells stained with Picogreen (DNA, green) and MitoTracker red (mitochondria, red) taken by S.B. of our laboratory. mtDNA nucleoids appear yellow. Right, a schematic diagram shows mtDNA wrapped in nucleoid-associated proteins, which form core and peripheral complexes within the nucleoid. These protein complexes are essential for the replication, transcription, translation and maintenance of mtDNA within mitochondria.