REACHING FOR THE RING - THE STUDY OF MITOCHONDRIAL GENOME STRUCTURE

The linear molecules that comprise most of the mitochondrial DNA (mtDN A) isolated from most organisms result from the artifactual degradatio n of circular genomes that exist within mitochondria. This view has be en adopted by most investigators and is based on DNA fragment mapping data as well as analogy to the genome-sized circular mtDNA molecules o btained in high yield from animals. The alternative view that linear m olecules actually represent the major form of DNA within mitochondria is supported by two observations: (1) over a 1000-fold range of genome size among fungi and plants we find the same size distribution of lin ear mtDNA molecules, and (2) linear mtDNA molecules much larger than g enome size can be found for some fungi and plants. The circles that re present only a small fraction of the mtDNA obtained from most eukaryot es could be optional sequence forms unimportant for mitochondrial func tion; they may also participate in mtDNA replication. The circles migh t result from incidental recombination events between directly repeate d sequences within or between tandemly arrayed genome units on linear mtDNA molecules.