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.