Mitochondrial DNA: I. Intramitochondrial distribution and structural relations of single- and double-length circular DNA

Abstract

The structure of the DNA of L-cell mitochondria was studied as it may relate to the organelle in vivo. Serial sections of mitochondria revealed two to three DNA-containing regions (nucleoids) per organelle; up to six regions were occasionally observed. Osmotic rupture of freshly isolated mitochondria released 2 to 4% circular DNA dimers (10.1μ, mol. wt 20 × 10 6) in a population of circular monomers (5.2 μ, mol. wt 10 × 10 6). The molecules were unbranched, usually folded in half, and either loosely coiled or extended. Most dimers were single circles, either lying free or attached at a polar region to another dimer or two monomers. It appears that each nucleoid may contain one to several monomers and dimers, to yield an average total of two to six molecules per mitochondrion. About 80% of the monomers were associated at polar regions with the mitochondrial membranes; experiments were performed that reduced the possibility of artifact formation. Osmotically shocked mitochondria of ascites tumor cells and adult rat and chicken liver were also compared. The observed membrane associations and the multiplicity of DNA molecules per mitochondrion or per nucleoid may be related to mitochondrial duplication; the high degree of variation in number of molecules per mitochondrion implies redundancy of informational content.