Since the elucidation of the structure of DNA it has been of central interest to determine those events that permit faithful duplication of the genetic material of a cell. Investigators have approached this fundamental problem from two main vantage points. One has been to characterize DNA molecules in various states of replication and thereby describe the mode of the replication cycle. The other has been to isolate and purify the enzymatic and other components involved in DNA replication, in efforts to define the biosynthetic reactions of initiation and elongation of new DNA daughter strands. The latter efforts have been particularly successful in the study of procaryotic viral systems, in which the investigator can employ a combined biochemical and genetic analysis. Although studies of mitochondrial DNA (mtDNA) biosynthesis in animal cells have relied solely on a biochemical approach, several features unique to this organelle system have permitted an extensive characterization of the major aspects of the replication process.

Structural Properties of Animal Mitochondrial DNA Even though the mtDNA of animal cells generally represents< I% of the total cellular DNA, it is conveniently isolated as a closed circular DNA species. Mitochondrial DNAs from different sources exhibit similar structural properties, and the entire nucleotide sequences of the human (16,569 bp) and mouse (16,295 bp) mtDNAs are known (Anderson et al., 1981; Bibb et al., 1981). In addition to the basic monomeric genome, all animal cells examined to date maintain a significant proportion of their mtDNA in the form of catenated circles, in which monomer units are joined as links in a chain. In the case of malignant human cells and certain tissue-culture lines, yet another topological variation is observed, in which two monomer units exist as a simple double-sized circle in a head-to-tail configuration (for a detailed description of the occurrence and properties of these oligomeric species, see Clayton and Smith, 1975). It has been known for some time that animal mtDNA is unusually susceptible to strand cleavage at high pH. Because animal mtDNA is also sensitive to strand breakage by RNAase, it has been assumed that this phenomenon results from the presence of ribonucleotides in closed circular mtDNA. Direct evidence for the presence of ribonucleotides in mouse L-cell mtDNA has been obtained; the major sites of ribosubstitution are at the two replication-origin regions (Bogenhagen