27 Mitochondrial DNA Replication and Human Disease

Abstract

Convincing evidence for the existence of a unique human mitochondrial genome was obtained almost four decades ago. The facts that mitochondrial DNA (mtDNA) was exclusive to the mitochondrial organelle and existed as a small closed-circular DNA species led to a straightforward and powerful strategy for its isolation to a very high degree of purity. Physical biochemical methods were available to identify and analyze the nature and kinetics of formation of replicative intermediates, and the unusual distribution of guanine and thymine between the two strands of human mtDNA provided a means to separate them physically at alkaline pH in high-salt buoyant density gradients. This latter attribute was important to the assignment of replication phenomena to each of the helical strands during a round of mtDNA replication. The central conclusions from the first three decades of investigation (1967–1996) are described in the previous volume of this book (Clayton 1996).

MODE OF REPLICATION OF HUMAN mtDNA
The basic mode of mtDNA replication in mammalian cells has been studied most extensively in human and mouse cells. In both cases, replication of mammalian mtDNA has been reported to occur asymmetrically, utilizing a limited number of strand-specific unidirectional origins of replication. Leading-strand synthesis begins with RNA priming at the light (L)-strand promoter (LSP; Clayton 1996), which marks the origin of heavy (H)-strand replication (O_H), and then proceeds unidirectionally, thereby displacing the parental H strand as single-stranded DNA. In the majority of cases, leading-strand synthesis is terminated after less than one kilobase of elongation. The exact...