so far studied has been shown by electron micro-scopy to comprise circular molecules with species-specific ontour lengths of 5-6/ ~ {for recent reviews see Rabinowitz and Swift, 1970; Wolstenholme et al., 1971; Borst, 1972). Circular mtDNA mole-cules, interpreted as replicative intermediates, were first isolated from normal rat liver (Kirschner et al., 1968). Each such molecule had the configur-ation of the replicating molecules demonstrated for Escherichia coli by Cairns (1963), and both of the daughter segments appeared to be totally double-stranded. In this paper, we describe a variety of replicative forms which we have since found in mtDNAs from a number of rat tissues and, based on our obser-vations, we present a general scheme for the replication of this circular molecule. Evidence obtained from a study of the lengths of daughter segments of replicative forms is also presented which indicates that replication of mtDNA is a discontinuous process. Materials and Methods Original stocks of Novikoff rat ascites hepatomas (Novikoff, 1957) and Chang rat solid hepatomas (Chang et al., 1967) were obtained from Drs.
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Wolstenholme et al. (1974) studied this question.