Studies on Early Intermediates in the Synthesis of DNA in Animal Cells

Discontinuous synthesis of DNA initially demonstrated by Okazaki et al. (1969) for Escherichia coli and coliphages, occurs by synthesis of short segments (Okazaki fragments) 1000–2000 nucleotides in length before joining to form high-molecular-weight DNA. In eukaryotes, the discontinuous mode of synthesis appears established for papovaviruses SV40 and polyoma (Fareed and Salzman 1972; Magnusson et al. 1973; Francke and Hunter 1974), and in several mammalian cell lines (Nuzzo et al. 1970; Huberman and Horwitz 1974; Tseng and Goulian 1975a). The mammalian nascent fragments are considerably smaller than the fragment observed for E. coli, and they have been identified both in vivo and in vitro as the rapidly labeled fraction of DNA that sediments in alkaline sucrose gradients around 4S–5S or 100–200 nucleotides in length by gel electrophoresis.