Abstract
BIOCHEMICAL studies of chromosome replication have been hampered by the unavailability of an adequate in vitro system with the basic features of in vivo DNA replication. The criteria for such a system are: (1) semiconservative replication; (2) normal biological activity of newly synthesized DNA; (3) normal advancement of the original replication fork; (4) rate of DNA replication equivalent to in vivo; and (5) expected phenotypic behaviour of temperature-sensitive dna mutants. Systems in Escherichia coli, a membrane-DNA fraction1, an agar-embedded cell lysate2 and toluene-treated cells3 have met two or three of the requirements. Several laboratories have also reported the expected behaviour of ts-dna E. coli mutants in toluenized cells3-5.