Author(s): Hobart P, Duncan R, Infante AA
THERE is considerable evidence implicating the cell membrane as the site for DNA replication in prokaryotic systems1–3. Several laboratories have considered the possibility that the nuclear membrane may serve a similar role in eukaryotes. This model has been supported by biochemical studies in various eukaryotic cell systems4–8. But, autoradiographic analyses in mammalian cells, that previously indicated that DNA synthesis was initiated at the nuclear membrane9,10, now provide the strongest evidence against an involvement of the nuclear membrane in DNA synthesis11–13. The sea urchin system presents several advantages for studies of DNA synthesis. On fertilisation a population of dormant unfertilised eggs begins a series of very rapid cellular divisions, the first three of which are naturally synchronous. Biochemical studies with this system have suggested that rapid replication of DNA occurs at a nuclear membrane site and that DNA may be associated with the site only during the period of DNA synthesis14. We therefore decided to undertake an autoradiographic study in the sea urchin embryo. The results presented here strongly support the role of the nuclear membrane in the synthesis of DNA.