Initiation and continuation of DNA replication are not associated with the nuclear envelope in mammalian cells

For determination of whether DNA replication is initiated at the nuclear envelope, synchronized Chinese hamster ovary cells labeled with [(3)H]thymidine were examined by electron microscope radioautography. The cells were synchronized initially by mitotic shake-off and held at the G(1)-S border by 5-fluorodeoxyuridine plus amethopterin. Cells were fixed at 1, 5, 10, and 30 min after the inhibitors were counteracted with [(3)H]thymidine. Radioautographic silver grains in each case were present over the more central parts of nuclei and were generally absent from the region of the nuclear envelope. We conclude that neither initiation nor continuation of DNA replication is associated with the nuclear envelope.