Localization of DNA replicator sites near the nuclear membrane in plant cells

By means of optical and electron microscope autoradiography, following the administration of [3H]thymidine, we have detected a DNA replication near or on the nuclear membrane of meristematic cells of Haplopappus gracilis. This replication was observed only in the latter part of the S period. No localization to the nuclear membrane was seen at the beginning of the S period. Localization of the grains to the nuclear membrane in late S was observed both after short pulses and pulses as long as 1 h, and with chases of up to 2 h. It is concluded that the peripherally localized DNA replication is due to a DNA fraction located at the nuclear membrane, which replicates in the last part of the S period. It is suggested that such a DNA fraction may in part correspond to the constitutive heterochromatin.