| Abstract: | The replication sites and morphological steps of chromosomal condensation during S period in the nuclei of Crepis capillaris root tip cells have been studied with light and electron microscopic autoradiography. From light microscopic autoradiographic observations, the S period can be divided with three portions, early S, mid S, and late S period. Labelled nuclei for each portion of the S period have also been found by using electron microscopic autoradiography. With electron microscopic autoradiography it has been found that in early, mid, and late S period, the replication sites are distributed in the electron transparent regions, interspersed with dense chromatin masses of variable size which are distributed throughout the nucleus. The time-dependent behavior of the label indicates that when compared with either mid or early replicated DNA, a majority of this chromatin, which contains predominantly late replicated DNA, is the earliest chromatin to be organized into the condensed chromatin. They are organized into the condensed chromatin within 15 min after the termination of replication. |
