Grain distribution in autoradiograms of nuclei of synchronized HeLa cells continuously labelled with 3H-thymidine

Changes in distribution of silver grains were evaluated in autoradiograms of nuclei of synchronized HeLa cells that were continuously labelled with 3H-TdR. Synchronization was achieved by repeated blocking with cold thymidine. Three patterns of grain distribution were distinguished: uniform, non-uniform, and a special case of non-uniform distribution, the S.C. clustering. The kinetics of changes in the proportion of individual types of grain distribution indicate that uniform and non-uniform distribution patterns result from 3H-TdR incorporation into DNA replicating during the entire S-phase, whereas DNA responsible for clustered grain distribution takes up 3H-TdR only during the second half of the S-phase. The rate of the increase in grain counts per nucleus of labelled cell and the kinetics of the exhaustion of radioactive precursor from the culture medium suggest that replication of DNA in DNA in the S-phase is not merely a linear function of time.