Pattern of DNA synthesis in the meristematic cells of sinapis

Summary High-resolution autoradiographs were made of ultrathin sections in the shoot apex and the very young leaves of Sinapis alba fed with tritiated thymidine for 4 hours. Three types of labeled nuclei were found. (1) Those labeled in both the dispersed and the condensed chromatin, (2) those labeled only in the dispersed chromatin, and (3) those labeled only in the condensed chromatin.A distinct cytoplasmic labeling was found. Proplastids and mitochondria were the only significantly labeled entities in the cytoplasm. DNA synthesis in these organelles seems to be synchronized with DNA synthesis in the nucleus.This work was carried out at the Department of Botany, University of California, Berkeley, during the tenure of a fellowship from I.R.S.I.A. (Institut pour l'Encouragement de la Recherche Scientifique dans l'Industrie et l'Agriculture), Belgium.