Timings of DNA synthesis in the nucleoli of Allium cepa L

The timing of nucleolar DNA synthesis has been studied in root tip cells of Allium cepa L. As in animal cells, nucleolar chromatin in this plant species replicates in two phases during S period, once in the early and again towards the end of S phase. Chromatin present in the dense nucleolar component and fibrillar centres, irrespective of being active or inactive, replicates in the early S - phase, whereas the nucleolar associated (hetero) chromatin replicates at the end of S-phase.