Experimental Analysis of Nucleolar Reorganization

The transition from telophase to the G I period of interphase includes the process of nucleolar reorganization in each of the daughter nuclei, and it has been demonstrated by the classic studies of Heitz (13) and McClintock (17) that only specific regions of particular chromosomes in each karyotype are involved in the formation of new nucleoli . On the other hand, a number of studies revealed the appearance in telophase of material among the chromosomes whose role in the reorganization of the nucleolus is controversial (27, 5, 20) . Biochemical studies carried out on cells in transition from telophase to GI have shown that protein and RNA syntheses, which are only slight in the middle stages of mitosis, are rapidly intensified towards the end of telophase, precisely during the period of nucleolar reorganization (2-4, 21) . In line with this finding, it is thought that mitosis plays a role in realigning the transcriptional pattern of the cell cycle . It was therefore considered that it would be of interest to study nucleolar reorganization under inhibition of either protein or RNA synthesis in order to determine the processes on which nucleologenesis depends . This was accomplished in Allium, where the whole set of ribosomal cistrons are clustered in the nucleolar organizer region (NOR) of one pair of homologous chromosomes, and these give rise to just one pair of nucleoli in the interphasic nuclei . Recent reports on nucleologenesis (25, 19) were done on different cells where this pattern of nucleolar formation is not followed at all. Unfortu-