Rate of nucleologenesis as a measure of gene activity

The nucleolus is not a permanent organelle in the life of a cell; at the initiation of a new cycle, the nucleolar formation seems to depend on RNA synthesis directed by nucleolar RNA polymerase1. We have compared the rate of nucleologenesis in meristem cells of Allium cepa L. and in those of four Vicia species with different numbers of ribosomal cistrons and different DNA contents3,4 as well as the rate of nucleologenesis in four different cell populations lying side by side in the root of Zea mays. The rate of nucleologenesis per ribosomal cistron seems to be constant in the different species growing in similar conditions, but this mean rate was modified in metabolically different subpopulations in the same root