Timing of nucleolar activity in meristems.

The nucleoli in 4 regions of the root meristem of Zea mays have been examined and the time spent in each of the phases of the nucleolar cycle has been calculated from estimates of the rates of mitosis and the fraction of cycling cells in each region. The nucleolar cycles have been fitted to the mitotic cycles of cycling cells in the same regions. The timing of activity of the r-RNA genes differs in relation to the chromosomal cycles of condensation and movement in the different regions of the meristem. In the stelar regions disorganization of the nucleolus starts in the middle of prophase and reorganization starts towards the end of telophase and is completed in the following interphase. In the quiescent centre of the meristem the phases of both cycles are lengthened except for telophase and reorganization of the nucleolus docs not start until after the end of telophase. The acceleration of telophase in the quiescent centre is discussed in relation to inhibition of protein synthesis. In the adjacent cap initials, which have the shortest mitotic cycle, the nucleolus starts to disorganize before the onset of prophase and reorganization is completed halfway through telophase. This fits with the fact that DNA synthesis is advanced into the telophase of the previous cycle in these cells.