Enhanced rRNA transport from nucleoli into cytoplasm at G1/S, S/G2, and G2/M transitory points of the cell cycle in root meristem of Helianthus annuus L.

Results obtained by use of cytophotometric, autoradiographic and electron-microscopic methods indicate that periodic changes occur during the course of the cell cycle in synthesis, accumulation and transport of rRNA. At the final periods of G1, S, and G2 phases an increased transport of rRNA into cytoplasm takes place; owing to an abrupt loss of nucleolar contents a transparent zone ("vacuole") appears and the majority of nucleoli becomes visible as ring-shaped. Due to an intensive synthesis and accumulation of rRNA some ring nucleoli transform into compact nucleoli during middle S and middle G2. Compact nucleoli which coexist with ring nucleoli also undergo periodic changes; they consist in the decrease in content and concentration of RNA by the final periods of G1, S, and prophase, as well as the reverse process, i.e. an increase in both content and concentration of RNA at early stages of S and G2.