The Dynamic Relationship Between Nucleolar Structure And RDNA Replication In Allium Cepa Cells

Nucleolus is the domain in which transcription and processing of rRNA and assembly of ribosomes take place in eukaryotes, its ultrastructure is composed of three basic subcompartments: fibrillar centers (FC), dense fibrillar component (DFC) and granular component (GC). In the cell cycle, the nucleolus is a highly dynamic structure, it is assembled at the end of mitosis and disassembled in prophase, with its structure and functions change largely. In Allium cepa , by using conventional electron microscopy and electron microscopy autoradiography, we investigated the morphological change of the nucleolar structure and the problem of rDNA replication in the cell cycle.The results are as follows:1. The nucleoli began to emerge at the early stage of G1 phase, there were several small nucleoli. In S and G2 phase, the cell had bigger but fewer nucleoli. In prophase, the nucleoli were smaller, and the shape of them changed, then the nucleoli disappeared.2. In S phase, the nucleolar rim showed more linking to the peripheral chromatins.3. In different phase, there was a dynamic process from condensed chromatin to decondensed chromatin in FCs. In G1 phase, the chromatin clumps were big and the chromatin condensed highly with high electronic density. In S phase, the chromatin clumps changed small, the chromatin were looser and less condensed than G1 phase. In G2 phase, the chromatin clumps were smaller with low electronic density, and the chromatin tend to decondensed. Then in prophase, the chromatin in FCs were decondensed.4. The structure of FCs and DFCs had a dynamic process in the cell cycle. In G1 phase, DFCs were small but not clear, and few big DFCs in them. In S phase, DFCs were big with high electronic density, and showed regionalized distribution. Some DFCs had FCs, while some big DFCs had not FCs in them, and several small FCs appeared. In G2 phase, DFCs were bigger with higher electronic density, and DFCs showed regionalized and centralized distribution obviously. There were many FCs in each DFCs, and more small FCs in them. In prophase, DFCs changed small and vague, and were not regionalized distribution, the number of FCs reduced, then FCs and DFCs disappeared.5. The rDNA replication mostly took place in the granular component and nucleolar associated chromatin of the border region of nucleoli. In the late-G1 phase, rDNA replication began. S phase was the chief stage in replication, and the replication of rDNA changed more active. Then in G2 phase, the rDNA replication did not stop at once, and the rDNA replication lasted until G2 phase.