| Abstract
| - Localization of intranucleolar DNA under hypoxic conditions was examined in excised root tips of Allium cepa. Application of a silver impregnation technique to serial ultrathin sections indicated that when excised root tips were subjected to hypoxia for at least 1 h, the nucleolus segregated into two regions and developed achromatic channels meandering within the DFC core. The two regions were the central core of the dense fibrillar component (DFC) and the periphery of the granular component (GC). Electron microscopy showed that the occurrence of achromatic channels was caused by the development and/or fusion of the fibrillar centres (FC5), which are believed to contain the temporarily resting ribosomal-RNA genes (rDNA). The anti-DNA antibody label was significantly stronger on the DFC than on the cytosol. Interestingly, the specific label (gold particles per μm2 of the FCs and their surrounding region in the specimens exposed to hypoxic conditions was almost the same as in those grown under normal conditions. Concurrently with the development of the FCs under hypoxic conditions, granules about 35 nm in diameter (35 nm granules) were formed in the FCs. They were not labelled with anti-DNA antibodies or RNase-gold complexes, indicating that they contain neither DNA nor RNA. Altogether, the DFC containing the intranucleolar DNA probably engaged in rDNA transcription may gradually collapse to turn into the FCs and simultaneously leave the inactivated rDNA as clusters of chromatin fragments and some nucleolar proteins as the 35 nm granules, when the cells are exposed to hypoxic conditions.
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