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Fig. 12. A model of the nucleolar ultrastructure of Allium sativum cells. rDNA is arranged in the DFC surrounding the FC, occupying about one-third of the total DFC, and the synthesis of rRNA is carried out in this region. The rDNA in the interior of the FC occurs in a condensed state and is possibly the anchoring site for the arrangement of rDNA and has no transcription activity. The distribution of rDNA is not uniform in the DFC. The splicing and processing of rRNA is carried in the other two-thirds of the DFC region, far away from FC and is transferred to the GC for the maturation processes of the ribosomal subunits. The white circles in the DFC represent splicing factors. FC, fibrillar center; DFC, Dense fibrillar component; GC, granular component; TChr, transcribing rRNA gene.





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