Nucleo-Cytoplasmic Exchange

ID #2423

What exactly is going on in each zone of the nucleolus? I cannot get this straight in my head just from my lecture notes. Does transcription of the rDNA occur in the fibrillar region, processing in the dense fibrillar region, and assembly of the subunits in the granular zone? Thanks!

The fibrillar center consists of the DNA for the ribosomal RNA genes. 
The dense fibrillar region is where most of the work is occurring; transcription, binding of transcription factors, and processing of the 45s rRNA all occur here. 
The granular zone is where the preribosomal particles (composed of the ribosomal proteins and the processed rRNA) are found. They are being prepared for export through the NPC into the cytoplasm. 

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