Practice question

Electron microscopy reveals nucleolus as darkly staining subnuclear domain due to high RNA protein density, tripartite with fibrillar centers containing inactive rDNA, dense fibrillar component where transcription and early processing occurs, granular component assembling pre-ribosomes. RNA polymerase I with transcription factor UBF drives high-rate synthesis of forty seven S precursor containing eighteen S, five point eight S, twenty eight S RNAs flanked by five prime external transcribed spacer and internal transcribed spacers ITS1 and ITS2. Early cleavages by U3 snoRNP at five prime ETS occur in dense fibrillar component, subsequent modifications include methylation by fibrillarin and pseudouridylation by dyskerin guided by snoRNAs. Pre-forty S and pre-sixty S assembled with imported ribosomal proteins one hundred copies transit to granular component before export. DNA replication initiates at many origins in nucleoplasm during S phase, ATP production mitochondrial oxidative phosphorylation, RNA modification widespread but concentrated for rRNA here. Actinomycin intercalates rDNA halting Pol I causing nucleolar disruption, confirming function as ribosomal RNA processing and assembly center coordinating growth with protein synthesis demand.