The dense fibrillar component (DFC) of the nucleolus is involved in:
Nucleolar organization reflects sequential ribosome assembly. Fibrillar centers contain transcriptionally inactive rDNA, border to dense fibrillar component active transcription. Dense fibrillar component appears as tight fibrillar mesh containing fibrillarin methyltransferase, Nop56, Nop58 and U3 snoRNP required for early cleavages of forty seven S precursor at sites A0, A1 and A2 within external and internal transcribed spacers, plus two prime O methylation directed by box C/D snoRNAs and pseudouridylation by box H/ACA snoRNAs. This processing trims precursor into eighteen S, five point eight S, twenty eight S RNAs. Granular component then hosts pre-ribosome assembly. In situ hybridization shows pre-rRNA rapidly moves from dense fibrillar component outward. Inhibition with five-fluorouridine labels dense fibrillar component preferentially, low actinomycin D disrupts its structure blocking processing while transcription persists transiently. DNA replication occurs at many foci throughout nucleoplasm S phase, protein folding ER, chromosome condensation at mitosis via condensin, distinguishing dense fibrillar component role specifically in ribosomal RNA maturation crucial for ribosome supply.
Ref: Boisvert Annu Rev Biochem; nucleolus DFC fibrillarin early cleavage methylation pseudouridylation.