Practice question

Ribosome biogenesis requires coordinated synthesis of forty seven S pre-ribosomal RNA containing eighteen S, five point eight S and twenty eight S sequences separated by internal transcribed spacers. This transcription occurs in nucleolus, a nonmembranous nuclear condensate nucleated by tandem repeats of ribosomal DNA located on nucleolar organizer regions of acrocentric chromosomes. RNA polymerase I machinery with upstream binding factor and selectivity factor SL1 concentrates in fibrillar centers, nascent transcripts enter dense fibrillar component where snoRNP-guided cleavages, two prime O methylation and pseudouridylation occur guided by fibrillarin and dyskerin. Granular component then assembles pre-forty S and pre-sixty S particles with imported ribosomal proteins. Pre subunits export via CRM1 for final maturation. Endoplasmic reticulum and Golgi modify secretory proteins, peroxisomes detoxify lipids, none produce rRNA. Actinomycin D at low doses selectively inhibits Pol I disrupting nucleoli, proving synthetic role. Consequently nucleolar hypertrophy correlates with growth rate and is upregulated in transformed cells requiring high protein synthesis.