Practice question

Nucleolus is non-membrane-bound condensate within nucleus formed by liquid-liquid phase separation around nucleolar organizer regions on chromosomes 13,14,15,21,22 containing tandem arrays of 45S rRNA genes 400 copies in humans. RNA polymerase I, with transcription factors UBF and SL1, transcribes 47S pre-rRNA precursor containing 18S, 5.8S, 28S sequences separated by internal and external transcribed spacers. Co-transcriptional processing involves snoRNPs C/D box guiding 2'-O-methylation and H/ACA box guiding pseudouridylation, endonucleolytic cleavages generating mature rRNAs. 5S rRNA transcribed by Pol III in nucleoplasm imports. Ribosomal proteins, 33 for 60S and 21 for 40S plus assembly factors, imported from cytoplasm, combine hierarchically to pre-60S and pre-40S particles exported via CRM1 and RanGTP. Tripartite morphology fibrillar center containing Pol I, dense fibrillar component where processing occurs, granular component for assembly reflects this vectorial flow. Nucleolar stress with impaired rRNA synthesis stabilizes p53 via MDM2 sequestration. Rough ER, Golgi, mitochondria perform translation, glycosylation, and respiration, not ribosomal RNA synthesis.