Practice question

Eukaryotic cells separate transcription spaces using distinct RNA polymerases. Precursor rRNAs eighteen S, five point eight S and twenty eight S originate from single forty seven S transcription unit driven by RNA polymerase I complex concentrated in nucleolus, organized around nucleolar organizer regions containing hundreds tandem rDNA repeats on acrocentric chromosomes. Accessory factors UBF and SL1 recruit Pol I to promoters within fibrillar centers, nascent transcripts emerge into dense fibrillar component where U3 snoRNP performs early cleavages and fibrillarin mediates methylation, dyskerin pseudouridylation guided by snoRNAs. Five S rRNA transcribed by Polymerase III in nucleoplasm later imports. Rough ER does not transcribe, mitochondria transcribe its own small rRNA for mitoribosomes, Golgi processes glycoproteins. Low-dose actinomycin D selectively inhibits Pol I leading to nucleolar segregation and stress activating p53. High rate of Pol I transcription meets demand for ribosome production, explaining prominent nucleolus in growing cells and upregulation in cancer driven by Myc and mTOR pathways.