Practice question

Eukaryotic nucleolar rDNA tandem repeats are transcribed by RNA polymerase I as large 45S pre-rRNA, about 13 kb in humans, containing 5'ETS, 18S, ITS1, 5.8S, ITS2, 28S and 3'ETS. Non-transcribed spacers flank units. This single precursor ensures equimolar production of small and large subunit rRNAs. Co-transcriptional modification by H/ACA and C/D snoRNPs adds pseudouridines and 2'-O-methyl groups. Stepwise cleavage by endonucleases and exonucleases excises spacers, separating 18S for 40S and 5.8S-28S complex for 60S, while 5S is made by Pol III elsewhere.