Practice question

rDNA is organized as tandem head-to-tail repeats. Between transcription units lies Non-Transcribed Spacer, NTS, historically called intergenic spacer. NTS is not transcribed by RNA Pol I and contains regulatory elements such as promoters, enhancers, terminators, replication origins and chromatin boundary sites controlling rDNA silencing and copy number stability. It contrasts with ETS, external transcribed spacer present at 5' and 3' ends of 45S transcript, and ITS, internal transcribed spacer between mature rRNAs, both initially transcribed then removed during processing. This refined regulation supports accurate ribosomal assembly, quality control and translational fidelity under diverse physiological conditions and growth states.