active rDNA resides inside of the nucleolus itself.

Humans

The human genome contains a total of 560 copies of the 45S rDNA transcriptional unit, spread across five chromosomes with nucleolus organizer regions. The repeat clusters are located on the acrocentric chromosomes 13 (RNR1), 14 (RNR2), 15 (RNR3), 21 (RNR4)