Figure 1.

The location of NORs and the nucleolar cycle in human cells. (A) Schematic showing a human rDNA array expanded to show the pre-rRNA-coding sequences that are transcribed by RNA Pol I. The positions of mature rRNA-coding sequences, ETSs, and ITSs are indicated. (B) The locations of NORs on the acrocentric chromosome are indicated. The short arms, circled in red, are missing from the current genome draft GRCh38.p7. (C) During cell division, transcription ceases, and nucleoli disappear. NORs can be observed as achromatic gaps on DAPI-stained metaphase chromosomes due to undercondensation of rDNA (red dotted line). Silent NORs (solid red) fail to show this morphology and do not contribute to nucleolar formation. Transcription resumes in anaphase, and nucleoli form around individual active NORs. In most cell types, these then fuse, producing characteristic large nucleoli surrounded by heterochromatin.