Organization of rDNA gene NOR clusters
comprise tandem arrays
of several hundred ribosomal
(rDNA) genes that code for the
major rRNA elements.
These elements are transcribed but not translated, and serve
structural elements of the ribosomes.
The rDNA clusters
are separated by non-transcribed
spacers (NTSs): post-transcriptional processing
removes the NTS elements.
are distinctive cytological features of the nucleus,
as darkly staining heterochromatic
As shown by the
frog and fly
NOR regions above,
highly conserved between vertebrate and invertebrate genomes.
Figure after © 2000
Klug & Cummings; text © 2011 by Steven M. Carr