Organization of rDNA gene NOR clusters
organizer regions (NORs) comprise tandem arrays of several
hundred ribosomal DNA (rDNA) genes that code for the
major rRNA elements.
These elements are transcribed as rRNA, which is of
course not translated, and serve as structural components of
the ribosomes. The rDNA clusters are
separated by non-transcribed
spacers (NTSs): post-transcriptional processing
removes the NTS elements.
NORs are distinctive
cytological features of the nucleus, and appear as darkly
staining heterochromatic regions.
As shown by the
comparison of frog and fly NOR
regions above, the basic arrangement is highly
conserved across evolution between invertebrate and vertebrate
Figure after © 2000
Klug & Cummings; text © 2014 by Steven M. Carr