Ribosomal DNA (
rDNA) is a DNA sequence that codes for
ribosomal RNA.
Ribosomes are assemblies of proteins and rRNA molecules that
translate mRNA molecules to produce proteins. As shown in the figure, rDNA of eukaryotes consists of a tandem repeat of a unit segment, an
operon, composed of NTS, ETS,
18S,
ITS1,
5.8S,
ITS2, and
28S tracts. rDNA has another gene, coding for
5S rRNA, located in the genome in most eukaryotes. 5S rDNA is also present in tandem repeats as in
Drosophila. In the nucleus, the rDNA region of the chromosome is visualized as a
nucleolus which forms expanded chromosomal loops with rDNA. These rDNA regions are also called
nucleolus organizer regions, as they give rise to the nucleolus. In the
human genome there are 5 chromosomes with nucleolus organizer regions: the
acrocentric chromosomes 13,14,15,21 and 22.