Ribosomal DNA (rDNA)

Ribosomal DNA is a highly repetitive genomic region that encodes ribosomal RNA genes, which are essential structural components of ribosomes. In human cells, rDNA consists of hundreds of tandem repeats clustered on the short arms of five acrocentric chromosomes (13, 14, 15, 21, and 22), forming nucleolar organizer regions. Each rDNA unit contains coding regions for ribosomal RNA (18S, 5.8S, and 28S in eukaryotes) and noncoding intergenic spacer regions. The rDNA is transcribed by RNA polymerase I in the nucleolus and represents the most abundant RNA in cells. It is characterized by a unique chromatin structure, with actively transcribed regions being largely depleted of nucleosomes due to dense RNA polymerase I occupancy. rDNA is intrinsically unstable and represents one of the most frequently rearranged chromosomal regions, undergoing continuous cycles of contraction and expansion through recombination mechanisms. The regulation of rDNA involves homogenization through gene conversion to maintain sequence quality, as well as mechanisms that control the number of transcriptionally active copies. DNA methylation patterns distinguish active (unmethylated) from inactive (methylated) rDNA copies, with cells maintaining stable active copy numbers despite variations in total rDNA content. Defects in DNA repair, as seen in Bloom syndrome, lead to hyper-instability of rDNA, contributing to genome-wide instability and cancer predisposition.

Other names

Ribosomal DNA arrayrRNA genesribosomal RNA genesrDNA clusterrDNA repeatsnucleolar organizer regions (NORs)

Not applicable - rDNA itself is not targeted by therapeutic interventions