Ribosomal DNA (rDNA) are sequences encoding ribosomal RNA. These sequences regulate amplification and transcription initiation and contain transcribed and nontranscribed spacer segments.

In some contexts, rDNA stands for recombinant DNA.

Eukaryotic rDNA

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The nucleolus consists of expanded chromosomal loops that carry transcriptional units encoding ribosomal RNA. These regions are called rDNA. The ribosomal RNA transcriptional units are clustered in tandem repeats. 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. DNA | Ribosomale DNA