Ribosomal DNA (rDNA)
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rDNA is a set of tandemly-repeated genes coding for preribosomal RNA. Because these genes have the ability to initiate the formation of nucleoli during interphase, these segments of the chromosomes are called nucleolus organizer regions or NORs. In the human genome, there are tandem repeats of the rDNA sequence on the short arms of each of the two copies of five different chromosomes. Ribosomal DNA (rDNA) are sequences encoding ribosomal RNA. These sequences regulate amplification and transcription initiation and contain transcribed and nontranscribed spacer segments. The nucleolus consists of expanded chromosomal loops that carry transcriptional units encoding ribosomal RNA These regions are called r-DNA. Remember that the ribosomal RNA transcriptional units are clustered in tandem repeats. These r-DNA 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(13,14,15,21,22).
In some contexts, rDNA stands for recombinant DNA.
Prokaryotic rDNA 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.
