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Arrangement of eukaryotic 18S, 5.8S and 28S rDNA in a tandem repetitive cluster; the 5S rDNA is coded separately. NTS, nontranscribed spacer, ETS, external transcribed spacer, ITS, internal transcribed spacer.

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 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: chromosomes 13,14,15,21 and 22.

The low rate of polymorphism in the rDNA transcription unit allows characterization of the rDNA of each species using only a few specimens, and makes this DNA useful for interspecific comparisons. In addition, the different coding regions of the rDNA repeats usually show distinct evolution rates. As a result, this DNA can provide information about almost any systematic level.[1]

See also


  1. ^ Hillis DM and Dixon MT (1991) Ribosomal DNA: molecular evolution and phylogenetic inference. Quart Rev Biol 66:411-453.


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