Abstract
In human cells, ribosomal DNA (rDNA) is arranged in ten clusters of multiple tandem repeats. Each repeat is usually described as consisting of two parts: the 13 kb long ribosomal part, containing three genes coding for 18S, 5.8S and 28S RNAs of the ribosomal particles, and the 30 kb long intergenic spacer (IGS). However, this standard scheme is, amazingly, often altered as a result of the peculiar instability of the locus, so that the sequence of each repeat and the number of the repeats in each cluster are highly variable. In the present review, we discuss the causes and types of human rDNA instability, the methods of its detection, its distribution within the locus, the ways in which it is prevented or reversed, and its biological significance. The data of the literature suggest that the variability of the rDNA is not only a potential cause of pathology, but also an important, though still poorly understood, aspect of the normal cell physiology.
Highlights
IntroductionRibosomal DNA (rDNA) is arranged in ten clusters of multiple tandem repeats
In human cells, ribosomal DNA is arranged in ten clusters of multiple tandem repeats
The variability of the locus is multiform, and it remains unknown whether certain DNA sequences cause the differentiation of the genes into active and silent, that is to say, why some of the ribosomal DNA (rDNA) repeats acquire a chromatin structure that is favorable for their transcription, while the rest remain in the silent state
Summary
Ribosomal DNA (rDNA) is arranged in ten clusters of multiple tandem repeats. These clusters are known as Nucleolus Organizing Regions (NORs) and situated on the short arms of the acrocentric chromosomes (#13, #14, #15, #21, #22). One prominent transcription unit includes a 2 kb long segment at the 3 end of IGS, and probably stretches into 5 ETS area [15,20,22,23,24] This region is transcribed by pol I and is involved in rDNA silencing [10,22,25,26,27,28,29,30,31,32]. The existing databases present only averaged assemblies of the locus processed by special software [15,56,57,58]
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