Analysis of fragments of human ribosomal intergenic spacer observed in chromosomes carrying no nucleolar organizer regions

Abstract

The tandem repeats of rDNA form so-called nucleolar organizer regions (NORs). At the same time, many rDNA-like segments are observed in some (NOR)− chromosomes. The nucleolus has been shown to perform many functions in addition to ribosome biogenesis: cell-cycle regulation, stress-induced response, and transcription regulation, often triggering cell cascades. The mechanism of the occurrence of rDNA segments in the (NOR)− chromosomes has not yet been studied and needs further research. About one-third of nucleolus-associated repeats are related to SINE/Alu repeats, homogenous sequences, and tandem repeats. The relative positions of the nucleolus and the chromosomes probably facilitate/prevent interaction between the chromosomes and rDNA clusters. The variability of two large repeats in the central part of rIGS, LR1 and LR2, which are ∼90% the same and separated from each other by several hundreds of base pairs, has been studied in our previous works. This work is devoted to searching for LR1-LR2 segments in other chromosomes and characterization of their terminal regions close to rupture points and the genomic regions where LR1-LR2 segments exist.