Cytogenetic instability of chromosomal nucleolar organizer regions (NORs) in cloned mouse L929 fibroblasts
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Ribosomal DNA (rDNA) gene codes for 18S, 5.8S, and 28S rRNA form tandem repetitive clusters, which occupy distinct chromosomal loci called nucleolar organizer regions (NORs). The number and position of NORs on chromosomes are genetic characteristics of the species although within a cell, the NOR sizes can significantly vary due to loss or multiplication of rDNA copies. In the current study, we used mouse L929 fibroblasts, the aneuploid cells which differ in the FISH- and Ag-NOR numbers, to examine whether the parental NOR variability is inherited in clones. By statistical analysis, we showed that the cloned fibroblasts were able to restore the NOR numerical characteristics of the parental cells after long-term culturing. These results support the idea that mammalian cells may have mechanisms which control the number and activity of NORs at the population level. In L929 fibroblasts, we also regularly observed laterally asymmetry of FISH-NORs that evidenced in an unequal distribution of the mother rDNA copies between the daughter cells in mitosis.
Keywordsmouse L929 fibroblasts clones nucleolar organizer regions rDNA-FISH Ag-NOR staining variability
Nucleolar organizer region
5′ External transcribed spacer
- ITS1 and ITS2
The first and second internal transcribed spacers
Fluorescence in situ hybridization
NOR hybridized with rDNA-FISH probes
The author is grateful to Prof. Ingrid Grummt (German Cancer Research Center, Heidelberg, Germany) for a kind provision of the rDNA plasmids, and to Drs. Maria Chaplina, Iana Riabukha, and Anastasiya Moraleva (Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry RAS, Moscow, Russia) for a valuable assistance in some experiments (MC) and statistical evaluation of the selected data (IR, AM).
The author designed the study, participated in the majority of experiments, performed a statistical analysis, wrote the paper, and prepared the illustrations.
The study was supported by the Russian Foundation for Basic Research (grant 16-04-01199).
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