scholarly journals Karyotype structure and NOR activity in Brazilian Smilax Linnaeus, 1753 species (Smilacaceae)

2019 ◽  
Vol 13 (3) ◽  
pp. 245-263
Author(s):  
Daniel Pizzaia ◽  
Vanessa M. Oliveira-Maekawa ◽  
Aline R. Martins ◽  
Mateus Mondin ◽  
Margarida L. R. Aguiar-Perecin
Keyword(s):  
Silver Staining ◽  
Nucleolus Organizer ◽  
Raw Material ◽  
Chromosome 1 ◽  
Rrna Gene ◽  
Dioecious Species ◽  
Chromosomal Structure ◽  
Nucleolus Organizer Regions ◽  
Karyotype Structure ◽  
Secondary Constrictions

The genus Smilax Linnaeus, 1753 (Smilacaceae) is a large genus of dioecious plants distributed in tropical, subtropical and temperate regions. Some Smilax species have medicinal importance and their identification is important for the control of raw material used in the manufacture of phytotherapeutical products. The karyotypes of seven Brazilian Smilax species were investigated. Mitotic metaphases of roots from young plants were analysed in Feulgen-stained preparations. The karyotypes were asymmetric and modal with 2n = 2x = 32 chromosomes gradually decreasing in size. In S. goyazana A De Candolle & C De Candolle, 1878, a polyploid species, 2n = 4x = 64. In all the species, the large and medium-sized chromosomes were subtelocentric and submetacentric and the small chromosomes were submetacentric or metacentric. Their karyotypes were quite similar, with differences in the arm ratio of some chromosomes. S. fluminensis Steudel, 1841 differed from the other species by having a large metacentric chromosome 1. These findings suggest that evolution occurred without drastic changes in the chromosomal structure in the species analyzed. Terminal secondary constrictions were visualized on the short arm of some chromosomes, but they were detected only in one homologue of each pair. Due to the terminal location and the degree of chromosome condensation, secondary constrictions were not visualized in some species. The nucleolus organizer regions (NORs) were mapped by silver-staining and fluorescent in situ hybridization (FISH) in S. rufescens Grisebach, 1842 and S. fluminensis. Silver-staining and FISH signals were colocalized on the short arms of six chromosomes in S. rufescens and four chromosomes in S. fluminensis. In FISH preparations, one of the largest chromosomes had the secondary constrictions highly decondensed in some cells. This finding and the heteromorphism observed in Feulgen-stained chromosomes suggest that differential rRNA gene expression between homologous rDNA loci can occur in some cells, resulting in different degrees of ribosomal chromatin decondensation. The presence of a heteromorphic chromosome pair in S. rufescens, S. polyantha Grisebach, 1842 and S. goyazana suggests a chromosomal sex determination in these dioecious species.

The nucleolus

2019 ◽  
pp. 147-152
Author(s):  
John C. Lucchesi
Keyword(s):  
Gene Silencing ◽  
Histone H1 ◽  
Nucleolus Organizer ◽  
Linker Histone ◽  
The Other ◽  
Rrna Genes ◽  
Rrna Gene ◽  
Dna Repeats ◽  
Gene Arrays ◽  
Nucleolus Organizer Regions

The nucleolus forms at nucleolus organizer regions (NORs) that consist of clusters of repeated rRNA genes. Transcription of the rRNA genes and processing of the transcripts yields the three types of RNAs necessary for the biogenesis of ribosomes. Only subsets of the rRNA genes present in cells are transcribed. The linker histone H1 plays a specific role in the repression of inactive rRNA genes and in many of the other functions of the nucleolus. One of these functions is gene silencing—the nucleolus is surrounded by a zone of heterochromatin consisting of silenced rRNA gene arrays, DNA repeats that flank the centromeres and chromatin domains that include gene-poor, as well as silent, regions of the genome; any gene associating with this zone is subjected to repression. Other functions include the assembly of telomerase, the regulation of p53 stability and the synthesis of 5S and tRNAs whose genes form clusters in the nucleolus.

scholarly journals Silver staining of the nucleolus organizer regions (NOR) requires clusters of sulfhydryl groups.

1982 ◽  
Vol 30 (9) ◽  
pp. 908-911 ◽  
Author(s):  
A De Capoa ◽  
M Ferraro ◽  
P Lavia ◽  
F Pelliccia ◽  
A Finazzi-Agrò
Keyword(s):  
Heavy Metals ◽  
Silver Staining ◽  
Reducing Agents ◽  
Nucleolus Organizer ◽  
The Other ◽  
Nuclear Proteins ◽  
Staining Reaction ◽  
Nucleolus Organizer Regions ◽  
High Affinity ◽  
Per Se

Silver stainability of nucleolus organizer regions (NORs) appears to be correlated with the presence of grouped sulfhydryl (SH) side chains of proteins. In fact, heavy metals with high affinity for SH groups, such as Hg and Cu, do prevent the silver staining reaction. Ferricyanide, which is known to oxidize SH to disulfides, also prevents any further reaction with silver. On the other hand, alkali and reducing agents (mercaptoethanol, cyanide) do not affect silver stainability of the NORs. These results show that the silver staining reaction is not related to disulfide or persulfide groups and that alkali-soluble, acidic nuclear proteins per se do not play a major role in this process.

Secondary constrictions and nucleolus organizer regions in man

1977 ◽  
Vol 104 (2) ◽  
pp. 428-430 ◽  
Author(s):  
M. Ferraro ◽  
N. Archidiacono ◽  
F. Pelliccia ◽  
M. Rocchi ◽  
A. Rocchi ◽  
...  
Keyword(s):  
Nucleolus Organizer ◽  
Nucleolus Organizer Regions ◽  
Secondary Constrictions

scholarly journals Selective nucleolus organizer inactivation in Arabidopsis is a chromosome position-effect phenomenon

2016 ◽  
Vol 113 (47) ◽  
pp. 13426-13431 ◽  
Author(s):  
Gireesha Mohannath ◽  
Frederic Pontvianne ◽  
Craig S. Pikaard
Keyword(s):  
Position Effect ◽  
Nucleolus Organizer ◽  
Rrna Genes ◽  
Rrna Gene ◽  
Chromosome 2 ◽  
Chromosome 4 ◽  
Base Pairs ◽  
Genomic Change ◽  
Nucleolus Organizer Regions ◽  
Chromosome Position

Nucleolus organizer regions (NORs) are chromosomal loci where hundreds of rRNA genes are clustered. Despite being nearly identical in sequence, specific rRNA genes are selected for silencing during development via choice mechanism(s) that remain unclear. In Arabidopsis thaliana, rRNA gene subtypes that are silenced during development were recently mapped to the NOR on chromosome 2, NOR2, whereas active rRNA genes map to NOR4, on chromosome 4. In a mutant line deficient for ATXR5 or ATXR6-dependent histone H3 lysine 27 (H3K27) monomethylation, we show that millions of base pairs of chromosome 4, including the telomere, TEL4N, and much of NOR4, have been converted to the corresponding sequences of chromosome 2. This genomic change places rRNA genes of NOR2, which are normally silenced, at the position on chromosome 4 where active rRNA genes are normally located. At their new location, NOR2-derived rRNA genes escape silencing, independent of the atxr mutations, indicating that selective rRNA gene silencing is chromosome 2-specific. The chromosome 2 position effect is not explained by the NOR2-associated telomere, TEL2N, which remains linked to the translocated NOR, implicating centromere-proximal sequences in silencing.

scholarly journals The Control of Natural Variation in Cytosine Methylation in Arabidopsis

Genetics ◽  
2002 ◽  
Vol 162 (1) ◽  
pp. 355-363 ◽  
Author(s):  
Nicole C Riddle ◽  
Eric J Richards
Keyword(s):  
Copy Number ◽  
Natural Variation ◽  
Cytosine Methylation ◽  
Gene Copy Number ◽  
Nucleolus Organizer ◽  
Flowering Plant ◽  
Gene Copy ◽  
Rrna Gene ◽  
Nucleolus Organizer Regions ◽  
Plant Arabidopsis Thaliana

Abstract We explore the extent and sources of epigenetic variation in cytosine methylation in natural accessions of the flowering plant, Arabidopsis thaliana, by focusing on the methylation of the major rRNA gene repeats at the two nucleolus organizer regions (NOR). Our findings indicate that natural variation in NOR methylation results from a combination of genetic and epigenetic mechanisms. Genetic variation in rRNA gene copy number and trans-acting modifier loci account for some of the natural variation in NOR methylation. Our results also suggest that divergence and inheritance of epigenetic information, independent of changes in underlying nucleotide sequence, may play an important role in maintaining natural variation in cytosine methylation.

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