Most mouse-human somatic cell hybrids show preferential loss of human chromosomes, absence of human 28S ribosomal RNA, and suppression of human nucleolus organizer activity, as visualized by the Ag-AS silver histochemical stain. In contrast, the mouse-human hybrids studied here show preferential loss of mouse chromosomes. The hybrids were made by fusion of HT-1080-6TG human fibrosarcoma cells w...

Around 2200 copies of genes encoding ribosomal RNA (rRNA) in pedunculate oak, Quercus robur, are organized into two rDNA loci, the major (NOR-1) and the minor (NOR-2) locus. We present the first cytogenetic evidence indicating that the NOR-1 represents the active nucleolar organizer responsible for rRNA synthesis, while the NOR-2 probably stays transcriptionally silent and does not participate ...

The replication of the rDNA complement of only one nucleolus organizer region during polytene chromosome formation (replicative dominance) was initially observed in Drosophila melanogaster. Here we demonstrate replicative dominance in Drosophila simulans and D. melanogaster/D. simulans interspecific hybrids. A second nucleolar phenomenon, nucleolar dominance, is observed in the diploid tissue o...

The nucleolus is required for ribosome biogenesis. Human cells have 2 or 3 nucleoli associated with nucleolar organizer region (NOR)-bearing chromosomes. An increase in number and altered nucleolar morphology define cancer cells. However, the mechanisms that modulate nucleolar morphology and function are unclear. Here we show that in addition to localizing at the nuclear envelope, lamin B2 loca...

The ultrastructure of embryonic root cells of Zea mays was studied from the quiescent stage (dry seed) to 72 h of germination. Semithin and ultrathin sections of tissues fixed with only glutaraldehyde and embedded in Epon were observed after usual section staining and after cytochemical reactions specific for DNA or preferential for ribonucleoproteiNS. In quiescent cells, dense chromatin forms ...

In the present study, the chromosomes numbers were confirmed, 2n = 34 for Amaranthus cruentus Linnaeus, 1759, and 2n = 32 for Amaranthus hypochondriacus Linnaeus, 1753, Amaranthus mantegazzianus Passer, 1864, and Amaranthus caudatus Linnaeus, 1753. The distribution and variability of constitutive heterochromatin were detailed using DAPI-CMA3 banding technique. The position of the nucleolus orga...

Cytogenetics studies in 12 specimens of Gymnothorax ocellatus reveled a diploid chromosome number of 2n 42 (16 metacentrics, 18 submetacentrics and 8 acrocentrics). The nucleolar organizer regions were located in a terminal position on the long arm of the chromosome pair number fifteen. Conspicuous blocks of constitutive heterochromatin were observed in the centromeric and pericentromeric regio...

Stages of meiosis from the bluebell Endymion non-scriptus (L.) were studied by electron microscopy. The nucleolus went through the process of segregation at the beginning of meiosis with the movement to its surface of a pale-staining region. This region was shown to be the same as that called the 'L zone' or lacunae of nucleoli. Its chromosomal nature was strongly suggested by the presence of t...

Dominance of a 5-azacytidine-substituted nucleolar organizer region (AZA NOR) over another with normal DNA when confined to a common nucleus or to different nuclei in a common cell was studied in Allium cepa L. root meristems. AZA administration took place for one S period. The assessment of NOR dominance was done either in early Gi (nucleologenesis) or later in interphase (mature nucleoli). At...