Timings of DNA synthesis in the nucleoli of Allium cepa L
01 Feb 1990-Cell Biology International Reports (No longer published by Elsevier)-Vol. 14, Iss: 2, pp 173-177
TL;DR: The timing of nucleolar DNA synthesis in root tip cells of Allium cepa LAs has been studied in this article, where the authors found that nucleolar chromatin in this plant species replicates in two phases during S period, once in the early and again towards the end of S phase.
About: This article is published in Cell Biology International Reports.The article was published on 1990-02-01. It has received 4 citations till now. The article focuses on the topics: Nucleolar chromatin & Chromatin.
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TL;DR: This review addresses the problem of understanding nucleolar morphology in terms of nucleolar function by reviewing in situ-cytochemical results that have not led to a generally accepted view on the structure-function correlation for nucleoli.
138 citations
01 Jan 1993
TL;DR: Its visibility both in the light-and the electron microscope is due to the fact that the genes for ribosomal RNA are not only transcribed but also processed and complexed with ribosomes in nucleoli, rendering them morphologically different from the rest of the karyoplasm.
Abstract: Nucleoli were noted by biologists very early (Fontana, 1781) and a lot of data has been accumulated on the structure of nucleoli in the last decades. Due to the work of Perry (1962; Perry et al., 1961) it is known that the nucleolus is the site of ribosome biogenesis. Its visibility both in the light-and the electron microscope is due to the fact that the genes for ribosomal RNA are not only transcribed but also processed and complexed with ribosomal proteins in nucleoli, rendering them morphologically different from the rest of the karyoplasm.
8 citations
TL;DR: In okadaic acid treated HeLa cells, the chromosomes sometimes condense without being accompanied by nuclear envelope breakdown, which strongly speak for the localization of nucleolar chromatin (ribosomal cistrons) within the dense nucleolar component of the interphase nucleolus.
6 citations
TL;DR: The nucleolar chromatin is traced from early prophase to the metaphase stage, where in metaphase it is fully condensed and in mitotic chromosomes this chromatin remains surrounded by achromatic materials resembling the fibrillar centre.
4 citations
References
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TL;DR: Observations in A. cepa and Aloe indicate clearly that the interphase chromosomes are non-random in their orientation and possibly maintain their telophase configuration through the attachment of telomeres and perhaps of kinetochores with the nuclear membrane.
Abstract: The orientation of Giemsa C-bands has been studied in mitotic and interphase cells of Allium cepa, A. sativum and of Aloe vera. The C-bands in these three species are located at the telomeres, secondary constriction region of the nucleolar chromosomes and the centromeric regions, respectively. Observations in A. cepa and Aloe indicate clearly that the interphase chromosomes are non-random in their orientation and possibly maintain their telophase configuration through the attachment of telomeres and perhaps of kinetochores with the nuclear membrane. Electron micrographs of onion cells also reveal that certain heterochromatic segments are associated with the nuclear membrane. — The nucleolar interstitial C-bands in A. sativum remain free in the nucleoplasm and may come close to each other due to heterochromatic attraction. Such a heterochromatic attraction is also evident between telomeric regions and between centromeres. However, a two by two attachment could not be noticed. A diagrammatic representation of the orientation of interphase chromosomes has been presented.
33 citations
TL;DR: The active ribosomal cistrons together with their primary transcripts and the associated proteins and enzymes forming the typical 'christmas tree' configuration in the dense nucleolar components can correlate the structural and functional organization of the nucleolus.
12 citations
TL;DR: The first aim was to determine the length of the S phase by observing the time between the first and the last appearance of a labeled cell in mitosis and the observation of the labeling pattern in interphase nuclei to provide information as to whether labeling of only one of two nucleoli might be observed with considerable frequency.
Abstract: No definite information exists on the time of synthesis of desoxyribonucleic acid (DNA) in nucleoli during the S phase. The few reports in the literature are not in agreement . Harris (3) concluded from his results with thymidine 3H that in rat fibroblasts DNA synthesis is initiated in the nucleolus. Ribas-Mundo (10) reported that DNA synthesis in human leukocytes occurs in the nucleolus-associated chromatin during the late S phase. In synchronized human tumor cells (strain CMP), Kasten and Strasser (5) observed two distinct periods of DNA synthesis in nucleoli : one period was at the beginning of the S phase, the other at about the middle of the second half of the S phase . In one of our previous publications (2), we reported thymidine-3H incorporation after a short pulse labeling in only one of the two nucleoli of chicken fibroblasts . Since there is no general agreement on the DNA synthesis pattern of nucleoli, we felt that further investigations should be undertaken . Our first aim was to determine the length of the S phase by observing the time between the first and the last appearance of a labeled cell in mitosis . The observation of the time between the end of incubation with thymidine3H and the appearance of prophase cells with labeled nucleoli allows us to decide whether the nucleolus synthesizes DNA at the beginning of the S phase or at its end . In addition, the observation of the labeling pattern in interphase nuclei was undertaken to provide information as to whether labeling of only one of two nucleoli might be observed with considerable frequency .
9 citations
TL;DR: A gradual drop occurs in the mitotic index during the first 6 hr after the change, while the percentage of prophases drops less steeply, and at the same time the phenomenon of nucleolar segregation is observed.
Abstract: . The dynamic adaptation of a meristem to a thermal change from 15 to 35°C has been studied. A gradual drop occurs in the mitotic index during the first 6 hr after the change, while the percentage of prophases drops less steeply. At the same time the phenomenon of nucleolar segregation is observed.
By making use of a cell population morphologically differentiated as binucleate (by ‘labelling’ with caffeine), it was possible (a) to prove that prophase is specifically prolonged and (b) to locate a highly thermosensitive segment of interphase, situated in the middle of the S period, and apparently coinciding with the sub-period S2, described in the same material as characterized by a decrease in the rate of DNA synthesis.
The lengthening of prophase and nucleolar segregation resemble the effects produced by drugs which inhibit RNA synthesis.
Intermittent thermal shocks are suggested as a possible means for synchronizing the division cycle of plant cells.
8 citations
TL;DR: To get more information as to whether the phenomenon of asynchrony in nucleolar labeling with thymidine 3H may also be found in other species, the authors decided to work with cells of the Chinese hamster, in which the cells show two to six nucleoli.
Abstract: In the preceding note (2), we showed an asynchronous pattern of DNA synthesis in the nucleoli of chicken fibroblast cells, in which the rRNA cistrons (about 100 per genome) (10) are accumulated on one chromosome (12) . We reported that in these cells nucleolar DNA synthesis takes place twice in the S phase, once at about the 4th hr and again at about the 10th hr, the total length of the S phase being 11 hr. However, we could not decide whether DNA in one of two nucleoli duplicates at the early S phase and in the other at the late S phase, or whether DNA in both nucleoli duplicates at both phases but asynchronously . That means: part of both polycistronic organizing regions may replicate at the early S phase, and the rest during the late S phase; in addition, it must be assumed that in both phases the respective segments of both chromosomes replicate one after the other. As was already mentioned in the previous paper, Kasten and Strasser (6) reported labeling of nucleoli, asynchronous with the majority of autosomes, twice during the S phase : once at the early, and again at the late, S phase . For their investigations, those authors used a human tumor cell line (CMP) . They did not mention that any asynchrony of labeling between nucleoli within one nucleus was observed . The photographs presented in their paper and in the other papers (3, 8) show, so far as we were able to see, labeling over both discernible nucleoli . In the human genome, the number of chromosomes which carry the information for the synthesis of rRNA is, so far, not precisely known . According to different sources (see 5, 11), it is supposed that in the diploid cell up to 10-12 nucleolus-producing sites are present . When cells that possess so many rDNA-regions have only two distinct nucleoli, nothing can be said about asynchrony in DNA synthesis between different organizing regions . To get more information as to whether the phenomenon of asynchrony in nucleolar labeling with thymidine 3H may also be found in other species, we decided to work with cells of the Chinese hamster, in which, in general, the cells show two to six nucleoli. Phillips and Phillips (7) have reported that in this species nucleolus-producing sites are located
5 citations