Showing papers on "Nucleolar chromatin published in 2013"

Chromatin Organization and the Mammalian Nucleolus

Abstract: The nucleolus is the largest compartment of the cell nucleus where ribosomal RNAs (rRNAs) are synthesized, processed and assembled with ribosomal proteins. In addition to rRNA gene clusters that build the core of this subnuclear structure, nucleoli are surrounded by a shell of condensed chromatin. Although the higher-order structures of rRNA genes and nucleolus-associated chromatin have been studied for decades, detailed molecular insights into the constituents and organization of the nucleolar genome are only beginning to emerge. Here, we summarize current views on the structural organization of nucleolar chromatin with a special emphasis on its protein components.

The Plant Nucleolus

Abstract: The majority of an actively dividing cell’s metabolic activity is devoted to ribosome biogenesis, and most traffic in and out of the nucleus is targeted to or from the nucleolus. It is therefore not surprising that the nucleolus is the most prominent and easily observable structure within the nucleus. It has been studied for more than two hundred years, and is still an active subject of research and is still generating surprising discoveries. This chapter summarizes the current state of knowledge of the nucleolus with particular reference to recent developments, and concentrating on work from plants where appropriate. Topics covered include the organization of ribosomal rDNA in nucleolar chromatin, epigenetic phenomena, nucleolar dynamics and non-conventional functions of the nucleolus, particularly in its involvement in various RNA pathways.

Nucleolar chromatin organization at different activities of soybean root meristematic cell nucleoli.

Abstract: Nucleolar chromatin, including nucleolus-associated chromatin as well as active and inactive condensed ribosomal DNA (rDNA) chromatin, derives mostly from secondary constrictions known as nucleolus organizer regions containing rDNA genes on nucleolus-forming chromosomes. This chromatin may occupy different nucleolar positions being in various condensation states which may imply different rDNA transcriptional competence. Sections of nucleoli originating from root meristematic cells of soybean seedlings grown at 25 °C (the control), then subjected to chilling stress (10 °C), and next transferred again to 25 °C (the recovery) were used to measure profile areas occupied by nucleolar condensed chromatin disclosed with sodium hydroxide methylation–acetylation plus uranyl acetate technique. The biggest total area of condensed chromatin was found in the nucleoli of chilled plants, while the smallest was found in those of recovered plants in relation to the amounts of chromatin in the control nucleoli. The condensed nucleolar chromatin, in the form of different-sized and different-shaped clumps, was mainly located in fibrillar centers. One can suppose that changes of condensed rDNA chromatin amounts might be a mechanism controlling the number of transcriptionally active rDNA genes as the nucleoli of plants grown under these experimental conditions show different transcriptional activity and morphology.

Quantitative analysis of nucleolar chromatin distribution in the complex convoluted nucleoli of Didinium nasutum (Ciliophora)

Abstract: We have earlier shown that the typical Didinium nasutum nucleolus is a complex convoluted branched domain, comprising a dense fibrillar component located at the periphery of the nucleolus and a granular component located in the central part. Here our main interest was to study quantitatively the spatial distribution of nucleolar chromatin structures in these convoluted nucleoli. There are no \"classical\" fibrillar centers in D.nasutum nucleoli. The spatial distribution of nucleolar chromatin bodies, which play the role of nucleolar organizers in the macronucleus of D.nasutum, was studied using 3D reconstructions based on serial ultrathin sections. The relative number of nucleolar chromatin bodies was determined in macronuclei of recently fed, starved D.nasutum cells and in resting cysts. This parameter is shown to correlate with the activity of the nucleolus. However, the relative number of nucleolar chromatin bodies in different regions of the same convoluted nucleolus is approximately the same. This finding suggests equal activity in different parts of the nucleolar domain and indicates the existence of some molecular mechanism enabling it to synchronize this activity in D. nasutum nucleoli. Our data show that D. nasutum nucleoli display bipartite structure. All nucleolar chromatin bodies are shown to be located outside of nucleoli, at the periphery of the fibrillar component.

Quantitative analysis of nucleolar chromatin distribution in the complex convoluted nucleoli of Didinium nasutum (Ciliophora)

Abstract: We have earlier shown that the typical Didinium nasutum nucleolus is a complex convoluted branched domain, comprising a dense fibrillar component located at the periphery of the nucleolus and a granular component located in the central part. Here our main interest was to study quantitatively the spatial distribution of nucleolar chromatin structures in these convoluted nucleoli. There are no "classical" fibrillar centers in D.nasutum nucleoli. The spatial distribution of nucleolar chromatin bodies, which play the role of nucleolar organizers in the macronucleus of D.nasutum, was studied using 3D reconstructions based on serial ultrathin sections. The relative number of nucleolar chromatin bodies was determined in macronuclei of recently fed, starved D.nasutum cells and in resting cysts. This parameter is shown to correlate with the activity of the nucleolus. However, the relative number of nucleolar chromatin bodies in different regions of the same convoluted nucleolus is approximately the same. This finding suggests equal activity in different parts of the nucleolar domain and indicates the existence of some molecular mechanism enabling it to synchronize this activity in D. nasutum nucleoli. Our data show that D. nasutum nucleoli display bipartite structure. All nucleolar chromatin bodies are shown to be located outside of nucleoli, at the periphery of the fibrillar component.

Endoreduplication and fruit growth in tomato:evidences in favour of the karyoplasmic ratio theory

Abstract: Endopolyploidy occurs in many plant species and supports the process of differentiation of cells and organs. The functional role of endopolyploidy in plant cells remains poorly understood, mainly because the analysis is hampered by the fact that complex polyploid tissues usually include cells with different ploidy levels. During the development of tomato fruit, cells from the (fleshy) pericarp tissue become highly polyploid reaching DNA content barely encountered in other plant species (between 2C and 512C). To investigate the spatial and temporal distribution of endopolyploidy, it is necessary to address the DNA content of individual nuclei in situ. Populations of nuclei with different ploidy levels were isolated to characterize at the cytological level the consequences of endopolyploidy on the ultrastructure of nuclear and nucleolar chromatin, the nuclear shape and the relationship with other cellular organelles such as mitochondria. We were able to develop a new method based on BAC-FISH to determine in situ the ploidy level of different nuclei and consequently establish a ploidy map of tomato fruit pericarp. Based on this map, we demonstrated a link between the ploidy level, the complexity of nuclear shape and the number of mitochondria in the vicinity of polyploid nuclei. We were able to provide the first direct evidence that endoreduplication plays a role in the increased transcription of rRNA and mRNA in plant cells. We thus provided quantitative data in favour of the ‘karyoplasmic ratio’ theory and showed that endoreduplication is associated with a complex cellular re-organization during development of tomato fruit.