Chromatin dispersion

The eukaryotic nuclei once referred to as merely a bag of chromatin has now been recognized to be a highly ordered structure or a hub of cellular activities. The nucleus is seen as a three dimensional mosaic of nucleolus, inter-chromatin regions and condensed chromatin, dispersed in a nuclear ground substance... 

Gurdon, J. B. (1976). Injected nuclei in frog oocytes Fate, enlargement, and chromatin dispersal. 

Glass depression slide with two concavities for chromatin dispersal (VWR Scientific Products, Micro slide, culture, two depression, cat. 48333-002). 

DNA is found in the nucleus of cells. In a cell that is not dividing, the DNA is in a dispersed form called chromatin. When the cell is preparing to divide, the chromatin reorganizes itself, forming pairs of thick, rodlike chromosomes. Chromosomes consist of DNA and protein. The genes, which control heredity, are positioned along the DNA in the chromosomes. Each gene consists of a specific section of DNA and directs the synthesis of a specific protein. 

The eukaryotic cell cycle (see Fig. 12-41) produces remarkable changes in the structure of chromosomes (Fig. 24-25). In nondividing eukaryotic cells (in GO) and those in interphase (Gl, S, and G2), the chromosomal material, chromatin, is amorphous and appears to be randomly dispersed in certain parts of the nucleus. In the S phase of interphase the DNA in this amorphous state replicates, each chromosome producing two sister chromosomes (called sister chromatids) that remain associated with each other after replication is complete. The chromosomes become much more condensed during prophase of mitosis, taking the form of a species-specific number of well-defined pairs of sister chromatids (Fig. 24-5). 

The effects of chromosome structure on gene regulation in eukaryotes have no clear parallel in prokaryotes. In the eukaryotic cell cycle, interphase chromosomes appear, at first viewing, to be dispersed and amorphous (see Figs 12-41, 24-25). Nevertheless, several forms of chromatin can be found along these chromosomes. About 10% of the chromatin in a typical eukaryotic cell is in a more condensed form than the rest of the chromatin. This form, heterochromatin, is transcriptionally inactive. Heterochromatin is generally associated... 

Fig. 1.—Nucleus (N) from D-Galactose-treated Orchid Seedling (Phalaenopsis cv. Doris Fi) Showing Dispersed Chromatin with Nuclear Envelope Envaginated (Arrows) into the Cytoplasm. [After treatment with D-galactose, seedlings were fixed in 2% glu-taraldehyde for 2 h followed by 2% OsO, for 12 h. Tissue was dehydrated in a graded concentration, embedded in Epon 812, sectioned with a diamond knife, and photographed with a Zeiss EM9A electron microscope x 13,340 (reproduced, by permission, from Ref. 533).]...

In undamaged cells, PARP-1 is found at nucleoli and actively transcribed regions of chromatin (Fakan et al., 1988). When isolated nuclei are treated with RNase, PARP-1 localized to nucleoli is dispersed (Kaufmann et al., 1991). These observations suggest that PARP-1 binds to RNA in cells. The damage sensing function of PARP-1, therefore, should be related to the binding of PARP-1 to RNA. 

Chromatin occupies the remaining part of the nucleus and consists of DNA, RNA, and specialized proteins. Between cell divisions, chromatin is dispersed in the nucleus but, immediately before cell division, chromatin is arranged in granular bodies named chromosomes. After replication, daughter chromosomes are separated and distributed to daughter cells during the mitosis process, after which the chromatin is dispersed again. 

The cell body contains many structures of importance. The nucleus is usually located in the center of the cell body. It contains widely dispersed, fine chromatin material. The chromatin is composed of deoxyribonucleic acid (DNA) and its associated histone proteins. The nucleolus contains the specific portion of DNA encoding the ribonucleic acid (RNA) of future ribosomes. 

Milgrom and co-workers [105] recently developed an immunogold method for detection of PR in the rabbit uterus and have examined the effect of hormone addition on receptor localization at the ultrastructural level. PR were found to be predominantly nuclear in the presence and absence of hormone, but a small amount was detectable in the cytoplasm which was not apparent at the light microscopical level. These cytoplasmic PR were localized over endoplasmic reticulum and clusters of free ribosomes and may likely represent newly synthesized protein. No PR were located in the plasma membrane. Within the nucleus, unoccupied PR were associated with condensed chromatin which became more dispersed after hormone addition. These ultrastructural studies indicate that steroid-free PR translocate from their site of synthesis in the cytoplasm to the nucleus in a hormone independent manner, and that addition of hormone changes their intranuclear localization. 

Shioda et al. [43,44] visualized by electron microscopy both regions of naked DNA and of DNA covered with particles in the chromosome of Halobacterium salinarium isolated from gently lysed cells. In a control experiment, they did not detect such particles in E. coli. They also reported the existence of nucleosome-like structures in S. acidocaldarius and methanogens (unpublished results cited in ref. [43]). The size of the particles detected in H. salinarium (9.5 nm) is similar to that of eukaryotic nucleosomes (10.3 nm) however, this putative archaebacterial chromatin is not as regular as eukaryotic chromatin, since not all of the DNA is covered with nucleosomes and since the length of the DNA spacer between the particles is not uniform. In contrast to these results, Bohrmann and coworkers [45] did not visualize nucleosome-like structures in isolated chromosome fibers of Thermoplasma acidophilum. These authors also reported that in situ the nucleoid of T. acidophilum appears to be highly dispersed in the cytoplasm. 

Cells from the control (embryos not exposed to freezing conditions) displayed features typically found in hydrated and metabolically active cells numerous mitochondria nuclei exhibiting fairly disperse chromatin and plastids with highly dense stroma could be seen (Figure 42.3a and Figure 42.4a and b). Starch, lipids, and proteins were the reserves present in plastids, lipid bodies, and vacuoles, respectively. 

By light microscopy, ES/PNETs are composed of uniform round cells, which often grow in a lobular configuration (Eig 9.14). The nuclei have finely dispersed chromatin, sometimes with barely discernible nucleoli. 

The DNAase-hypersensitive sites are small and dispersed throughout the chromatin they are interpreted as being due to the absence of histones or nucleosomes. Often, the binding of other proteins can be detected in the hypersensitive sites, using more sophisticated "footprinting" techniques. There is growing evidence for the existence of multi-subunit complexes of proteins bound at hypersensitive sites which may control the initiation of transcription. 

When replication is complete, the cell enters the G2 phase. It has a DNA content four times the haploid amount (4C). In most eukaryotic cells, the total time required for Gl, S, and G2 phases will be many hours. During this whole period, which is termed interphase, the chromatin is dispersed throughout the nucleus and is actively engaged in transcription. 

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