Chromatin, nucleolus associated

Intragenomic Distribution of Satellite DNAs. There is sufficient satellite DNA in the mouse to occupy 4 out of 40 complete chromosomes, but it is in fact distributed about equally among the different size classes of chromosomes (Maio and Schildkraut, 1969). Schildkraut and Maio (1968), however, provide evidence for enrichment in nucleolus-associated chromatin. Cytologic in-situ hybridization studies show that mouse satellite DNA is present in all chromosomes with the possible exception of the Y chromosome (Jones, 1970 Pardue and Gall, 1970). These studies also reveal that the satellite sequences are primarily localized in centromeric heterochromatin, although they do not preclude the possibility that lower concentrations of satellite sequences are dispersed throughout other regions of chromosomes. Indeed the studies of Flamm et al. (1969), which show that a small number of mouse satellite sequences are retained in main band... 

Close observations of immunofluorescence signals showed that there are three different staining patterns, which correspond to three different nucleolar compartments FC (Fibrillar center), DFC (dense fibrillar component), and GC (granular component). Nucleolus is surrounded by heterochromatin. When the cells are in very active state of its proliferation, the nucleolar compartments and heterochromatin are integrated into a highly intricate structure called nucleolonema . A recent study has suggested that the chromatin associated with the nucleolus is less mobile than... 

The nucleus is not capable of synthesizing proteins. All of the nuclear proteins therefore have to be imported—the histones with which DNA is associated in chromatin, and also the so-called non-histone proteins (DNA polymerases and RNA polymerases, auxiliary and structural proteins, transcription factors, and ribosomal proteins). Ribosomal RNA (rRNA) already associates with proteins in the nucleolus to form ribosome precursors. 

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. 

A FIGURE 12-32 Electron micrograph of pre-rRNA transcription units from nucleolus of a frog oocyte. Each "feather" represents a pre-rRNA molecule associated with protein in a pre-ribonucleoprotein particle (pre-RNP) emerging from a transcription unit. Pre-rRNA transcription units are arranged in tandem, separated by nontranscribed spacer regions of nucleolar chromatin. [Courtesy of Y. Osheim and O. J. Miller, Jr.]... 

Unlike DNA, RNA is found mainly in the cytoplasm where it is present in three different forms. The nucleus contains about 5% of the total RNA. Most of this is found in the nucleolus, although small amounts are found associated with the chromatin. 

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