Nucleoli structure

Drugs Used to Study Nucleolus Structure and Function... 

Microscopic examination of the mature neutrophils reveals two striking features a single multilobed nucleus and a dense, granular appearance of the cytoplasm (see Fig. 1.1a). The nucleus typically comprises two to four segments, and within this organelle the chromatin is coarsely clumped. Until recently, this abnormal chromatin structure was taken as evidence that the nucleus was transcriptionally inactive however, it is now appreciated that the mature neutrophil does perform active transcription ( 7.3), although rates of biosynthesis are somewhat lower than those observed in cells such as monocytes. There is no detectable nucleolus, so there can be only limited synthesis of ribosomal RNA in these cells. 

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... 

Figure 7. A series of monoclonal antibodies raised against nuclear proteins. HeLa cells fixed with 4% paraformaldehyde were immunostained with monoclonal antibodies raised against nuclear proteins. The intracellular localization of these antigens are (a) dot inside the nucleolus, (b) whole nucleolus, (c) nuclear foci, (d) nucleoplasm, (e) the edge of the nucleus, (f) cytoplasm, (g) cytoskeleton, (h) plasma membrane, (i) mitochondria, (j) nucleus and cytoplasm, (k) nucleus and the paranuclear structure, and (1) paranuclear structure and nucleoplasm...

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... 

Cellular RNAs vary widely in their size, structure, and lifespan. The great majority of them are ribosomal RNA (rRNA), which in several forms is a structural and functional component of ribosomes (see p.250). Ribosomal RNA is produced from DNA by transcription in the nucleolus, and it is processed there and assembled with proteins to form ribosome subunits (see pp.208, 242). The bacterial 16S-rRNA shown in Fig. A, with 1542 nucleotides (nt), is a component of the small ribosomae subunit, while the much smaller 5S-rRNA (118 nt) is located in the large subunit. 

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. 

Nucleolus. A spherical structure visible in the nucleus during interphase. The nucleolus is associated with a site on the chromosome that is involved in ri-bosomal RNA synthesis. 

The nucleus of eukaryotic cells is a very complex structure, containing various components. It is separated from the rest of the cell by two membranes named the nuclear envelope. At regular intervals, the two membranes of the nuclear envelope form pores with a diameter of around 90 nm. These pores regulate flux of macromolecules to and from the cytoplasm. Inside the nucleus is located the nucleolus, which acts to produce ribonucleic acid (RNA), which is the first step for ribosome synthesis. 

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. 

The nerve cell body contains a nucleus, a nucleolus, and cytoplasm containing the cell (such as mitochcndria, endoplasmic reticulum, and so on). Unique to the nerve cell body are Nissl bodies, which are rough surfaced vesicles in the endoplasmic reticulum (cytcplaan located near the nucleus), and are involved with protein mthe-sis. Another characteristic stracture of nerve cells are the neurofibrils, which are delicate threadlike structures that help to maintain the shape of the cell, and which transport substances between the cell body and the axcn terminals. The plasma merrrbrane around the cell separates the cytoplasm on the inside of the cell from the extracel-... 

Hepatic lesions in the case of yellow fever are more likely to correspond to those of hepatosis, (s. p. 404) There is also evidence of distinct acidophilic hepatocellular necrosis as well as microvesicular fatty degeneration of the hepatocytes. Hyaline, eosinophilic inclusions in the cytoplasm of degenerated hepatic cells (so-called Councilman bodies) are characteristic and were first identified by w.T. Councilman in 1890 in yellow fever (s. p. 396). Acidophilic inclusion bodies in the hepatocellular nuclei which are arranged concentrically around the nucleolus (so-called Torres corpuscles) correspond to the yellow fever virus (C.M. Torres, 1928). The liver does not present any significant signs of inflammation. The reticular fibre structure is maintained, so that the liver architecture is usually completely restored-provided the outcome of the disease is favourable, (s. fig. 23.4)... 

Flgure 12.1 Electron micrograph of an apoptotic human neutrophil showing the characteristic chromatin aggregation, prominent nucleolus and dilated cytoplasmic vacuoles. Note that the cell membrane is intact and the granule structure appears normal. (EM taken by Jan Henson), (x 11 000). 

Finally, 80 proteins localized to an abnormal intranuclear microtubule structure upon LMB treatment, while others simply occupied nucleus or nucleolus. Crml-dependent dynamic interchange of proteins between the nucleus and the microtubules may regulate microtubule organization during the cell cycle in fission yeast. Further analysis of individual proteins will reveal the involvement of Crml in the process for microtubule assembly and disassembly. 

Very often found within the nucleus is a small densely stainin body called the nucleolus the number of these vary in differer cell types they are full of a particular class of RNA. Some RNA i also present in the nucleus in the form of ribosomes, althoug these are mainly to be found in the cytoplasm (see below). Th whole of the nucleus is constrained by a well-defined boundary the nuclear membrane, which has the same type of lipoproteii bilayer structure as the cell membrane, except that it is perforatei by pores whose role is obscure but which are believed to allow th passage of RNA into the cytoplasm (Plate 2). The significance o many of these structures and arrangements will become cleare when we discuss the mechanisms of protein synthesis in Chapte 10. 

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