Nucleolus synthesis

Like other cells, a neuron has a nucleus with genetic DNA, although nerve cells cannot divide (replicate) after maturity, and a prominent nucleolus for ribosome synthesis. There are also mitochondria for energy supply as well as a smooth and a rough endoplasmic reticulum for lipid and protein synthesis, and a Golgi apparatus. These are all in a fluid cytosol (cytoplasm), containing enzymes for cell metabolism and NT synthesis and which is surrounded by a phospholipid plasma membrane, impermeable to ions and water-soluble substances. In order to cross the membrane, substances either have to be very lipid soluble or transported by special carrier proteins. It is also the site for NT receptors and the various ion channels important in the control of neuronal excitability. 

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. 

Bose S, Basu M, Banerjee AK (2004) Role of nucleolin in human parainfluenza virus type 3 infection of human lung epithelial cells. J Virol 78 8146-8158 Bouche G, Caizergues-Ferrer M, Bugler B, Amalric F (1984) Interrelations between the maturation of a 100 kDa nucleolar protein and pre rRNA synthesis in CHO cells. Nucleic Acids Res 12 3025-3035 Bouche G, Gas N, Prats H, Baldin V, Tauber JP, Teissie J, Amalric F (1987) Basic fibroblast growth factor enters the nucleolus and stimulates the transcription of ribosomal genes in ABAE cells undergoing GO-Gl transition. Proc Natl Acad Sci U S A 84(19) 6770-6774. 

Eukaryotic ribosomal RNA is transcribed in the nucleolus by RNA polymerase I as a single piece of 45S RNA, which is subsequently deaved to yield 28S rRNA, 18S rRNA, and 5.8S rRNA. RNA polymerase III transcribes the 5S rRNA unit from a separate gene. TTie ribosomal subunits assemble in the nudeolus as the rRNA pieces combine with ribosomal proteins. Eukaryotic ribosomal subunits are 60S and 40S. They join during protein synthesis to form the whole SOS ribosome. 

Visible in the nuclei of most cells, especially those actively synthesising protein, is a nucleolus. It consists of a mass of incomplete ribosome particles and DNA molecules that code for ribosomal RNA this is the site of synthesis of the ribosomal subunits. 

The final principal component of the cell is the nucleus. This is located in the center of the cell and is surrounded by a double membrane, the outer layer being derived from the ER of the cytoplasm and the inner layer coming from the nucleus itself. The two leaflets of the double membrane are fused in places, producing nuclear pores that enable the transfer of macromolecules from the cytoplasm to the nucleus. Two important components of the nucleus are chromatin and the nucleolus. Chromatin represents polymers of DNA complexed with protein. The nucleolus is a complex substructure, composed of ribonucleoprotein granules, that controls the synthesis of RNA destined to form the ribosomes of the cytoplasm. Cells engaged heavily in protein synthesis have... 

As is indicated in Fig. 28-15, transcription is thought to occur from the loops of DNA that form the nucleolar organizing region. The 100-kDa nucleolin, the major protein of the nucleolus, binds to the non-transcribed spacer sequences in the DNA.529-530 It also binds to the newly formed transcripts, as do various proteins that enter the nucleus from the cytoplasm.524531 More than 270 proteins, many of which participate in synthesis of ribosomes, have been detected in the nucleolus.5313 Some of these proteins, acting together with the snoRNAs discussed in the next section, catalyze hydrolytic cleavage of the pre-rRNA molecules. For completion of pre-ribosomal particles additional protein molecules enter the nucleolus and associate with the pre-rRNA particles, then diffuse out of the nucleus. 

RNA polymerase I is located in the nucleolus and synthesizes a large precursor that is later processed to form rRNA. It is completely resistant to inhibition by a-amanitin. RNA polymerase II is located in the nucleoplasm and synthesizes large precursor RNAs (sometimes called heterogeneous nuclear RNA, or hnRNA) that are processed to form cytoplasmic mRNAs. It is also responsible for the synthesis of most viral RNA in virus-infected cells. PolII is very sensitive to a-amanitin, being inhibited by 50% at 0.05 /u,g/ml. RNA polymerase III is also located in the nucleoplasm and synthesizes small RNAs, such as 5S RNA and the precursors to tRNAs. This enzyme is somewhat resistant to a-amanitin, requiring about 5 /u,g/ml to reach 50% inhibition. 

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 stores the cell s genetic information as DNA in chromosomes. It is bounded by a double membrane but pores in this membrane allow molecules to move in and out of the nucleus. The nucleolus within the nucleus is the site of ribosomal ribonucleic acid (rRNA) 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. 

Nucleus Membrane-limited area of cell containing nucleolus and chromatin Site of synthesis of RNA and chromatin, involved in cell division... 

The nucleus of the cell (Figure 1.2) is composed of a porous nuclear membrane, the nucleolus, and soluble materials. The nucleolus contains ribonucleic acids (RNA) and genetic materials also termed chromatin that code for the proteins synthesized upon the ribosomes in the cell cytoplasm. The nuclear membrane is continuous with the outer membrane of the endoplasmic reticulum. Messenger RNA synthesized in the nucleus is transported across the nuclear membrane and is involved in protein synthesis. It fits into the groove between the large and small rRNA subunits (Figure 1.2)... 

The nucleolus is composed of 5 to 10 percent RNA, and the remainder of the mass is protein and DNA. In light microscopy it appears to be spherical and basophilic (Prob. 1.1). Its function is the synthesis of ribosomal RNA (Chap. 17). There may be more than one per nucleus. 

Eukaryotic cells contain three classes of RNA polymerases, denoted I, II, and III, which are distinguished by their requirements for particular ions and by their sensitivity to various toxins. All are found in the nucleus. Minor RNA polymerases are found in mitochondria and chloroplasts. Polymerase I molecules are located in the nucleolus and are responsible for synthesis of 5.8S, 18S, and 28S rRNA molecules. Polymerase II synthesizes all... 

Nucleolar organizing region (NOR) Area of the nucleolus where a great deal of rRNA transcription and synthesis occurs. 

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. 

When nuclei are stained with certain dyes, one or more spherical structures called nucleoli (singular nucleolus) become visible. The nucleolus is the site of ribosomal RNA synthesis. Its high content of RNA makes it stain differently than the rest of the nucleus. 

The nucleus contains the cell s genetic information and the machinery for converting that information into protein molecules. The nucleolus plays an important role in the synthesis of ribosomal RNA. RER is primarily involved in protein synthesis. SER lacks attached ribosomes and is involved in lipid synthesis and biotransformation. 

A FIGURE 12-34 Processing of pre-rRNA and assembly of ribosomes in higher eukaryotes. Ribosomal and nucleolar proteins associate with 45S pre-RNA as it is synthesized, forming an SOS pre-rRNR Sites of cleavage and chemical modifications are determined by small nucleolar RNAs (not shown). Note that synthesis of 5S rRNA occurs outside the nucleolus. 

Synthesis and processing of pre-rRNA occur in the nucleolus. The 5S rRNA component of the large ribosomal subunit is synthesized in the nucleoplasm by RNA polymerase III and is not processed. 

Ribosome A granule of protein and RNA, synthesized in the nucleolus and found in the cytoplasm of cells. Ribosomes are the main sites of protein synthesis. Messenger RNA attaches to them and there receives molecules of transfer RNA bearing amino adds, [nih]... 

Fig. 14.15. rRNA and ribosome synthesis. The 5S rRNA is transcribed in the nucleoplasm and moves into the nucleolus. The other rRNAs are transcribed from DNA and mature in the nucleolus, forming the 40S and 60S ribosomal subunits, which migrate to the cytoplasm. 

The main feature of normal animal cell is its compartmentalization [7]. The DNA of the animal cell is restricted to the nucleus at all cell cycle stages except during metaphase when no nucleus exists. The synthesis of RNA occurs in the nucleus and most of it remains there, but messenger RNA and transfer RNA migrate to the cytoplasm. Ribosomal RNA is synthesized in the nucleolus the two ribosomal subunits are partly assembled in the nucleolus and nucleus then migrate to the cytoplasm. All protein synthesis proceed in the cytoplasm. The mitochondria, which is located only in the cytoplasm, contains DNA-s, RNA- and protein-synthesizing systems of their own [7]. 

---