Question

neucleolus notes in 4000 word​

Answer 1

Answer:

Explanation:

The nucleolus was identified by bright-field microscopy during the 1830s. Little was known about the function of the nucleolus until 1964, when a study of nucleoli by John Gurdon and Donald Brown in the African clawed frog Xenopus laevis generated increasing interest in the function and detailed structure of the nucleolus. They found that 25% of the frog eggs had no nucleolus and that such eggs were not capable of life. Half of the eggs had one nucleolus and 25% had two. They concluded that the nucleolus had a function necessary for life. In 1966 Max L. Birnstiel and collaborators showed via nucleic acid hybridization experiments that DNA within nucleoli code for ribosomal RNA

Answer 2

nucleolus notes

The nucleolus is the dominant internal feature of the cell nucleus and was among the first subcellular or­ganelles described by microscopy (by Fontona in 1774).

The nucleolus is not separated from the remain­ing nucleoplasm by a membrane, but in many cells its margins are associated with chromatin (i.e., the peri­nucleolar chromatin, see Figs. 20-15 and 20-16).

The nucleolus is composed of RNA and proteins and is the site of formation of ribosomal constituents.

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In cells that are actively engaged in protein synthesis and which therefore have greater demands for ribosomes, the nucleoli are larger and more numerous.

Types and Distribution of Chromatin in the Interphase Nucleus

The Interphase Nucleus  

When the cell undergoes nuclear division, protein syn­thesis is decreased and ribosome production is halted. These changes are accompanied by a reduction in the size and number of nucleoli. In fact, at the metaphase stage of mitosis, no nucleoli can be distin­guished.

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Parts of one or more chromosomes are looped into a nucleolus forming what is known as a nucleolar organizing region (NOR). These regions contain the genes that are transcribed into rRNA. Dense fibrillar regions surrounding the NOR are associated with small ribonucleoprotein particles and frequently re­veal mature ribosomes.

The Nuclear Envelope:

The presence of a membrane separating the contents of the nucleus from the surrounding cytosol is one of the principal characteristics distinguishing eukaryotic organisms from prokaryotic organisms. The existence of a membrane delimiting the nucleus was first dem­onstrated by O. Hertwig in 1893. However, little inter­est was shown in studying this membrane until elec­tron microscopy revealed that it was not simply a single membrane, but rather a double membrane in which the outer membrane had features that clearly distinguished it from the inner membrane.

The two membranes are concentric and separated by a small space (called the perinuclear space) varying in size from about 10 to 50 nm; collectively, the two mem­branes form the nuclear envelope. A fibrous material called the lamina densa lies just under the inner nu­clear membrane and is believed to give structural sup­port to the envelope and also provide anchorage for the condensed peripheral heterochromatin. The inner and outer nuclear membranes fuse together at the margins of the nuclear pores (Figs. 20-15 and 20-16).

The functions of the nuclear envelope are diverse. Clearly, it serves to compartmentalize the nucleo­plasm, acting as a barrier to the .movement of large subcellular components between the nucleus and the cytosol. However, ribosome precursors from the nucle­olus and mRNA and tRNA molecules do exit the nu­cleus through the nuclear pores. Molecules also enter the nucleus from the cytoplasm, triggering a number of nucleocytoplasmic interactions.

The nuclear envelope is not just a physical barrier, but functions also in connection with cellular activi­ties taking place on either side. Ribosomes may be at­tached to the cytosol face of the outer nuclear mem­brane, which thereby takes on a structural appearance like rough endoplasmic reticulum; in­deed, the outer membrane may have periodic continui­ties with the endoplasmic reticulum. The intimate re­lationship between the nuclear membranes and the endoplasmic reticulum is not surprising because a new nuclear envelope forms at the end of mitosis by the coalescence of membranes of the endoplasmic re­ticulum.

The inner membrane of the nuclear envelope may have special functions associated with the chromo­somes. As noted earlier, in the interphase nuclei of many cells, heterochromatin is closely appressed to the inner membrane.

This association does not occur at the nuclear pores; instead, the chromosomes seem to be firmly attached to the inter-pore areas. The role of the association between chromatin and the nuclear envelope is unclear but may be related to chromosome replication and orientation during interphase and the early prophase of mitosis and meiosis.

Some materials such as ribonucleoprotein particles are exchanged between the nucleoplasm and the cyto­sol through open nuclear pores. However, the nuclear pores are not the only avenues for nucleocytoplasmic exchanges (Fig. 20-17). For example, small molecules and ions readily permeate both nuclear membranes. Larger molecules and particles may pass through the membrane by formation of small pockets and vesicles that traverse and envelope and empty on the other side.

Various Avenues for Transport of Material from the Nucleus to the Extranuclear Cytoplasm