Nucleolus

The nucleolus, a nuclear inclusion that is not surrounded by a membrane

It is observed in interphase cells that are actively synthesizing proteins (some nuclei house several nucleoli).

It contains mostly rRNA and protein, such as nucleostemin, nucleolin, and fibrillarin, along with a modest amount of DNA. Each nucleolus is formed around the nucleolar organizer regions (NORs), portions of the chromosomes (in humans, chromosomes 13, 14, 15, 21, and 22) where rRNA genes are located, along with the ribonucleoprotein SRP and RNA polymerase I, the enzyme required for the transcription of rRNA. Additionally, the presence of 15-nm maturing ribosomal precursor particles should be noted because they represent the assembly of the 5.8S, 18S, and 28S rRNA subunits. Ribosomal proteins, manufactured in and imported from the cytoplasm, are combined with rRNA to form the small and large ribosomal subunits, which are then individually exported into the cytoplasm where ribosomal assembly is completed. In order to complete the formation of the large ribosomal subunit the 5S rRNA is also required. However, this is coded by a region of the DNA outside the nucleolus. RNA polymerase III transcribes the 5S rRNA, which is transported into the nucleolus to become incorporated into the large subunit. NORs are also involved in reconstituting the nucleolus during the G1 phase of the cell cycle. Although it has been demonstrated by electron microscopy that the nucleolus is composed of three to four distinct regions, recent studies have dismissed the importance of these morphologic distributions.

The nucleolus also sequesters certain nucleolar proteins, such as nucleostemin, that function as cell cycle checkpoint signaling proteins. These cell cycle regulator proteins remain sequestered in the nucleolus until their release is required for targets in the nucleus and/or the cytoplasm. Additional functions include sensing and repair of damaged DNA, managing the fate of noncoding RNAs, as well as managing responses of cells to various stresses. Following prophase of the cell cycle the nucleolus disintegrates because the NORs of chromosomes 13, 14, 15, 21, and 22 are unavailable for transcription. Subsequent to telophase the NORs unwind and facilitate the reconstitution of the nucleolus.

The nucleolus is where where (1) rRNA forms and (2) ribosomal subunits are assembled for export into the cytoplasm.

rRNA is transcribed in the nucleus by RNA polymerases pol I, II, and III, and often methylated or shortened. The resulting rRNA sequences are targeted to the nucleolus where they join with the r-proteins to make the 40S and 60S subunits of mammalian ribosome, which is subsequently exported through a nuclear pore to the cytoplasm.

The protein components of ribosomes, often called “r-proteins,” are made in the cytoplasm and transported to the nucleolus via a connected network of nuclear membrane pores and nucleolar channels.

One or more highly basophilic nucleoli are visible within each nucleus, indicating intense protein synthesis by these cells. Most of the chromatin is light staining or euchromatic, with small areas of more darkly stained heterochromatin scattered throughout the nucleus and just inside the nuclear envelope.

This superficial heterochromatin allows the boundary of the organelle to be seen more easily by light microscopy.

One cell here has two nuclei, which is fairly common in the liver. (X500; Pararosaniline–toluidine blue)

Reference

https://www.ncbi.nlm.nih.gov/books/NBK22433/#:~:text=In%20eukaryotes%2C%20transcription%20and%20translation,the%20nucleus%20in%20the%20cytoplasm.

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Nucleus

Update May 10, 2021