Nucleolus: Definition, Function, and Examples

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Nucleolus Definition

Nucleolus is a spherical granular structure within a cell’s nucleus that is made up of proteins, DNA, and RNA and is responsible for the production of ribosomes.

What is Nucleolus?

The nucleus is the organelle in eukaryotic cells that is responsible for maintaining DNA integrity as well as managing cellular functions including metabolism, growth, and reproduction through regulating gene expression.

It’s a two-membraned organelle that houses nuclear structures such as chromatin and nuclear bodies. Any of the major non-membraned structures in the nucleus of a eukaryotic cell is referred to as a nuclear body.

Nuclear bodies are proteinaceous structures made up of non-chromatinic fibrillary material. One of the most well-known nuclear structures is the nucleolus, a spherical granular structure within a cell’s nucleus that is made up of proteins, DNA, and RNA and is responsible for the production of ribosomes.

Nuclear bodies are typified by Cajal bodies and gems (Gemini of Cajal bodies), polymorphic interphase karyosomal association (PIKA) domains, promyelocytic leukaemia protein (PML) bodies, splicing speckles and paraspeckles, perichromatin fibrils, and clastosomes.

Nucleolus Characteristics

The nucleolus is sometimes alluded to as an organelle, especially one that is not membrane-bound (as opposed to membrane-bound organelles). Because the nucleolus lacks a lipid bilayer membrane, some people do not consider it an organelle. It is instead regarded as one of the nuclear entities located within the nucleus.

The nucleolus is a granular, spherical structure found inside the nucleus. It is the nucleus’ most conspicuous subnuclear structure. Proteins, DNA, and RNA make up this molecule. The fibrillar centre (FC), the dense fibrillar component (DFC), and the granular component are the three primary components of higher eukaryotes (GC).

The transcription of rDNA is aided by the FC. The Fibrillarin protein, which is involved in rRNA processing, is found in the DFC. Nucleophosmin, a protein involved in ribosome synthesis, is found in the GC.

Another structure in plants is the nucleolar vacuole, which is a clear region in the middle of the nucleolus. In contrast to the pale staining and fibrillar zones, the granular component of the nucleolus refers to the part of the nucleolus that looks granular when examined under an electron microscope.

The presence of 15nm-diameter mature ribosomes accounts for the granular appearance. Nucleophosmin is a protein that transports fluid between the nucleus and the cytoplasm and is involved in a variety of cellular activities, such as protein synthesis, DNA replication, and the cell cycle.

The nucleolar protein, which is involved in RNA splicing and suppresses apoptosis to reduce enzyme activity, is another protein found in the nucleolus. The nucleolus organiser region (NOR) is a chromosomal region that the nucleus develops around. The NORs are found on the acrocentric chromosomes’ satellite stalks.

Biological Reactions in Nucleolus

The biosynthesis of ribosomes is referred to as ribosome biogenesis. The cytoplasm and nucleus of eukaryotes are where ribosomes are formed. Eukaryotic ribosomes have an 80S ribosome, whereas prokaryotic ribosomes have a 70S ribosome.

A big subunit (60S) plus a small subunit (80S) make up the 80S ribosome (40S). Ribosomal protein and ribosomal RNA make up each of these components (s). Ribosomal proteins are made in the same way as other proteins, with transcription taking place in the nucleus and then translation and maturation taking place in the cytoplasm.

The nucleus, especially the nucleolus, receives mature ribosomal proteins for ribosomal subunit assembly, such as the 60S or 40S assembly. The rRNA components of the 60S and 40S genes are produced in the nucleus.

The catalytic activity of RNA polymerase I transcribes the rRNAs 18S, 28S, and 5.8S into a single unit pre-rRNA (referred to precisely as 45S pre-RNA) in the nucleolus organiser area of mammals. The outcome is a huge pre-rRNA molecule made up of 18S, 28S, and 5.8S, which will be released separately following processing.

The genes that code for 5S rRNA are translated into pre-5S rRNA by the RNA polymerase III. The pre-5S rRNA transcript, on the other hand, is made outside the nucleolus, in the nucleoplasm.

Regardless, it makes its way to the nucleolus to begin the assembly process. 5S rRNA joins with 28S and 5.8S rRNA to create the major subunit (60S) of the ribosomal complex.

By joining with ribosomal proteins, 18S creates a small subunit (i.e. 40S). These subunits would subsequently be transported from the nucleolus to the cytoplasm, where they would be assembled into a fully functional 80S ribosome.

Nucleolus Function

In eukaryotes, the nucleolus is important for ribosomal RNA production and ribosome formation. It also plays a role in signal recognition particle assembly and the cell’s stress response.

Nucleolus Citations
  • Genome Organization in and around the Nucleolus. Cells . 2019 Jun 12;8(6):579.
  • The nucleolus as a multiphase liquid condensate. Nat Rev Mol Cell Biol . 2021 Mar;22(3):165-182.
  • The nucleolus. Cold Spring Harb Perspect Biol . 2011 Mar 1;3(3):a000638.
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