Nucleolus: What is, function, structure and cycle

The nucleolus, an essential cellular organelle, is distinguished by its presence as a spherical structure of dark staining within the core of the cells. It is a remarkably heterochromatic region, however, it lacks a membrane that separates it from the surrounding nucleoplasm. This peculiarity differentiates it from other cell organelles that are generally delimited by membranes. From a morphological perspective, the nucleolus adopts an oval or rounded shape in histological preparations, although its appearance can suggest net limits, There is no physical membrane that delimits the nucleolus of the nucleoplasmic environment.

In terms of size, variability is notable between animal and vegetable species, but in general terms, its diameter usually ranges between 1 and 2 micrometers. However, this measure can vary according to the cell type and physiological conditions. In addition, the number of nucleolors present in a cell can fluctuate significantly, even within the same cell type, being common to find one or two nucleolialthough in certain circumstances this number may be greater.

The nucleolus, an intracellular structure located in the nucleus of eukaryotic cells, performs a variety of critical functions:

• Ribosomal subunit synthesis: It produces both the small and the great subunits of the ribosomes, essential for the synthesis of protein in the cytoplasm after transport from the nucleus.
• Participation in key cellular processes: It is involved in cell aging, responses to cell stress and telomerase activity, a crucial enzyme to maintain the integrity of DNA telomers and, therefore, for duplication and cell division.
• Biosynthesis of ribosomas: Its main function lies in the synthesis of Ribosomal RNA (ARNR) from DNA components, which contributes directly to protein production. The amount of nucleoli can increase in cells with a high protein synthesis rate.
• RNA traffic regulation: It facilitates the movement of small RNA segments and participates in its maturation and transport to its final destination in the cell.
• Possible regulation of the cell cycle: Although the nucleolus is not visible during cell division, recent investigations suggest their role in the regulation of the cell cycle.

The nucleolus is a prominent and dynamic nuclear structure that plays a crucial role in the synthesis and maturation of ribosomes, molecular factories responsible for translation of the messenger to protein.

Inside, the nucleolo houses an aggregate of ribosomal genes, Ribosomal RNA (Arnr) freshly synthesized, ribosomal and ribonucleoprotein proteins. The high concentration of this densely packaged RNA in ribosomal subunits is responsible for the intense basophilic staining (dark) characteristic of the nucleolus when observed at the microscope.

Structurally, the nucleolus is divided into two main regions:

1. amorphous: Electrons are not very dense when examined with electronic microscopy and is constituted by interconnected spaces, equivalent to the surrounding nucleoplasm.

2. Dense part: Form nucleolonema. It presents a more complex internal organization, which can be subdivided into three different subregions:

• Fibrillar Center: It contains the DNA with multiple copies of the genes coding for the precursor of the RNA 45s. It also houses various associated factors, mainly protein, which participate in the transcription of these ribosomal genes.
• Fibrillar dense component: It surrounds the fibrillar center and is the region where the initial processing of the primary pre-arnr-45s transcript occurs. Here the maturation of this arnr precursor in the different RNR species that will be part of the ribosomal subunits takes place.
• Granular Center: less dense than the fibrillar region, but denser than the fibrillar center. It is the site where the late RNA processing and the final assembly of ribosomal subunits occurs.

The nucleolus does not remain constant throughout the cell cycle, but experiences a series of morphological changes according to the phase in which the cell is located, either in interface or in division. In the different phases of cell division, the nucleolus does have more notable transformations. The stages of the nucleolus cycle are:

• Profásic disorganization: In the prophase, the nucleolus decreases in size and becomes irregular. Small accumulations of nucleolar material appear among the chromosomes that are condensing.
• Metapasic and anaphasic transport: During metaphase and anaphase, the nucleolus loses its individuality and its components are integrated into metaphasic chromosomes.
• Telophasic Organization: In the telophase, when the chromosomes are discouraged, spherical structures called laminar bodies and pre -neolar bodies reappear, the latter larger by the fusion of the former. These pre -Nucleolar bodies increase in volume and begin to organize to form one or more nucleol, depending on the number of nucleolar organizers present.

Now that you know what it is and the function of the nucleolus, we invite you to read this article about the nucleoid: what is, function and structure.

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