Nucleolus – Concept, function and cycle of the nucleolus

We explain what the nucleolus is and some of its characteristics. In addition, its function and how the nucleolus cycle is carried out.

The nucleolus is within the nucleus, but it is not separated from it by any membrane.

What is the nucleolus?

In cell biology, a region of the cell nucleus (of the cell organelle that contains the organism’s genetic material) is called a nucleolus where the synthesis of ribosomes takes place (cellular genetic translation machines, responsible for the assembly of proteins) and that takes care of various cellular tasks.

The nucleolus is inside the core, but it is not separated from it by any membrane, since it is considered a supra-macromolecular structure, that is, composed of macromolecules.

His first observations accidentally took place in 1781, a time when cellular observation techniques were far from as powerful as they are today, and it was not recognized. The formulation of its name and discovery itself would occur in 1836, when Rudolph Wagner and Gabriel Gustav Valentin made the first direct observations of it.

The nucleolus is a structure or region, although it can also be defined as a macromolecular grouping (from biochemistry), which is organized around specific chromosomes that contain repeating parts of DNA called Nucleolar Organizing Regions (NORs). From them, the rest of the chromosomes necessary for the synthesis of ribosomal RNA (rRNA) are organized, during the formation of ribosomes.

Regarding their location, the nucleoli are usually inside the nucleus but not exactly in the center, but slightly outwards. They are found in all eukaryotic cells, with the exception of sperm and certain amphibian cells. Its size varies depending on the animal or plant in question (usually between 1 and 3 micrometers), and they are usually one or two per cell, although this can also vary by species.

Nucleolus function

The nucleolus participates in cellular aging and cellular stress responses.

The main role of this structure is the biosynthesis of ribosomes, to form ribosomal RNA, vital for protein synthesis. In fact, the more intense the protein synthesis activity of a cell, the more nucleoli it will tend to have. Once synthesized, the RNA will mature and be transported from the nucleolus to its destination.

Other functions of the nucleolus involve cellular aging, cellular stress responses and the activity of telomerase, an essential ribonucleic enzyme for the elongation of DNA telomeres, that is, vital for genetic duplication and cell division.

This enzyme is abundant in fetal tissue, stem cells, and germ cells. Hence, the nucleolus intervenes in the regulation of the cell cycle itself, despite the fact that during these stages the nucleolus remains invisible, as if it disappeared. This, of course, during the phases of cell division.

Nucleolus cycle

The nucleolus undergoes an intense series of changes during cell division.

Like the chromosomes in the cell nucleus, the nucleolus undergoes an intense series of changes during cell division, a process in which it cannot be seen. During cell division, the nucleolus cycle occurs, which involves three distinct phases:

  • Prophetic disorganization. The nucleolus loses size and volume, becoming irregular, to allow the appearance of small masses of its own material, between the prophasic chromosomes that are condensing.
  • Metaphasic and anaphase transport. The nucleolus loses its individual character and allows its components to join the metaphase chromosomes.
  • Telophasic organization. Nucleoli reappear after, in telophase, the chromosomes are decondensed and laminar and prenucleolar bodies appear, which will gradually increase in size until one or more nucleoli form again.