CELL NUCLEUS: CHARACTERISTICS, FUNCTIONS, STRUCTURE - BIOLOGY - 2025

The cell nucleus is a fundamental compartment of eukaryotic cells. It is the most conspicuous structure of this cell type and has the genetic material. It directs all cellular processes: it contains all the instructions encoded in DNA to carry out the necessary reactions. It is involved in the processes of cell division.

All eukaryotic cells have a nucleus, except for a few specific examples such as mature red blood cells (erythrocytes) in mammals and phloem cells in plants. Similarly, there are cells with more than one nucleus, such as some muscle cells, hepatocytes, and neurons.

The nucleus was discovered in 1802 by Franz Bauer; However, in 1830 the scientist Robert Brown also observed this structure and became popular as its main discoverer. Due to its large size, it can be clearly observed under a microscope. In addition, it is an easy staining structure.

The nucleus is not a homogeneous and static spherical entity with dispersed DNA. It is a complex and intricate structure with different components and parts inside. In addition, it is dynamic and constantly changes throughout the cell cycle.

characteristics

The nucleus is the main structure that allows the differentiation between eukaryotic and prokaryotic cells. It is the largest cell compartment. Generally, the nucleus is close to the center of the cell, but there are exceptions, such as plasma cells and epithelial cells.

It is a sphere-shaped organelle of about 5 µm in diameter on average, but can reach 12 µm, depending on the type of cell. I can occupy approximately 10% of the total cell volume.

It has a nuclear envelope formed by two membranes that separates it from the cytoplasm. The genetic material is organized together with proteins within it.

Although there are no other membranous subcompartments within the nucleus, a series of components or regions within the structure that have specific functions can be distinguished.

Features

The nucleus is attributed an extraordinary number of functions, since it contains the collection of all the genetic information of the cell (excluding mitochondrial DNA and chloroplast DNA) and directs the processes of cell division. In summary, the main functions of the kernel are as follows:

Gene regulation

The existence of a lipid barrier between the genetic material and the rest of the cytoplasmic components helps to reduce the interference of other components in the functioning of DNA. This represents an evolutionary innovation of great importance for groups of eukaryotes.

Cutting and splicing

The splicing process of messenger RNA occurs in the nucleus, before the molecule travels to the cytoplasm.

The objective of this process is the elimination of introns (“pieces” of genetic material that are not coding and that interrupt the exons, areas that are coding) from the RNA. Later, the RNA leaves the nucleus, where it is translated into proteins.

There are other more specific functions of each kernel structure that will be discussed later.

Structure and composition

The nucleus consists of three defined parts: the nuclear envelope, the chromatin, and the nucleolus. We will describe each structure in detail below:

Nuclear envelope

The nuclear envelope is composed of membranes of a lipid nature and separates the nucleus from the rest of the cellular components. This membrane is double and between these there is a small space called the perinuclear space.

The inner and outer membrane system forms a continuous structure with the endoplasmic reticulum

This membrane system is interrupted by a series of pores. These nuclear channels allow the exchange of material with the cytoplasm because the nucleus is not completely isolated from the rest of the components.

Nuclear pore complex

Through these pores the exchange of substances occurs in two ways: passive, without the need for energy expenditure; or active, with energy expenditure. Passively, small molecules such as water or salts, smaller than 9 nm or 30-40 kDa, can enter and leave.

This occurs in contrast to high molecular weight molecules, which require ATP (energy-adenosine triphosphate) to move through these compartments. Large molecules include pieces of RNA (ribonucleic acid) or other biomolecules of a protein nature.

Pores are not simply holes that molecules pass through. They are large protein structures, which can contain 100 or 200 proteins and are called "nuclear pore complex". Structurally, it looks a lot like a basketball hoop. These proteins are called nucleoporins.

This complex has been found in a large number of organisms: from yeasts to humans. In addition to the cellular transport function, it is also involved in the regulation of gene expression. They are an indispensable structure for eukaryotes.

In terms of size and number, the complex can reach a size of 125 MDa in vertebrates, and a nucleus in this animal group can have about 2000 pores. These characteristics vary according to the taxon studied.

Chromatin

Chromatin is found in the nucleus, but we cannot consider it as a compartment of it. It is named for its excellent ability to color and be observed under the microscope.

DNA is an extremely long linear molecule in eukaryotes. Its compaction is a key process. The genetic material is associated with a series of proteins called histones, which have a high affinity for DNA. There are also other types of proteins that can interact with DNA and they are not histones.

In histones, DNA rolls up and forms chromosomes. These are dynamic structures and are not constantly found in their typical shape (the Xs and Ys that we are used to seeing in illustrations in books). This arrangement only appears during the processes of cell division.

In the rest of the stages (when the cell is not in the process of dividing), the individual chromosomes cannot be distinguished. This fact does not suggest that chromosomes are homogeneously or disorderly dispersed throughout the nucleus.

At the interface, the chromosomes are organized into specific domains. In mammalian cells, each chromosome occupies a specific “territory”.

Chromatin types

Two types of chromatin can be distinguished: heterochromatin and euchromatin. The first is highly condensed and is located on the periphery of the nucleus, so the transcription machinery does not have access to these genes. Euchromatin is organized more loosely.

Heterochromatin is divided into two types: constitutive heterochromatin, which is never expressed; and facultative heterochromatin, which is not transcribed in some cells and is in others.

The most famous example of heterochromatin as a regulator of gene expression is the condensation and inactivation of the X chromosome. In mammals, females have XX sex chromosomes, while males are XY.

For reasons of gene dosage, females cannot have twice as many genes in X as males. To avoid this conflict, an X chromosome is randomly inactivated (becoming heterochromatin) in each cell.

Nucleolus

The nucleolus is a very relevant internal structure of the nucleus. It is not a compartment delimited by membranous structures, it is a darker area of ​​the nucleus with specific functions.

The genes that code for ribosomal RNA, transcribed by RNA polymerase I, are grouped in this area. In human DNA, these genes are found in the satellites of the following chromosomes: 13, 14, 15, 21 and 22. These are nucleolar organizers.

In turn, the nucleolus is separated into three discrete regions: fibrillar centers, fibrillar components, and granular components.

Recent studies have accumulated more and more evidence of possible additional functions of the nucleolus, not only restricted to the synthesis and assembly of ribosomal RNA.

It is currently believed that the nucleolus may be involved in the assembly and synthesis of different proteins. Post-transcriptional modifications have also been evidenced in this nuclear zone.

The nucleolus is also involved in regulatory functions. One study showed how it was related to tumor suppressor proteins.

Cajal's bodies

The Cajal bodies (also called coiled bodies) bear this name in honor of their discoverer, Santiago Ramón y Cajal. This researcher observed these corpuscles in neurons in 1903.

They are small structures in the form of spheres and exist from 1 to 5 copies per nucleus. These bodies are very complex with a fairly high number of components, including these transcription factors and splicing-related machinery.

These spherical structures have been found in different parts of the nucleus, since they are mobile structures. They are generally found in the nucleoplasm, although in cancer cells they have been found in the nucleolus.

There are two types of Box bodies in the core, classified according to their size: large and small.

PML bodies

PML bodies (promyelocytic leukemia) are small spherical subnuclear areas of clinical importance, since they have been associated with viral infections and oncogenesis.

They are known by a variety of names in the literature, such as nuclear domain 10, Kremer bodies, and PML oncogenic domains.

A nucleus has 10 to 30 of these domains and has a diameter of 0.2 to 1.0 µm. Among its functions, the regulation of genes and RNA synthesis stands out.

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