12 julio, 2024

Nucleoplasm: what it is, characteristics, structure, functions, composition

What is the nucleoplasm?

He nucleoplasm It is the substance in which DNA and other nuclear structures, such as nucleoli, are immersed. It is separated from the cell cytoplasm through the nuclear membrane, but it can exchange materials with it through the nuclear pores.

Its components are mainly water and a series of sugars, ions, amino acids, and proteins and enzymes involved in gene regulation, including more than 300 different histone proteins. In fact, its composition is similar to that of the cell cytoplasm.

Within this nuclear fluid are also nucleotides, which are the «building blocks» used to build DNA and RNA, with the help of enzymes and cofactors. In some large cells, such as acetabulariathe nucleoplasm is clearly visible.

Previously it was thought that the nucleoplasm consisted of an amorphous mass enclosed in the nucleus, excluding the chromatin and nucleolus. However, inside the nucleoplasm there is a protein network responsible for organizing the chromatin and other components of the nucleus, called the nuclear matrix.

New techniques have been able to better visualize this component and identify new structures such as intranuclear sheets, protein filaments emerging from nuclear pores, and RNA processing machinery.

General characteristics of the nucleoplasm

– Nucleoplasm, also called “nuclear juice” or caryoplasm, is a protoplasmic colloid with properties similar to cytoplasm, relatively dense and rich in different biomolecules, mainly proteins.

– In this substance is the chromatin and one or two corpuscles called nucleoli. There are also other immense structures in this fluid such as Cajal bodies, PML bodies, spiral bodies or specks nuclear, among others.

– The structures necessary for the processing of pre-messenger RNAs and transcription factors are concentrated in the bodies of Cajal.

– The specks Nuclear cells appear to be similar to Cajal bodies, they are very dynamic and move towards regions where transcription is active.

– PML bodies appear to be markers of cancer cells, as their numbers increase dramatically within the nucleus.

– There is also a series of spherical nucleolar bodies ranging between 0.5 and 2 µm in diameter made up of globules or fibrils that, although they have been reported in healthy cells, are much more frequent in pathological structures.

Nucleoplasm structure

The most relevant nuclear structures that are embedded in the nucleoplasm are described below:

nucleoli

The nucleolus or nucleolus is a protruding spherical structure located inside the cell nucleus and is not delimited by any type of biomembrane that separates them from the rest of the nucleoplasm.

It is made up of regions called NORs (chromosomal nucleolar organizer regions) where the sequences that code for ribosomes are located. These genes are found in specific regions of the chromosomes.

In the specific case of humans, they are organized in the satellite regions of chromosomes 13, 14, 15, 21 and 22.

A series of essential processes occur in the nucleolus, such as transcription, processing, and assembly of the subunits that make up the ribosomes.

On the other hand, leaving aside its traditional function, recent studies have found that the nucleolus is related to proteins that suppress cancer cells, regulators of the cell cycle, and proteins from viral particles.

subnuclear territories

The DNA molecule is not scattered randomly in the cell nucleoplasm, it is organized in a highly specific and compact way with a set of highly conserved proteins throughout evolution called histones.

The DNA organization process allows almost four meters of genetic material to be inserted into a microscopic structure.

This association of genetic material and protein is called chromatin. This is organized in regions or domains defined in the nucleoplasm, being able to distinguish two types: euchromatin and heterochromatin.

Euchromatin is less compact and encompasses genes whose transcription is active, since transcription factors and other proteins have access to it in contrast to heterochromatin, which is highly compact.

The heterochromatin regions are found at the periphery and the euchromatin more in the center of the nucleus, and also close to the nuclear pores.

Similarly, chromosomes are distributed in specific areas within the nucleus called chromosomal territories. In other words, chromatin is not found floating around haphazardly in the nucleoplasm.

nuclear matrix

The organization of the different nuclear compartments seems to be dictated by the nuclear matrix.

It is an internal structure of the nucleus composed of a sheet coupled to the nuclear pore complexes, nucleolar remnants and a set of fibrous and granular structures that are distributed throughout the nucleus, occupying a significant volume of it.

Studies that have attempted to characterize the matrix have concluded that it is too diverse to define its biochemical and functional constitution.

The lamina is a kind of layer composed of proteins that spans from 10 to 20 nm and is juxtaposed to the inner face of the core membrane. The protein constitution varies depending on the taxonomic group studied.

The proteins that make up the lamina are similar to intermediate filaments and, in addition to nuclear signaling, have globular and columnar regions.

As for the internal nuclear matrix, it contains a high number of proteins with a binding site for messenger RNA and other types of RNA. In this internal matrix, DNA replication, non-nucleolar transcription, and post-transcriptional premessenger processing occur.

Nucleoskeleton

Inside the nucleus there is a structure comparable to the cytoskeleton in cells called the nucleoskeleton, made up of proteins such as actin, αII-spectrin, myosin and the giant protein called titin. However, the existence of this structure is still debated by researchers.

Composition

One of the main components of the nucleoplasm are ribonucleoproteins, composed of proteins and RNA made up of a region rich in aromatic amino acids with affinity for RNA.

Ribonucleoproteins found in the nucleus are specifically called small nuclear ribonucleoproteins.

biochemical composition

The chemical composition of the nucleoplasm is complex, including complex biomolecules such as nuclear proteins and enzymes, and also inorganic compounds such as salts and minerals such as potassium, sodium, calcium, magnesium, and phosphorus.

Some of these ions are essential cofactors for enzymes that replicate DNA. It also contains ATP (adenosine triphosphate) and acetyl coenzyme A.

Embedded in the nucleoplasm are a series of enzymes necessary for the synthesis of nucleic acids, such as DNA and RNA. Among the most important are DNA polymerase, RNA polymerase, NAD synthetase, pyruvate kinase, among others.

One of the most abundant proteins in the nucleoplasm is nucleoplasma, which is an acidic and pentameric protein that has unequal domains in the head and tail. Its acid characteristic manages to shield the positive charges present in the histones and manages to associate with the nucleosome.

Nucleosomes are those bead-like structures on a necklace, formed by the interaction of DNA with histones. Small molecules of a lipid nature have also been detected floating in this semi-aqueous matrix.

Functions of the nucleoplasm

The nucleoplasm is the matrix where a series of reactions that are essential for the proper functioning of the nucleus and the cell in general take place. It is the site where the synthesis of DNA, RNA and ribosomal subunits occurs.

It functions as a kind of «mattress» that protects the structures immersed in it, as well as providing a means of transporting materials.

It serves as a suspension intermediate for subnuclear structures and also helps to keep the shape of the nucleus stable, giving it rigidity and hardness.

Several metabolic pathways have been shown to exist in the nucleoplasm, as in the cell cytoplasm. Within these biochemical pathways are glycolysis and the citric acid cycle.

The pentose phosphate pathway, which supplies the pentoses to the nucleus, has also been reported. Similarly, the nucleus is a zone for the synthesis of NAD+, which functions as coenzymes for dehydrogenases.

Pre-messenger RNA processing

Pre-mRNA processing takes place in the nucleoplasm and requires the presence of small nucleolar ribonucleoproteins, abbreviated as snRNP.

Indeed, one of the most important active activities that occurs in the eukaryotic nucleoplasm is the synthesis, processing, transport, and export of mature messenger RNAs.

Ribonucleoproteins group together to form the spliceosome or splicing complex, which is a catalytic center responsible for removing introns from messenger RNA. A series of RNA molecules with a high uracil content is responsible for recognizing introns.

The spliciosome is composed of about five small nucleolar RNAs termed snRNA U1, U2, U4/U6, and U5, in addition to other proteins.

Let us remember that in eukaryotes the genes are interrupted in the DNA molecule by non-coding regions called introns that must be eliminated.

The reaction of splicing It integrates two consecutive steps: the nucleophilic attack in the 5′ cut zone by interaction with an adenosine residue contiguous to the 3′ zone of the intron (step that releases the exon), followed by the union of the exons.

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