Cell Nucleus: Structure and Function

Posted on May 12, 2024 in Biology

The Cell Nucleus

The cell nucleus, an organelle first described by Franz Bauer in 1802, is a structure found in eukaryotic cells. It contains the cell’s DNA and is bounded by the nuclear envelope, communicating with the cytoplasm through nuclear pores. The nucleus has two primary functions:

  1. To regulate chemical reactions within the cell.
  2. To store the cell’s genetic information.

Its diameter can range from 2 to 25 μm.

Besides genetic material, the nucleus also contains proteins that regulate gene expression. This involves complex processes like transcription, pre-processing of mRNA (messenger RNA), and transport of mRNA to the cytoplasm. Inside the nucleus is a structure called the nucleolus, responsible for producing ribosome subunits. The nuclear envelope separates chemical reactions occurring within the cytoplasm from those inside the nucleus while allowing communication between the two. This communication occurs through nuclear pores formed by the merging of the inner and outer membranes of the nuclear envelope.

The inner nucleus is composed of a matrix called the nucleoplasm, a gelatinous liquid similar to the cytoplasm. It contains various substances necessary for the nucleus’s operation, including nitrogenous bases, enzymes, proteins, and transcription factors. A fiber network within the nucleoplasm (called the nuclear matrix) has a function that is still being researched.

The DNA within the nucleus is generally organized as chromatin (euchromatin or heterochromatin) during interphase. During cell division, however, the genetic material is organized into chromosomes. Their position is usually central, mirroring the cell’s shape, but this can vary. Mammalian erythrocyte nuclei are absent.

Structure

The nucleus is the largest organelle in an animal cell.[4] In mammalian cells, the average diameter is typically around 11–22 μm, occupying 10% of the total volume.[5] The viscous liquid within the nucleus is called nucleoplasm and is similar to that found in the cytoplasm outside the nucleus.

Cytoskeleton

In animal cells, two networks of intermediate filaments provide structural support to the nucleus:

  • The nuclear lamina forms an organized network on the inner side of the envelope.
  • A less organized support system exists on the cytosolic face of the envelope.

Both systems provide structural support for the nuclear envelope and act as anchor points for chromosomes and nuclear pores.[5]

The nuclear lamina is mainly composed of proteins called lamins. Like all proteins, lamins are synthesized in the cytoplasm and transported into the nucleus, where they aggregate before being incorporated into the existing nuclear lamina network.[6] [7] Lamins can also be found within the nucleoplasm, where they form a regular structure[8] visible with fluorescence microscopy. The function of this structure is not yet fully understood, although it is known to be excluded from the nucleolus and present during interphase.[9] The structures formed by these lamins bind to chromatin, and their disruption inhibits the transcription of genes encoding proteins.[10]

Like other intermediate filament components, the lamin monomer contains an alpha-helix region. Two coiled monomers use this region to bind to one another, forming a coiled-coil dimer. Two of these dimeric structures align side-by-side in an antiparallel arrangement, forming a tetramer called a protofilament. Eight of these protofilaments arrange laterally and twist together, forming a rope-like structure. These filaments can assemble and disassemble dynamically, meaning their length depends on the rates of filament addition and removal.

Nuclear Envelope and Nuclear Pores

the nuclear envelope is composed of two membranes arranged in parallel (one indoor and one outdoor) and separated by 10 to 50 nanometers. The nuclear envelope completely surrounding the nucleus and separates the genetic material of the cell cytoplasm, serving as a barrier to free diffusion of macromolecules between the nucleoplasm and the cytoplasm [18] The nuclear membrane is continuous with the outer membrane of rough endoplasmic reticulum (RER), they were also covered with ribosomes . The space between the nuclear membrane is called the perinuclear space and is continuous with the lumen of the RER.

The nuclear pores provide aqueous channels through the casing, being composed of multiple proteins, collectively termed nucleoporinas. The pores are about 125 million daltons molecular weight and consist of about 50 (in yeast ) to 100 proteins (in vertebrates ). [4] The pores are 100 nm in overall diameter, however, the space through which substances diffuse freely is only 9 nm wide, due to the presence of regulatory systems in the center of the pore. This size allows the free passage of small molecules soluble in water while it prevents large molecules such as nucleic acids and proteins into or out inappropriately. These larger molecules to be transported into the nucleus of active way. The core of a typical mammalian cell is around 3000-4000 pores through its entire envelope, [19] with each one containing a ring structure, octagonal symmetry, the point where the inner and outer membranes are fused. [ 20] On this ring there is a basket-shaped structure which extends towards the nucleoplasm, and a series of filamentous extensions that reach the cytoplasm. Both structures serve to mediate binding to nuclear transport proteins.[4]

Most proteins, ribosomal subunits and some RNA are transported through the pores of a complex process mediated by a family of factors of transport called karyopherins . Those karyopherins that mediate movement into the nucleus are also called importins, while those that mediate the outward movement of the nucleus are called exportins. Most karyopherins interact directly with its cargo, although some use adapter proteins. [21] steroid hormones such as cortisol and aldosterone , like other small soluble soléculas, involved in signaling intecelular, can diffuse through the cell membrane to the cytoplasm, where they bind to nuclear receptors that are transported to the nucleus. In the nucleus, serve as transcription factors when together with its ligand, in the absence of ligand, many receptors function as histone deacetylases that repress gene expression. [4]

Nucleolus

The nucleolus is a structure present within the nucleus, not surrounded by membrane. It is sometimes classified as suborganelo. Formed around repeats of rDNA, DNA coding for ribosomal RNA (rRNA). These regions are called nucleolar organizing regions. The main role of the nucleolus is to synthesize rRNA and form the ribosomes. The structural cohesion of the nucleolus depends on its activity, since the formation of ribosomes results in the temporary association of nucleolar component, thereby facilitating formation of more ribosomes and then a larger association. This model is supported by observations that inactivation of rDNA results in a mixture of nucleolar components.[22]

The first step in the formation of the ribosome is the transcription of rDNA, performed by a protein called RNA polymerase I , resulting in a pre-rRNA precursor, large. This is cleaved into subunits 5.8S, 18S and 28S rRNA’s. [23] The transcription, post-transcriptional processing and formation of ribosome, occurs in the nucleolus, aided by small nucleolar RNA molecules (snoRNA, in English), some of which derived from splicing of introns in genes coding messenger RNA related to ribosomal functions. Ribosomal subunits are already formed larger structures that pass through the nuclear pores. [4]

When observed by electron microscope , the nucleolus can be seen as consisting of three distinct regions: an inner region (fibrillar center), surrounded by dense fibrillar component, which in turn is surrounded by the granular component. The transcription of rDNA occurs in the fibrillar center or at the border between the fibrillar center and dense fibrillar component. When rDNA transcription is increased, there is a detection of more fibrillar centers. Most of the cleavage and modification of rRNA occurs in the dense fibrillar component, whereas the later steps involving the assembly of proteins into ribosomal subunits, occurs in central granular

Function

The main function of the cell nucleus is to control gene expression and mediate DNA replication during the cell cycle . The core provides the site for transcription , which is separate from the site of translation in the cytoplasm. This allows a level of genetic regulation that is not available inprokaryotes.

Nuclear transport

The entry and exit of large molecules from the nucleus is closely controlled by nuclear pore complexes. Although small molecules can enter the nucleus without regulation, [41] macromolecules such as RNA and proteins require association with karyopherins called importins to enter the nucleus and exportins to leave. Cargo proteins that must be transferred from the cytoplasm to the nucleus contain nuclear localization signals linked by exportins. The ability of importins and exportins in transporting their cargo is regulated by GTPases , enzymes that hydrolyze the molecule of guanosine triphosphate to release energy. A more important GTPase involved in nuclear transport is called Ran , which can bind to GTP or GDP, depending on whether you are located in the nucleus or cytoplasm. While the importins depend on RanGTP to dissociate from its cargo, exportins require RanGTP to be able to connect to your load. [21]

The nuclear import depends on the importin bind to their cargo in the cytoplasm, and transport it through the nuclear pore to the nucleus. Within the nucleus, RanGTP acts to separate the cargo from importin, allowing it to leave the nucleus to be reused. The nuclear export is similar, in that the exportins binds to the cargo inside the nucleus, a process facilitated by RanGTP, after leaving through the nuclear pore, separated after their cargo in the cytoplasm.

Protein export, specialized, exist to effect the transfer of mature mRNA and tRNA to the cytoplasm after post-transcriptional modification is complete. This mechanism of quality control is important due to the central role of these molecules in the translation process of protein, a wrong expression of a protein due to incomplete excision of introns or incomporação incorrect amino acid, may have negative effects on the cell, RNA amended so incomplete that reaches the cytoplasm is degraded instead of being used for translation into proteins.[4]