The cell nucleus, or nucleus, is found in every cell of the so-called eukaryotes (living organisms with a nucleus). It is separated from the cytoplasm, the liquid substance within the cell by a membrane, but is capable of selective mass transfer with the cytoplasm via nuclear pores in the nuclear membrane. The nucleus, with its contained chromatin (DNA plus other proteins), functions as the control center of the cell.
What is the nucleus?
The nucleus is an organelle present in (almost) all cells of higher organisms. Living things with nuclei are called eukaryotes. The nuclei act as the control or command center of the cell and contain all the hereditary information except for the hereditary information for the mitochondria, which have their own DNA. The genetic information is present as so-called chromatin, which consists of double-helix filaments and certain proteins. During the division phase of the nucleus and the cell, the filaments arrange themselves into chromosomes. Compared to the cytoplasm, the interior of the cell, the usually spherical nucleus is separated by a two-layer membrane of lipids. Since the nucleus must be capable of selective mass transfer with the cytoplasm, the nuclear membrane is interspersed with so-called nuclear pores through which selective mass transfer takes place. Approximately in the center of the nucleus is the nuclear corpuscle (nucleolus), which copies instructions for protein assembly from the genes as so-called mRNA and tRNA. The mRNA and tRNA are virtually passed through the nuclear pores to the ribosomes in the cytoplasm as instructions for building proteins.
Anatomy and structure
Nuclei, which are usually spherical in shape, are separated from the cytoplasm of the cell by the nuclear membrane. In mammals, the nucleus reaches a diameter of 5 to 16 µm. The approximately 35 nm thick nuclear membrane consists of a bilayer of lipids and is nearly impermeable to aqueous solutions due to its hydrophobic properties. The cell membrane contains about 2,000 nuclear pores through which a selective, bilateral, exchange of substances takes place. The outer side of the membrane merges into the rough endoplasmic reticulum, while the inward side of the membrane is lined by a layer of microfilaments that provide stability to the membrane and form a clear boundary with chromatin. Chromatin is the main component of the nuclear interior and consists of disordered chromatin filaments that contain DNA and other proteins and arrange themselves into the species-specific chromosomes prior to nuclear and cell division. At about the center of the nucleus is the nuclear corpuscle (nucleolus), which is composed of a conglomerate of ribosomal RNA.
Function and tasks
The main functions of the nucleus are to store the genetic information of the whole organism and to control the metabolic processes of the cell, including nuclear and cell division during growth processes. The control of metabolic processes occurs in accordance with the genetic instructions available to the nucleus for this purpose. The genetic information for the entire organism is located in the cell nucleus in the form of chromatin filaments. The cell nuclei of all occurring tissue types always contain the entire blueprint of the organism except for that of the mitochondria, the power plants of the cell. The mitochondria contain their own DNA and are independent of the control center of the nucleus. The nucleus can selectively replicate or transcribe DNA sequences by means of its nuclear corpuscle and transport them through the nuclear pores into the cytoplasm, where within the ribosomes the RNA sequences are converted into “real” amino acid sequences for building proteins. To control the task of cell division, the nucleus causes chromatin filaments to assemble into species-specific chromosomes prior to division. This makes it easier to distribute the DNA to the daughter cell, and the genes can be held together better because the nuclear membrane dissolves during the division phase, so there is practically no recognizable nucleus left. After the division phase is complete, the endoplasmic reticulum again develops a nuclear membrane, and the structure of the chromosomes dissolves. Hereditary information is now again selectively available to the nucleus in the form of the chromatin filaments.
Diseases and disorders
Malfunctions originating from the nucleus can cause serious health problems.Specific symptoms can be directly or indirectly related to a malfunction in the cell nucleus. Mitochondriopathy, which is based on certain hereditary genetic defects, initially manifests itself in the fact that one or more proteins encoded in the cell nucleus, which are channelled into the mitochondria via the nuclear pores, lead to malfunctions in the mitochondria. Mitochondriopathy can lead to serious problems even at a young age because the energy supply via the mitochondria is impaired. This is less a true malfunction in coding and more a faulty “instruction” by a mutated DNA sequence. Another group of diseases, triggered by genetic defects and known as Hutchinson-Gilford syndrome (HGPS), is due to the fact that a certain protein that provides stability to the nuclear membrane is incorrectly coded. This leads to deformations of the cell nucleus with serious consequences. All known forms of HGPS trigger a dramatically accelerated aging process, so that the average life expectancy is only about 14 years. The extremely rare HGPS is triggered by gene defects and subsequently leads to direct malfunctions of the nuclear membrane. A German-Belgian research group links amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) to a failure of the protein TDP-43, which normally plays a role in protein coding in the nucleus. The research team found that TD-43 is deposited outside the nucleus and can no longer enter the nucleus through the nuclear pores, preventing it from performing its function there.
