The cell nucleus is the largest organelle of eukaryotic cells and is located in the cytoplasm, separated by a double membrane (nuclear envelope). As a carrier of genetic information, the cell nucleus contains the genetic information in the form of chromosomes (DNA strand) and thus plays an essential role in heredity. Most mammalian cells have only one nucleus; this is roundish and has a diameter of 5 to 16 micrometers. Certain cell types, such as muscle fibres or specialised cells in bone, may have more than one nucleus.
Functions of the cell nucleus
The cell nucleus is the most important organelle of a cell and makes up 10 -15 % of the cell volume. The cell nucleus contains most of the genetic information of a cell. In humans, in addition to the cell nucleus, the mitochondria also contain DNA (“mitochondrial DNA”).
However, the mitochondrial genome only codes for a few proteins, which are mainly required in the respiratory chain for energy production. As a store of deoxyribonucleic acid (DNA), the cell nucleus is considered the control centre of the cell and regulates many important processes of cell metabolism. The cell nucleus is essential for the function of a cell.
Cells without a cell nucleus usually cannot survive. An exception to this is the nucleusless red blood cells (erythrocytes). In addition to regulatory functions, the tasks of the cell nucleus include the storage, duplication and transmission of DNA.
The DNA is located in the cell nucleus in the form of a long, strand-like double helix and is compactly packed with nuclear proteins, the histones, to form chromosomes. Chromosomes consist of chromatin, which only condenses into microscopically visible chromosomes during cell division. Every human cell contains 23 chromosomes, each of which is duplicated and inherited from both parents.
One half of the genes in a cell therefore comes from the mother, the other from the father. The cell nucleus controls metabolic processes within the cell through messenger molecules of RNA. The genetic information codes for proteins that are responsible for the function and structure of the cell.
When necessary, certain sections of DNA, known as genes, are transcribed into a messenger substance (messenger RNA or mRNA). The mRNA formed leaves the cell nucleus and serves as a template for the synthesis of the respective proteins. One can imagine that the DNA is a kind of encoded language consisting of four letters.
These are the four bases: adenine, thymine, guanine and cytosine. These letters form words, each consisting of three bases, called codons. Each codon codes for a specific amino acid and thus forms the basis for protein biosynthesis, because the sequence of bases of the genes is translated into a protein by linking the respective amino acids.
All this encoded information is called the genetic code. The specific sequence of bases makes our DNA unique and determines our genes. But not only bases are involved in the construction of DNA.
DNA is made up of nucleotides that are strung together, which in turn consist of a sugar, a phosphate and a base. The nucleotides form the backbone of the DNA, which is in the form of a helical double helix. In addition, this strand is further condensed so that it fits into the small cell nucleus.
This is also referred to as chromosomes as the packaging form of DNA. With each cell division, the complete DNA is copied so that each daughter cell contains the complete identical genetic information. A chromosome is a specific packaging form of our genetic material (DNA) that is only visible during cell division.
DNA is a linear structure that is far too long to fit into our cell nucleus in its natural state. This problem is solved by various space-saving spirals of the DNA and the incorporation of small proteins around which the DNA can wrap itself further. The most compact form of DNA are the chromosomes.
Under the microscope, these appear as rod-shaped bodies with a central constriction. This form of DNA can only be observed during cell division, i.e. during mitosis. Cell division in turn can be divided into several phases, whereby the chromosomes are best represented in the metaphase.
Normal somatic cells have a double set of chromosomes, consisting of 46 chromosomes. RNA describes ribonucleic acid, which has a similar structure to DNA. However, it is a single-stranded structure that differs from DNA in individual building blocks.
In addition, RNA is also much shorter than DNA and has several different tasks compared to DNA. RNA can be further divided into different RNA subgroups that perform different tasks. Among other things, mRNA plays an important role during cell nucleus division.
Like tRNA, it is also used in the production of proteins and enzymes. Another subgroup of RNA is rRNA, which is a component of ribosomes and is therefore also involved in the production of proteins. The first step in protein biosynthesis is the transcription of DNA into mRNA (transcription) and takes place in the cell nucleus.
During this process, one strand of DNA serves as a template for a complementary RNA sequence. However, since no proteins can be produced within the cell nucleus, the mRNA formed must be discharged into the cytoplasm and transported to the ribosomes, where the actual synthesis of the proteins ultimately takes place. Within the ribosomes, the mRNA is converted into a sequence of amino acids that are used for the construction of proteins.
This process is called translation. However, before the messenger RNA can be transported out of the nucleus, it is first processed in many steps, i.e. certain sequences are either attached or cut out and reassembled. In this way, different protein variants can be produced from one transcript.
This process enables humans to produce a large number of different proteins with relatively few genes. Another important function of the cell, which takes place in the cell nucleus, is the duplication of DNA (replication). In a cell, there is a constant cycle of building up and breaking down: old proteins, pollutants and metabolic products are broken down, new proteins have to be synthesised and energy has to be produced.
In addition, the cell grows and divides into two identical daughter cells. However, before a cell can divide, the entire genetic information must first be duplicated. This is important because the genetic material of all cells within an organism is absolutely identical.
Replication takes place at a precisely defined time of cell division in the cell nucleus; both processes are closely linked and are regulated by certain proteins (enzymes). First the double-stranded DNA is separated and each single strand serves as a template for the subsequent duplication. For this purpose, various enzymes dock to the DNA and complete the single strand to form a new double helix.
At the end of this process, an exact copy of the DNA has been produced, which can be passed on to the daughter cell during division. However, if errors occur in one of the cell cycle phases, different mutations can develop. There are several types of mutations that can occur spontaneously during different cell cycle phases.
For example, if a gene is defective, this is called a gene mutation. However, if the defect affects certain chromosomes or parts of chromosomes, then it is called a chromosomal mutation. If the number of chromosomes is affected, it then leads to a gene mutation.
The topic might also be of interest to you: Chromosomal aberration – what is meant by this? The double membrane of the nuclear envelope has pores that serve the selective transport of proteins, nucleic acids and signalling substances out of or into the nucleus. Through these pores, certain metabolic factors and signalling substances enter the nucleus where they influence the transcription of certain proteins.
The conversion of genetic information into proteins is strictly monitored and is regulated by many metabolic factors and signalling substances, this is called gene expression. Many signalling pathways that occur in a cell end in the nucleus where they influence the gene expression of certain proteins. Within the nucleus of eukaryotic cells is the nucleolus, the nucleic body.
A cell can contain one or more nucleoli, whereas cells that are very active and divide frequently can contain up to 10 nucleoli. The nucleolus is a spherical, dense structure that is clearly visible under the light microscope and is clearly delineated within the cell nucleus. It forms a functionally independent area of the nucleus, but is not surrounded by its own membrane.
The nucleolus consists of DNA, RNA and proteins, which lie together in a dense conglomerate. The maturation of the ribosomal subunits takes place in the nucleolus. The more proteins are synthesized in a cell, the more ribosomes are needed and therefore metabolically active cells have several nucleoli.
The nucleus in a nerve cell has a variety of functions. The nucleus of a nerve cell is located in the cell body (soma) together with other cell components (organelles), such as the endoplasmic reticulum (ER) and the Golgi apparatus. As in all body cells, the cell nucleus contains the genetic information in the form of DNA.
Due to the presence of DNA, other body cells are able to duplicate themselves via mitosis. Nerve cells, however, are very specific and highly differentiated cells that form part of the nervous system. As a result, they are no longer able to duplicate themselves.
However, the cell nucleus performs another important task. Among other things, the nerve cells are responsible for the excitation of our muscles, which ultimately leads to muscle movement. The communication between the nerve cells among themselves and between the nerve cells and the muscles takes place via messenger substances (transmitters).
These chemical substances and also other important life-sustaining substances are produced with the help of the cell nucleus. Not only the cell nucleus but also the other components of the soma play an important role in this process. Furthermore, the cell nucleus controls all metabolic pathways in all cells, including nerve cells. For this purpose, the cell nucleus contains all our genes, which can be read and translated into the required proteins and enzymes depending on their use. Further information about the special feature of the nerve cell can be found at Nerve cell