The nucleus
introduction
The cell nucleus or nucleus is the largest organelle in a cell and is located in the cytoplasm of eukaryotic cells. The rounded cell nucleus, bounded by a double membrane (nuclear envelope), contains the genetic information packed in chromatin, the deoxyribonucleic acid (DNA). As a store of genetic information, the cell nucleus is of central importance for heredity.
Function of the cell nucleus
All human cells except erythrocytes have a nucleus in which the DNA is in the form of chromosomes. The cell nucleus regulates and controls all processes that take place in a cell. For example, the instructions for the synthesis of proteins, the transmission of genetic information, cell division and various metabolic processes.
In addition to storing the genetic information, doubling (Replication) of DNA and the synthesis of ribonucleic acids (RNA) by transcribing the DNA (transcription), as well as the modification of this RNA (processing) to the most important functions of the cell nucleus.
In addition to the DNA in the cell nucleus, humans also have mitochondrial DNA in the mitochondria, the replication of which is completely independent of the nucleus. The information of many proteins that are necessary for the respiratory chain is stored here.
Find out more about this topic: Cellular respiration in humans
Illustration of a cell nucleus
- Cell nucleus -
Nucleus - Outer nuclear membrane
(Nuclear envelope)
Nucleolemma - Inner nuclear membrane
- Nuclear corpuscles
Nucleolus - Nuclear plasma
Nucleoplasm - DNA thread
- Nuclear pore
- Chromosomes
- cell
Celulla
A - nucleus
B - cell
You can find an overview of all images from Dr-Gumpert under: medical images
What is the nuclear substance?
Nuclear substance is the genetic information encoded in the nucleus. This is also known as DNA (deoxyribonucleic acid). A molecule of DNA or RNA is in turn made up of basic chemical building blocks, the nucleotides, and consists of a sugar (deoxyribose for DNA or ribose for RNA), an acidic phosphate residue and a base. The bases are called adenine, cytosine, guanine or thymine (or uracil in the case of RNA). DNA is unique because of the fixed sequence of the four bases, which differs in every person.
The DNA is not in the form of a free strand, but is wrapped around special proteins (histones), which are collectively known as chromatin. If this chromatin is compressed further, the chromosomes are ultimately formed, which are visible under the microscope in the metaphase of mitosis. The rod-shaped corpuscles are therefore the carriers of the genetic information and are involved in the division of the nucleus. A normal human body cell has 46 chromosomes that are arranged in pairs (double or diploid chromosome set). 23 chromosomes come from the mother and 23 chromosomes from the father.
Learn more about the DNA
Furthermore, the nucleus contains the nucleolus, which is particularly noticeable as a compacted zone. It consists of ribosomal RNA (rRNA).
Read more on the topic Ribosomes
What is the caryoplasm?
The caryoplasm is also known as the nuclear plasma or nucleoplasm. It describes the structures that lie within the nuclear membrane. In contrast, there is also the cytoplasm, which is bounded by the outer cell membrane (plasmalemm).
Read also: Cell plasma in the human body
These two rooms consist largely of water and various additives. An important difference between caryoplasm and cytoplasm are the different concentrations of electrolytes, such as Cl- (chloride) and Na + (sodium). This special milieu in the karyoplasm represents the optimal environment for the processes of replication and transcription. Chromatin, which contains the genetic material, and the nucleolus are also stored in the karyoplasm.
Nucleus size
Eukaryotic cell nuclei usually have a rounded shape and a diameter of 5-16 µm. The conspicuous nucleolus can be clearly seen in the light microscope and has a diameter of 2–6 µm. In general, the appearance and size of the cell nucleus strongly depend on the cell type and species.
The double membrane of the cell nucleus
The cell nucleus is separated from the cytoplasm by a double membrane. This double membrane is called the nuclear envelope and consists of an inner and an outer nuclear membrane, with the perinuclear space in between. Both membranes are connected to one another by pores and thus form a physiological unit (see next section).
In general, double membranes always consist of a lipid bilayer in which various proteins are embedded.These proteins can be modified with various sugar residues and enable the specific biological functions of the nuclear membrane.
Like all double membranes, the nuclear envelope has both a water-loving (hydrophilic) as well as a water-avoiding (hydrophobic) Portion and is therefore fat and water soluble (amphiphilic). In aqueous solutions, the polar lipids of the double membrane form aggregates and are arranged in such a way that the hydrophilic part faces the water, whereas the hydrophobic parts of the double layer are attached to one another. This special structure creates the conditions for the selective permeability of the double membrane, which means that cell membranes are only permeable to certain substances.
In addition to the regulated exchange of substances, the nuclear envelope also serves to delimit (Compartmentalization) of the cell nucleus and forms a physiological barrier so that only certain substances can get into and out of the cell nucleus.
Read more on the topic: Cell membrane
What are the nuclear pores needed for?
The pores in the membrane are complex channels with a diameter of 60 to 100 nm that form a physiological barrier between the nucleus and the cytoplasm. They are needed for the transport of certain molecules to or from the cell nucleus.
These molecules include, for example, mRNA, which is of great importance in replication and subsequent translation. The DNA is first copied in the cell nucleus so that the mRNA is created. This copy of the genetic material leaves the cell nucleus through a nuclear pore and reaches the ribosomes, where translation takes place.
Functions of the cell nucleus
Two elementary biological processes take place in the cell nucleus: on the one hand the replication of the DNA and on the other hand the transcription, i.e. the transcription of DNA into RNA.
During cell division (mitosis), the DNA doubles (replication). Only after the entire genetic information has been doubled can the cell divide and thereby form the basis for growth and cell renewal.
During transcription, one of the two strands of DNA is used as a template and converted into a complementary RNA sequence. A variety of transcription factors determine which genes are transcribed. The resulting RNA is modified in many further steps. The stable end product that can be exported into the cytoplasm and ultimately translated into protein building blocks is called messenger RNA (mRNA).
Find out more about this: Tasks of the cell nucleus
What happens when the cell nucleus divides?
Cell nucleus division is understood to be the division of a cell nucleus, which can take place in two different ways. The two types, mitosis and meiosis, differ in their process and also in their function. Depending on the type of nucleus division that has taken place, different daughter cells are obtained.
After mitosis has ended, you have two daughter cells that are identical to the mother cell and that also have a diploid set of chromosomes. This type of cell nucleus division predominates in the human organism. Their function is the renewal of all cells, such as those of the skin cells or those of the mucous membrane cells. Mitosis takes place in several phases, but there is only one real division of chromosomes.
In contrast to this, meiosis consists of a total of two core divisions. The result of a completed meiosis are four cells that contain a haploid set of chromosomes. These germ cells are necessary for sexual reproduction and are therefore only found in the genital organs.
In women, it is the egg cells that are present in the ovaries from birth. In male organisms, the sperm are produced in the testes and are ready for fertilization.
If you are further interested in this topic, then read our next article below: Meiosis - simply explained!
When the egg cell and sperm fuse during fertilization, two haploid sets of chromosomes form a cell with one set of diploids.
Read more on the topic: Cell nucleus division
What is a cell nucleus transfer?
A nucleus transfer (synonym: nucleus transplantation) is the introduction of a nucleus into an egg cell without a nucleus. This was artificially produced in advance, for example using UV radiation. The now nucleated egg cell can then be inserted into a sexually mature individual and carried to term. In this way, the formerly nucleated cell receives genetic information and changes as a result.
This procedure represents a type of asexual fertilization and was first used in 1968. There are therapeutic approaches that aim to produce specific tissues from stem cells that can be used for transplants. In addition, somatic cell nuclear transfer can be used for cloning. However, for ethical reasons, this is only allowed for animals, although it is also controversial here, as many animals die during this process or are born sick. The most famous example is the cloned sheep Dolly. This cloned sheep was genetically identical to its mother animal.
The nucleus of a nerve cell
Nerve cells (neurons) are terminally differentiated cells. In contrast to other cells, they can no longer divide. However, neurons have the ability to regenerate and specific repetition of tasks ("brain training") increases the plasticity of the brain.
The cell nucleus sits in the cell body (soma) of the nerve cell. The nuclear envelope contains myelin, a substance that occurs specifically in the nervous system, and has only a lower protein content than other double membranes.
The reception and transmission of information in the form of electrical impulses (action potentials) is the most important task of neurons. Neurotransmitters are chemical messengers that allow nerve cells to communicate with one another. As the control center of the neuron, the cell nucleus primarily regulates the production of the various messenger substances and the expression of the respective receptors.
By binding a neurotransmitter to the appropriate receptor, the corresponding effect is transferred to the nerve cell. It is crucial that there are no transmitter-specific effects, but only receptor-specific effects. This means that the effect of the messenger substance depends on the receptor.
