There are trillions of cells in our bodies, and each one is special. Some cells have important functions, such as transporting substances. Other cells only absorb nutrients. But no matter what their task, almost all cells have one thing in common: they need a nucleus.
The nucleus is the center of a cell. It is a reaction site where many vital processes take place. A cell nucleus is also called a nucleus. The special on eukaryotes is that only they carry cell nuclei. These contain the DNA of the organism. In contrast, prokaryotes do not have a nucleus because the chromosomal DNA is free in the cytoplasm.
The naming of eukaryotes from ancient Greek means: eu = “true” and karyon = “nucleus”. In contrast, the naming of prokaryotes means as follows: pro = «before» and karyon = «nucleus».
Cell nucleus structure – drawing with inscription
The nucleus can be seen under the microscope as spherical to elliptical and, depending on the cell type, has a very different diameter, which can be between 5 and 600 microns. It is surrounded by a double membrane, i.e. two biomembranes, but contains no other, further compartments inside, in contrast to the mitochondrion.
Since the cell nucleus corresponds to about 10% of the total volume of an animal cell, it can also be called the largest cell organelle!
In fact, almost all cells have a nucleus. There are of course some exceptions: while mature red blood cells (erythrocytes) do not have a cell nucleus, liver cells, for example, can have several cell nuclei. In the case that there are several cell nuclei in the cytoplasm in a cell, they are called syncytium.
The origin of the cell nucleus has not yet been fully explained!
The following structural elements can be distinguished:
- nuclear envelope
- nuclear pores
- nuclear plasma
- chromatin
- chromosome
- nucleolus
The cell nucleus has no fixed compartment. It is an organelle that often breaks down.
cell membrane
the nuclear envelope or nuclear membrane consists of a double membrane and separates the interior of the nucleus from the cytoplasm.
The outer membrane of the nucleus merges into the rough endoplasmic reticulum (ER). Between the outer and the inner membrane there is an approx. 60 nm wide gap, the perinuclear cistern. The interior of the nuclear membrane is lined with a dense network of protein filaments that form the nuclear lamina. The task of the core lamina is to ensure the shape and stability of the membrane.
During cell division, the membrane is loosened. Later, the membrane is regenerated in the daughter cells.
nuclear pores
The nuclear envelope is with nuclear pores interspersed. nuclear pore complexesalso nuclear pore complex (NPC), consist of at least 30 different proteins and usually have a diameter of about 120 nanometers.
The nuclear pores are essential because they allow exchanges between the cytoplasm and the nucleus. This means that they represent a kind of membrane channel. This channel is surrounded by certain proteins that direct the entry and exit of the molecules.
All core proteins required for DNA replication and DNA transcription, as well as the DNA envelope proteins and ribosomal proteins, are synthesized on free ribosomes in the cytoplasm and must be imported in large quantities through the cytoplasm into the cell nucleus. Such large molecules as mRNA can pass through the membrane through the nuclear pores. A difference in this case is that after transcription, mRNA leaves the nucleus and is transported to the cytoplasm, while proteins are conversely transported to the nucleus.
nuclear plasma
That nuclear plasma is the viscous, also viscous or slimy, liquid that is inside the cell nucleus. She is also called nucleoplasm designated.
The core plasma mainly consists of water and contains all genetic information (chromatin) as well as core bodies (nucleoli). The fluid is not the same as the cytoplasm. Both differ in the composition of their ions.
chromatin
In general, chromatin can only be found in the nucleus of eukaryotic cells. A chromatin has two main functions:
- The packaging of the DNA to protect it from nucleases, as well as to transport the DNA and
- condensation and decondensation, which regulate gene activity.
The DNA in the nucleus of a eukaryote can never be free like in prokaryotes because it could attract any positively charged proteins. This would create a disordered structure, which would be very unstable. In eukaryotic cells DNA is packaged in proteins called histones to prevent structural instability. That’s how it is DNA protein complex before, the so-called chromatin. In this sense, the chromatin packages the large amount of eukaryotic DNA.
Histones are basic polypeptides around which DNA can wrap.
So no chromatin is formed in the prokaryotic cell, but the DNA is in the nucleoid with so-called Nucleoid-Associated Proteins (NAPs) that change the shape and structure of DNA and thus prevent instability of chromosomal DNA. Since prokaryotic DNA is very small, it does not need to be packaged.
The chromatin can be present in different degrees of compression:
- euchromatin is active at the level of transcription or replication and has a relatively loose structure.
- heterochromatin refers to the highly convoluted and gene-poor parts of the DNA strand.
histones are packaging proteins of the DNA and thus form a Histone-DNA complex. Histones are synthesized on free ribosomes in the cytoplasm and transported in large numbers to the cell nucleus.
Because histones have a strong positive base charge, they are immediately attracted to DNA, which is negatively charged, and a DNA-protein complex is formed. Histones are highly efficient DNA-binding proteins, but their association is independent of specific DNA sequences.
chromosomes
chromosomes are the compact transport form of DNA during cell division. Like heterochromatin, they are highly convoluted and genetically inactive.
Chromosomes are easily seen under a microscope and have a curved, rod-like appearance. You save all parts of the genetic information in the nucleus. In principle, all chromosomes have the same basic shape, but they are still different because they only carry specific parts of the genetic information.
Based on this slightly different structure, they can also be distinguished under the microscope. Chromosomes usually come in homologous (similar) pairs. This is known as a diploid set of chromosomes. This is due to the sexual reproduction that most eukaryotes engage in. The offspring receive one set of chromosomes from each parent.
nucleolus (nucleolus)
nucleoli are core bodies and not the actual cell core (nucleus).
There can be one or more nucleoli in a cell nucleus. Like the membrane, these dissolve during cell division. After cell division, the nucleoli are separated from specific sections of the chromosomes, called nucleolus Oorganizer regions (NOR), established.
Nucleoli mainly participate in the production of ribosomes.
Cell nucleus – function
The nucleus is a reaction space for many reactions. The cell nucleus is particularly important when it comes to the transmission of genetic information. Since the genetic information in the form of DNA and RNA for the reproduction of an organism as a whole is stored in the cell nucleus, the genetic material must be transported out of the cell nucleus without damaging the cell nucleus and the genetic material.
transcription, i.e. reading the DNA e.g. B. for protein production, and replication of the DNA, i.e. the copying of the DNA e.g. B. for cell division, take place directly in the cell nucleus. Here, the DNA is read and transcribed into a so-called mRNA, which is a form of RNA that can be transported out of the cell nucleus.
However, not all genetic information is present as chromosomes in the cell nucleus. Humans also have mitochondrial DNA, plants have chloroplast DNA. The replication of this genome is independent of processes in the nucleus.
You can find more information on translation and replication of DNA under the articles of the same name!
Nuclear Division – Phases
The cell division of eukaryotic organisms is also called mitosis. Nuclear division is necessary to form tissue cells that are constantly renewing themselves. The body’s ability to function and heal is based on the fact that dead cells are replaced with new cells.
Mitosis is divided into different stages depending on the state of the chromosomes: prophase, prometaphase, metaphase, anaphase and telophase.
prophase
During the prophase The chromosomes condense and shorten. At the same time, a spindle apparatus forms at the poles of the cell. At the end of prophase, the cell membrane breaks down.
Before the chromosomes are compressed, they are not visible under the light microscope. Only when the chromosomes are condensed are they separated from each other, seen under the microscope once they have been stained.
prometaphase
The second stage of mitosis is called prometaphase designated. During the prometaphase, the cell membrane completely disintegrates and the spindle apparatus forms at the poles of the cell.
metaphase
In the metaphase the spindle apparatus, which was formed before that in the prometaphase, is of great importance. At this stage of mitosis, the chromosomes are aligned at the equatorial plane of the cell.
anaphase
The next phase will anaphase called. During this process, the chromosomes separate, resulting in two identical daughter chromosomes. Also, the resulting daughter chromosomes separate.
telophase
The final stage of mitosis is called telophase denoted in which the nuclear envelope is restored. In addition, the spindle fibers dissolve completely and a new cell membrane forms. In the end, there are two functional cell nuclei.
What if the cell nucleus divides incorrectly?
There are several in the cell cycle checkpoints, which are intended to prevent errors occurring during the division of the nucleus. These «checkpoints» are located at different phases of the cell cycle where important processes take place. during the crossing-overs it can happen that the chromosomes are not separated. If something goes wrong, defective cells can develop, including tumor cells. These faulty cells are usually killed by programmed cell death (apoptosis), so that only healthy cells can continue dividing.
Cell nucleus and cell nucleus division – plant cell
Like all other eukaryotes, plant cells also have a nucleus and they divide according to the same principle of mitosis.
You can find other differences between plant and animal cells in the articles of the same name!
Cell nucleus – the most important things at a glance
- The cell nucleus is surrounded by a double membrane that encloses the nuclear plasma, in which the nucleoli and the genetic information in the form of chromosomes are located.
- The nucleus is not a fixed compartment. In the…