All living organisms in the world (except the viruses) are divided into two groups – prokaryotes, whose nucleic acid simply lies in the thickness of the cytoplasm, and eukaryotes, whose nucleic acid is located in a special compartment called the nucleus. In this topic, we will talk about the origin of the nucleus, its structure, and its main functions.
Origin of the nucleus
Initially, all organisms on Earth were nuclear-free. It is believed that nuclear cells appeared about 2.7 billion years ago – it is in such rocks that traces of the first eukaryotes are found. Despite the fact that the nucleus is one of the most important compartments of the eukaryotic cell, the mechanism of its origin has not yet been clarified. There are several main theories, each has both supporters and critics.
One theory suggests that the nucleus arose as a result of endosymbiosis, from an archaea or another bacterium. There is also a viral theory, which says that at some point a virus-infected cell did not destroy the virus, but "left" it as a convenient storage place. This theory is based on the fact that some viruses do have common features with the nucleus – for example, the structure of DNA polymerases. Lastly, there is a theory that the nucleus is the "original" cell, which at some point formed an additional membrane. Subsequently, the membrane increased, filled with cytoplasm and other organelles.
The structure of the nucleus
The nucleus is a large organelle surrounded by a double membrane – nuclear envelope. There is a nucleus in almost all cells of the body, with the exception of narrowly specific ones, for example, erythrocytes. In addition, there are cells with many nuclei – they are formed due to the fusion of several cells.
Inside the nucleus there is a nuclear matrix, and in it is a network of filaments that create a nuclear lamina – a frame for maintaining the architecture of the nucleus. Transport is constantly taking place between the nucleus and the cytoplasm of the cell – both various molecules and a large RNA molecule. Therefore, there are nuclear pores in the nuclear membrane and carrier proteins. Some substances pass through the nuclear membrane passively, without energy expenditure – these are small molecules and ions. Large molecules, like RNA, require energy. The pores provide selective transport. Pores are generally the same in all living organisms. Proteins-nucleoporins penetrate the entire membrane, forming a channel.
The DNA inside the nucleus is usually tightly condensed and forms chromosomes. A human individual has two copies of each chromosome. The chromosomes are made up of 2 sister chromatids which are attached by a centromere. Centromeres have a disc-shaped structure known as kinetochores. Condensation of chromosomes is achieved by twisting the nucleic acid around special proteins – histones. Histones are a very broad family of proteins. All eukaryotes and archaea have them, but not bacteria. Histones are basic and bind DNA (acid) well. There are two main types of histones – core (they form a dense structure) and linker (they sit at the "end" of the complex, fixing the complex).
The total condensed DNA is called chromatin. There are 2 types of chromatin – euchromatin, which is very dense, and heterochromatin, which is loose. Looser DNA is used for protein synthesis more often, so it is energetically beneficial to store it in a non-super-condensed state. In addition, super-condensation will allow you to "hide" important parts of the DNA, which helps to avoid mutations. This can be compared to winter clothes and trekking clothes. Winter clothes can be needed at any time of the year, so they are stored folded in a closet, but in an accessible area. Trekking items may next be needed in five years, so it's more convenient to put them far away in the attic.
Ribosomes are created in a special structure called the nucleolus and then transferred to the cytosol. In addition to the nucleolus, there are other formations in the nucleus, the function of which is to synthesize certain molecules – for example, the Cajal body, in which small RNAs are synthesized.
DNA and regulation of transcription and translation
The main function of the nucleus is the storage of DNA, as well as the formation of a system for its "transfer" to a copy (RNA). It allows convenient and safe storage of DNA, preventing unnecessary mutations and interaction with dangerous molecules. In the nucleus, DNA is not only stored, but also undergoes replication – doubling of the nucleic acid, and is also repaired. Accordingly, there are many enzyme complexes in the nucleus – for example, a complex of polymerases and reparation enzymes.
Moreover, the nucleus actually regulates the rate of protein synthesis. In prokaryotes, the processes of transcription and translation occur simultaneously due to the physical location of all participants in the same place. The nucleus, more precisely, the nuclear envelope, helps to spread these processes geographically. In addition, this helps to avoid the situation when the protein begins to read from unprepared RNA that has not undergone, for example, splicing or some other processing.
Besides, not all DNA is equally "useful": exons (parts of DNA containing genes) and introns ("junk" DNA) are isolated. For DNA synthesis, introns are removed from the "final" RNA. This avoids the situation of "immediate" reading of RNA and synthesis of incorrect proteins.
Nucleus as a ribosome center
Ribosomes are also assembled in the nucleus – molecular machines that "translate" RNA into protein. Protein synthesis on ribosomes occurs in the cytosol, outside the nucleus, but the subunits themselves are synthesized in the nucleus. Subunits are synthesized in the nucleolus, after which the processing of ribosomal RNA takes place there. After that, the synthesized subunits are ready for assembly and are transported through nuclear carrier proteins into the cytoplasm.
The nucleolus is a temporary structure (it disappears during mitosis) that gathers around those sections of DNA in which special nucleolar RNA genes are encoded. A special component is formed around the nucleolus that binds RNA polymerase.
Conclusion
The nucleus is a large organelle that stores DNA. DNA can be in the form of slightly condensed heterochromatin, and very condensed euchromatin. In addition, the synthesis of ribosome subunits occurs in the nucleolus. The nucleus helps separate the processes of translation and transcription in space, which contributes to the finer tuning of protein synthesis.