Cell nucleus
As a nucleus, nucleus or nucleus (Latin nucleus " core " ) is defined as a located in the cytoplasm, usually roundish shaped organelle of the eukaryotic cell which contains the genetic material. The nucleus is expressed with a reference to the cell nucleus. Likewise, with or Karyo Karyo ( ancient Greek κάρυον Karyon "core" ), for example in eukaryotes or karyotype.
The nucleus is the main feature to distinguish between eukaryotes ( organisms with membrane bound nucleus ) and prokaryotes (living without defined nucleus, ie bacteria and archaea ). It contains the major part of the genetic material of the eukaryotic cells in the form of multiple chromosomes. Other genes are found in the mitochondria and in plants in chloroplasts. Most cells contain exactly one core. However, there are exceptions. For example, contain myotubes caused by fusion of myoblasts, multiple cores. In embryos of the fruit itself cores share very quickly without first result separating cell membranes. Mature erythrocytes of mammals contain no core anymore, he is repelled during maturation.
Important processes that occur within the cell nucleus, DNA replication ( duplication of the present in the form of DNA genetic material ) and transcription (equivalent to creating an mRNA copy of a given DNA segment, which is often, but not always, a gene ).
Construction
Nuclei can look very different depending on the cell type. Mostly they are spherical or oval. In some cells, they look more antler shaped. Sometimes the nucleus can be subdivided into nodular sections, so the rosary -like nucleus of the trumpet animals. The granulocytes of mammals contain lobed nuclei.
The cell core, which in mammals is typically a diameter of 5 to 16 micrometre is the easiest in the microscope to be recognized organelle of the cell. It is composed of two biological membranes, the inner and outer nuclear membrane is limited by the nuclear envelope, the so-called perinuclear which cistern ( 10-15 nm width, consolidated microfilaments - thickness of 2 to 3 nm) enclose. The total thickness of the core sleeve is about 35 nm, the outer nuclear membrane merges into the rough endoplasmic reticulum and ribosomes, and this has as on its surface. The inner nuclear membrane is adjacent to a 20-100 nm wide " felt " of the nuclear lamina (lamina fibrosa nuclei ), which consists of lamins, a group of intermediate filaments, the nucleus supports and separates the inner membrane of the chromatin of the cell nucleus. By the contained nuclear pores in the nuclear envelope, covering approximately 25% of the surface active agent is of the exchange (for example, rRNA or mRNA) from the nucleus and the cytoplasm, driven by a nuclear pore complex instead. Regulatory proteins get out of the cytoplasm into the cell nucleus, the transcription products such as the mRNA for protein synthesis, taking place on the ribosomes of the cytoplasm are exported from the nucleus into the plasma. The fluid in the core is referred to as nucleoplasm. Nuclei can be highlighted by light microscopy by staining the DNA, for example by the Feulgen staining with Giemsa or with fluorescent dyes such as DAPI.
Light microscopy fall into many nuclei, one or more roundish formations on the nucleolus or nucleoli. They contain the genes for ribosomal RNA. Here the subunits of ribosomes are formed which pass through the nuclear pores into the cytoplasm. Nucleoli included in comparison to the rest of the core, only low levels of DNA, but instead more RNA. Other " corpuscles " of the nucleus can be represented only by special staining techniques, such as by antibody staining. The function of these bodies is often still unknown. These include for example " speckles " ( collections of factors that are required for splicing ), Cajal bodies or PML bodies.
The presence of a nuclear matrix was first proposed in the 1970s. However, their existence is still controversial.
The existing genetic material in the nucleus of the cell is located in the chromosomes, several packaged into chromatin DNA strands, which in addition to the DNA and proteins such as histones. In addition to the histones and other nuclear proteins such as DNA polymerases and RNA polymerases, transcription factors, as well as more ribonucleic acids in the core occur.
Arrangement of chromosomes
Take chromosomes during interphase demarcated areas in the nucleus, which chromosome territories. Their existence was first described by Carl Rabl (1885 ) and Theodor Boveri proposed ( 1909), the direct detection was achieved only in 1985 with the help of fluorescent in situ hybridization.
The distribution of chromatin and thus the chromosomes within the nucleus appears random at first glance: the arrangement of chromosomes changes to each other from nucleus to nucleus, neighbors in a can in the next are far apart. Since the 1990s, however, some principles of order could be found. The DNA replication occurs during the S phase is not uniform, but earlier in some places of the chromosomes, to others later. Early or late replication are properties that are constant for all sections of the chromosomes in a given cell type. It turned out that early replicated regions are predominantly in the interior of the core, while late replicated regions are predominantly localized to the nuclear envelope and the nucleoli around. For the arrangement of the chromosome territories in the nucleus was observed that chromosomes with high gene density preferably lie in the center of the core are low during chromosomal gene density often found at the periphery. For some cell types has also been reported that small chromosomes are rather large in the middle while the outside are. Both motifs are compatible.
Nuclear division
In mitosis and meiosis, occurring at the eukaryotic cell types of nuclear division, the nucleus disappears temporarily, because the nuclear envelope is dissolved during the division process. While chromosomes do not form a light- microscopically visible boundaries in the interphase, they condense for nuclear division to the compact metaphase chromosomes. In this form of transport the genetic material is distributed to the daughter cells. After the division of the nuclear envelopes form around the chromosomes of the daughter cells from again and the chromosomes decondense again.
History of Research
The nucleus is first discovered organelle of the cell. The oldest preserved drawing dates back to the early microscopist Antoni van Leeuwenhoek ( 1632-1723 ). The examined red blood of the salmon, and described therein, a " lumen " of the nucleus. As opposed to the red blood cells of mammals have those of the other vertebrate cell nuclei. A further mention was made in 1804 by Franz Andreas Bauer .. 1831, the nucleus of the Scottish botanist Robert Brown in a lecture to the Linnaean Society in London was described as " areola " He did not mention Possible meanings. Such was first proposed in 1838 by Matthias Schleiden, namely that it plays a role in the formation of the cell. Therefore Schleiden bore the name '' ' cytoblast ''' ( Zellenbildner ) a. He said to have observed that new cells formed in these Cytoblasten. Franz Julius Ferdinand Meyen disagreed strongly believes that "the core -producing cells, the cell itself ." He had already described above, that the proliferation of cells by division. However, Meyen was also of the view that many cells would have no nuclei. Was overcome the idea of formation of new cells only by the work of Robert Remak (1852 ) and Rudolf Virchow ( " Omnis cellula e cellula ", 1855), which represented the new doctrine of the exclusive formation of cells from cells offensively. The function of the nucleus remained unclear.
1876-1878 Oscar Hertwig published, which showed a number of studies on the fertilization of the sea urchin egg operations that the nucleus of the sperm enters the egg and merges there with the nucleus of the egg. This was asserted for the first time that an individual from a ( single ) nucleated cell was developing. This contradicted the view advocated by Ernst Haeckel, that during embryonic development the entire evolutionary history would be repeated, especially the emergence of the first nucleated cell from a " Monerula ", a structureless mass of primordial slime. The necessity of the sperm nucleus for fertilization was therefore still discussed controversially for a long time. However, Hertwig confirmed his findings to other groups of animals, such as amphibians and molluscs. Eduard Strasburger came to plant the same result (1884 ). This paved the way to assign the nucleus an important role in heredity. 1873, August Weismann postulated the equivalence of the maternal and paternal germ cells for heredity. The role of the core in this case was later revealed, after mitosis was described in the early 20th century, Mendel's laws were rediscovered: The chromosome theory of inheritance has been developed (see there and chromosome).
1874 the Friedrich Miescher isolated from nuclei of a substance which he called nuclein (see history of discovery in the article DNA). Only a few years later, other ingredients such as histones and adenine (Albrecht Kossel ) were added. It is only now possible, the genome, the transcriptome or the proteome by mass spectrometry also a cell to determine at any given time to make a full picture of this system.