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cells stained for DNA with the Blue Hoechst dye. The central and rightmost cell are in Interphase , thus their entire nuclei are labeled. The left cell is in the process of nuclear division ( Mitosis ) where separated Chromosome s can be identified.]] s: (1) Nucleolus (2) nucleus (3) Ribosome (4) Vesicle (5) rough Endoplasmic Reticulum (ER) (6) Golgi Apparatus (7) Cytoskeleton (8) Smooth ER (9) Mitochondria (10) Vacuole (11) Cytoplasm (12) Lysosome (13) Centriole s]] In Cell Biology , the nucleus (pl. ''nuclei''; from Latin or , kernel) is a membrane-enclosed Organelle found in most Eukaryotic Cells . It contains most of the cell's Genetic Material , organized as multiple long linear DNA molecules in complex with a large variety of Protein s, such as Histone s, to form Chromosome s. The Gene s within these chromosomes make up the cell's Nuclear Genome . The function of the nucleus is to maintain the integrity of these genes and to control the activities of the cell by regulating Gene Expression . The main structural elements of the nucleus are the Nuclear Envelope , a double membrane that encloses the entire organelle and keeps its contents separated from the cellular Cytoplasm , and the Nuclear Lamina , a meshwork within the nucleus that adds mechanical support much like the Cytoskeleton supports the cell as a whole. Because the nuclear membrane is impermeable to most molecules, Nuclear Pore s are required to allow movement of molecules across the envelope. These pores cross both membranes of the envelope, providing a channel that allows free movement of small molecules and Ion s. The movement of larger molecules such as proteins is carefully controlled, and requires active transport facilitated by carrier proteins. Nuclear Transport is of paramount importance to cell function, as movement through the pores is required for both gene expression and chromosomal maintenance. Although the interior of the nucleus does not contain any membrane-delineated bodies, its contents are not uniform, and a number of ''subnuclear bodies'' exist, made up of unique proteins, RNA molecules, and DNA conglomerates. The best known of these is the Nucleolus , which is mainly involved in assembly of Ribosome s. After being produced in the nucleolus, ribosomes are exported to the cytoplasm where they translate mRNA. HISTORY The nucleus was the first organelle to be discovered, and was first described by Franz Bauer in 1802.1 It was later described in more detail by Scottish Botanist Robert Brown in 1831 in a talk at the Linnean Society Of London . Brown was studying Orchid s microscopically when he observed an opaque area, which he called the areola or nucleus, in the cells of the flower's outer layer.2 He did not suggest a potential function. In 1838 Matthias Schleiden proposed that the nucleus plays a role in generating cells, thus he introduced the name "Cytoblast" (cell builder). He believed that he had observed new cells assembling around "cytoblasts". Franz Meyen was a strong opponent of this view having already described cells multiplying by division and believing that many cells would have no nuclei. The idea that cells can be generated de novo, by the "cytoblast" or otherwise, contradicted work by Robert Remak (1852) and Rudolf Virchow (1855) who decisively propagated the new paradigm that cells are generated solely by cells ("Omnis cellula e cellula"). The function of the nucleus remained unclear.3 Online Version here Between 1876 and 1878 Oscar Hertwig published several studies on the Fertilization of Sea Urchin eggs, showing that the nucleus of the Sperm enters the Oocyte and fuses with its nucleus. This was the first time it was suggested that an individual develops from a (single) nucleated cell. This was in contradiction to Ernst Haeckel 's theory that the complete Phylogeny of a species would be repeated during embryonic development, including generation of the first nucleated cell from a "Monerula", a structureless mass of primordial mucus ("Urschleim"). Therefore, the necessity of the sperm nucleus for fertilization was discussed for quite some time. However, Hertwig confirmed his observation in other animal groups, e.g. Amphibians and Molluscs . Eduard Strasburger produced the same results for plants (1884). This paved the way to assign the nucleus an important role in heredity. In 1873 August Weismann postulated the equivalence of the maternal and paternal germ ''cells'' for heredity. The function of the nucleus as carrier of genetic information became clear only later, after Mitosis was discovered and the Mendelian Rules were rediscovered at the beginning of the 20th century: the chromosome theory of heredity was developed. STRUCTURE The nucleus is the largest cellular Organelle in animals.4 In Mammal ian cells, the average diameter typically varies from 11 to 22 micrometers (μm) and occupies about 10% of the total volume.5 The viscous liquid within it is called Nucleoplasm , and is similar to the Cytoplasm found outside the nucleus. Nuclear envelope and pores See Also: Nuclear envelope Nuclear pores The Nuclear Envelope consists of two Cellular Membranes , an inner and an outer membrane, arranged parallel to one another and separated by 10 to 50 nanometers (nm). The nuclear envelope completely encloses the nucleus and separates the cell's genetic material from the surrounding cytoplasm, serving as a barrier to prevent Macromolecule s from diffusing freely between the nucleoplasm and the cytoplasm.6 The outer nuclear membrane is continuous with the membrane of the Rough Endoplasmic Reticulum (RER), and is similarly studded with Ribosomes . The space between the membranes is called the perinuclear space and is continuous with the RER Lumen . Nuclear Pores , which provide aqueous channels through the envelope, are composed of multiple proteins, collectively referred to as nucleoporins. The pores are about 125 million Daltons in Molecular Weight and consist of around 50 (in Yeast ) to 100 proteins (in Vertebrate s). The pores are 100 nm in total diameter; however, the gap through which molecules freely diffuse is only about 9 nm wide, due to the presence of regulatory systems within the center of the pore. This size allows the free passage of small water-soluble molecules while preventing larger molecules, such as Nucleic Acid s and proteins, from inappropriately entering or exiting the nucleus. These large molecules must be actively transported into the nucleus instead. The nucleus of a typical mammalian cell will have about 3000 to 4000 pores throughout its envelope,7 each of which contains a donut-shaped, eightfold-symmetric ring-shaped structure at a position where the inner and outer membranes fuse.8 Attached to the ring is a structure called the ''nuclear basket'' that extends into the nucleoplasm, and a series of filamentous extensions that reach into the cytoplasm. Both structures serve to mediate binding to nuclear transport proteins. Most proteins, ribosomal subunits, and some RNAs are transported through the pore complexes in a process mediated by a family of transport factors known as Karyopherin s. Those karyopherins that mediate movement into the nucleus are also called importins, while those that mediate movement out of the nucleus are called exportins. Most karyopherins interact directly with their cargo, although some use adaptor proteins.9 Steroid Hormone s such as Cortisol and Aldosterone , as well as other small lipid-soluble molecules involved in intercellular Signaling can diffuse through the cell membrane and into the cytoplasm, where they bind Nuclear Receptor proteins that are trafficked into the nucleus. There they serve as Transcription Factor s when bound to their Ligand ; in the absence of ligand many such receptors function as Histone Deacetylase s that repress gene expression. Cytoskeleton See Also: Nuclear lamina In animal cells, two networks of forms an organized meshwork on the internal face of the envelope, while less organized support is provided on the cytosolic face of the envelope. Both systems provide structural support for the nuclear envelope and anchoring sites for chromosomes and nuclear pores. The nuclear lamina is mostly composed of Lamin proteins. Like all proteins, lamins are synthesized in the cytoplasm and later transported into the nucleus interior, where they are assembled before being incorporated into the existing network of nuclear lamina.1011 Lamins are also found inside the nucleoplasm where they form another regular structure, known as the ''nucleoplasmic veil'',12 that is visible using Fluorescence Microscopy . The actual function of the veil is not clear, although it is excluded from the Nucleolus and is present during Interphase .13 The lamin structures that make up the veil bind Chromatin and disrupting their structure inhibits transcription of protein-coding genes.14 Like the components of other Intermediate Filament s, the lamin Monomer contains an Alpha-helical domain used by two monomers to coil around each other, forming a Dimer structure called a Coiled Coil . Two of these dimer structures then join side by side, in an Antiparallel arrangement, to form a Tetramer called a ''protofilament''. Eight of these protofilaments form a lateral arrangement that is twisted to form a ropelike ''filament''. These filaments can be assembled or disassembled in a dynamic manner, meaning that changes in the length of the filament depend on the competing rates of filament addition and removal. Mutations in lamin genes leading to defects in filament assembly are known as '' Laminopathies ''. The most notable laminopathy is the family of diseases known as Progeria , which causes the appearance of premature Aging in its sufferers. The exact mechanism by which the associated biochemical changes give rise to the aged Phenotype is not well understood.15 Chromosomes See Also: Chromosome nucleus in which DNA is stained blue. The distinct chromosome territories of chromosome 2 (red) and chromosome 9 (green) are visible stained with Fluorescent In Situ Hybridization .]] The cell nucleus contains the majority of the cell's genetic material, in the form of multiple linear DNA molecules organized into structures called Chromosome s. During most of the Cell Cycle these are organized in a DNA-protein complex known as Chromatin , and during cell division the chromatin can be seen to form the well defined Chromosome s familiar from a Karyotype . A small fraction of the cell's genes are located instead in the Mitochondria . There are two types of chromatin. Euchromatin is the less compact DNA form, and contains genes that are frequently Expressed by the cell.16 The other type, Heterochromatin , is the more compact form, and contains DNA that are infrequently transcribed. This structure is further categorized into ''facultative'' heterochromatin, consisting of genes that are organized as heterochromatin only in certain cell types or at certain stages of development, and ''constitutive'' heterochromatin that consists of chromosome structural components such as Telomere s and Centromere s.17 During interphase the chromatin organizes itself into discrete individual patches,18 called ''chromosome territories''.19 Active genes, which are generally found in the euchromatic region of the chromosome, tend to be located towards the chromosome's territory boundary.20 Antibodies to certain types of chromatin organization, particularly Nucleosome s, have been associated with a number of Autoimmune Disease s, such as Systemic Lupus Erythematosus .21 These are known as Anti-nuclear Antibodies (ANA) and have also been observed in concert with Multiple Sclerosis as part of general immune system dysfunction.22 As in the case of progeria, the role played by the antibodies in inducing the symptoms of autoimmune diseases is not obvious. Nucleolus See Also: Nucleolus of a cell nucleus, showing the darkly stained Nucleolus .]] | ||||||||||||||||||||||||||||||
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