Cell nucleus HeLa cells stained for the cell nucleus DNA with the Blue Hoechst dye. The central and rightmost cell are in interphase, thus their entire nuclei are labeled. On the left, a cell is going through mitosis and its DNA has condensed. Cell biology The animal cell Components of a typical animal cell: 1. Nucleolus 2. Nucleus 3. Ribosome (little dots) 4. Vesicle 5. Rough endoplasmic reticulum 6. Golgi apparatus (or Golgi body) 7. Cytoskeleton 8. Smooth endoplasmic reticulum 9. Mitochondrion 10. Vacuole 11. Cytosol (fluid that contains organelles) 12. Lysosome 13. Centrosome 14. Cell membrane In cell biology, the nucleus (pl. nuclei ; from Latin nucleus or nuculeus , meaning kernel) is a membrane -enclosed organelle found in eukaryotic cells. It contains most of the cells genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins , such as histones , to form chromosomes . The genes within these chromosomes are the cells 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 nucleus is, therefore, the control center of the cell. The main structures making up the nucleus are the nuclear envelope , a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm , and the nucleoskeleton (which includes nuclear lamina ), a network within the nucleus that adds mechanical support, much like the cytoskeleton , which supports the cell as a whole. Because the nuclear membrane is impermeable to large molecules, nuclear pores are required that regulate nuclear transport of molecules across the envelope. The pores cross both nuclear membranes, providing a channel through which larger molecules must be actively transported by carrier proteins while allowing free movement of small molecules and ions . Movement of large molecules such as proteins and RNA through the pores is required for both gene expression and the maintenance of chromosomes. The interior of the nucleus does not contain any membrane-bound sub compartments, its contents are not uniform, and a number of sub-nuclear bodies exist, made up of unique proteins, RNA molecules, and particular parts of the chromosomes. The best- known of these is the nucleolus , which is mainly involved in the assembly of ribosomes . After being produced in the nucleolus, ribosomes are exported to the cytoplasm where they translate mRNA. History Oldest known depiction of cells and their nuclei by Antonie van Leeuwenhoek, 1719 Drawing of a Chironomus salivary gland cell published by Walther Flemming in 1882. The nucleus contains Polytene chromosomes . The nucleus was the first organelle to be discovered. What is most likely the oldest preserved drawing dates back to the early microscopist Antonie van Leeuwenhoek (1632– 1723). He observed a Lumen, the nucleus, in the red blood cells of salmon. [1] Unlike mammalian red blood cells, those of other vertebrates still possess nuclei. The nucleus was also described by Franz Bauer in 1804 [2] and in more detail in 1831 by Scottish botanist Robert Brown in a talk at the Linnean Society of London. Brown was studying orchids under microscope when he observed an opaque area, which he called the areola or nucleus, in the cells of the flowers outer layer. [3] 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. [4] Between 1877 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 therefore developed. [4] Structures The nucleus is the largest cellular organelle in animal cells. [5] In mammalian cells, the average diameter of the nucleus is approximately 6 micrometers (μm), which occupies about 10% of the total cell volume. [6] The viscous liquid within it is called nucleoplasm , and is similar in composition to the cytosol found outside the nucleus. [7] It appears as a dense, roughly spherical organelle. Nuclear envelope and pores Main articles: Nuclear envelope and Nuclear pores The eukaryotic cell nucleus. Visible in this diagram are the ribosome -studded double membranes of the nuclear envelope, the DNA (complexed as chromatin), and the nucleolus . Within the cell nucleus is a viscous liquid called nucleoplasm , similar to the cytoplasm found outside the nucleus. A cross section of a nuclear pore on the surface of the nuclear envelope (1). Other diagram labels show (2) the outer ring, (3) spokes, (4) basket, and (5) filaments. The nuclear envelope, otherwise known as nuclear membrane, 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 cells genetic material from the surrounding cytoplasm, serving as a barrier to prevent macromolecules from diffusing freely between the nucleoplasm and the cytoplasm. [8] The outer nuclear membrane is continuous with the membrane of the rough endoplasmic reticulum (RER), and is similarly studded with ribosomes . [8] 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 several hundred proteins (in vertebrates). [5] 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 selectively allows the passage of small water-soluble molecules while preventing larger molecules, such as nucleic acids and larger 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, [9] each of which contains an eightfold-symmetric ring-shaped structure at a position where the inner and outer membranes fuse. [10] 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. [5] Most proteins, ribosomal subunits, and some DNAs are transported through the pore complexes in a process mediated by a family of transport factors known as karyopherins . Those karyopherins that mediate movement into the nucleus are also called importins, whereas those that mediate movement out of the nucleus are called exportins. Most karyopherins interact directly with their cargo, although some use adaptor proteins . [11] Steroid hormones 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 factors when bound to their ligand ; in the absence of ligand, many such receptors function as histone deacetylases that repress gene expression...
Posted on: Wed, 05 Nov 2014 14:45:49 +0000
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