HeLa cells stained for DNA with the Blue Hoechst dye. A HeLa cell (also Hela or hela cell) is an immortal cell line used in medical research Deoxyribonucleic acid ( DNA) is a Nucleic acid that contains the genetic instructions used in the development and functioning of all known Blue is a Colour, the Perception of which is evoked by The Hoechst stains are part of a family of Fluorescent stains for labelling DNA in Fluorescence microscopy. The central and rightmost cell are in interphase, thus their entire nuclei are labeled. Interphase is the phase of the Cell cycle in which the cell spends the majority of its time and performs the majority of its purposes including preparation for Cell On the left a cell is going through mitosis and its nucleus has disintegrated in preparation of division. Mitosis is the process in which a Eukaryotic cell separates the Chromosomes in its Cell nucleus, into two identical sets in two daughter nuclei

Schematic of typical animal cell, showing subcellular components. Organelles: (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) centrioles

In cell biology, the nucleus (pl. In Cell biology, an organelle (pronunciation /ɔː(rgəˡnɛl/ is a specialized subunit within a cell that has a specific function and is usually separately enclosed The nucleolus (also called nucleole) is a structure found within the nucleus in which Ribosomal RNA is transcribed. Ribosomes ( from ribo nucleic acid and "Greek soma ( meaning body") are complexes of RNA and Protein that A vesicle is a small bubble of liquid within a cell A more formal definition in Cell biology, would be that a vesicle is a relatively small intracellular membrane-enclosed The endoplasmic reticulum (Greek endo = "within" (prefix plásma = "formed entity" Latin reticulum = "little net" or ER, is an Organelle The Golgi apparatus (also called the cytoskeleton (also CSK is a cellular " Scaffolding " or " Skeleton " contained within the Cytoplasm. The endoplasmic reticulum (Greek endo = "within" (prefix plásma = "formed entity" Latin reticulum = "little net" or ER, is an Organelle In Cell biology, a mitochondrion (plural mitochondria) is a membrane-enclosed Organelle found in most eukaryotic cells. In general vacuole functions include Removing unwanted structural debris Isolating materials that might be harmful or a threat to the cell Containing The cytoplasm is the contents of a cell that is enclosed within the Plasma membrane. Lysosomes are Organelles that contain Digestive enzymes (acid Hydrolases. A Centriole is a barrel shaped Organelle found in most animal Eukaryotic cells though absent in Higher plants and Fungi. See also List of basic cell biology topics. Cell biology (also called cellular biology or formerly cytology, from the nuclei; from Latin nucleus or nuculeus, "little nut" or kernel) is a membrane-enclosed organelle found in most eukaryotic cells. Latin ( lingua Latīna, laˈtiːna is an Italic language, historically spoken in Latium and Ancient Rome. In Cell biology, an organelle (pronunciation /ɔː(rgəˡnɛl/ is a specialized subunit within a cell that has a specific function and is usually separately enclosed Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. Genetics (from Ancient Greek grc-Latn genetikos, “genitive” and that from grc-Latn genesis, “origin” a discipline of Biology, is Deoxyribonucleic acid ( DNA) is a Nucleic acid that contains the genetic instructions used in the development and functioning of all known Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl In Biology, histones are the chief Protein components of Chromatin. A chromosome is an organized structure of DNA and Protein that is found in cells. The genes within these chromosomes are the cell's nuclear genome. History See also History of genetics The existence of genes was first suggested by Gregor Mendel (1822-1884 who in the 1860s studied inheritance In classical genetics the genome of a Diploid Organism including Eukarya refers to a full set of chromosomes or genes in a Gamete, thereby 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. Gene expression is the process by which inheritable information from a Gene, such as the DNA sequence, is made into a functional Gene product, such

The main structures making up the nucleus are the nuclear envelope, a double membrane that encloses the entire organelle and separates its contents 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. The nuclear envelope (NE(also known as the perinuclear envelope, nuclear membrane, nucleolemma or karyotheca) is a double lipid bilayer that The cytoplasm is the contents of a cell that is enclosed within the Plasma membrane. In Eukaryotic cells during Interphase, there is a bilayered Nuclear envelope surrounding the nucleus, which separates the Cytoplasm and its cytoskeleton (also CSK is a cellular " Scaffolding " or " Skeleton " contained within the Cytoplasm. Because the nuclear membrane is impermeable to most molecules, nuclear pores are required to allow movement of molecules across the envelope. Nuclear pores are large Protein complexes that cross the Nuclear envelope, which is the double membrane surrounding the eukaryotic These pores cross both of the membranes, providing a channel that allows free movement of small molecules and ions. An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge The movement of larger molecules such as proteins is carefully controlled, and requires active transport regulated by carrier proteins. Nuclear transport is crucial to cell function, as movement through the pores is required for both gene expression and chromosomal maintenance. The entry and exit of large molecules from the Cell nucleus is tightly controlled by the Nuclear pore complexes (NPCs

Although the interior of the nucleus does not contain any membrane-bound subcompartments, its contents are not uniform, and a number of subnuclear bodies exist, made up of unique proteins, RNA molecules, and particular parts of the chromosomes. Ribonucleic acid ( RNA) is a Nucleic acid that consists of a long chain of Nucleotide units The best known of these is the nucleolus, which is mainly involved in the assembly of ribosomes. The nucleolus (also called nucleole) is a structure found within the nucleus in which Ribosomal RNA is transcribed. Ribosomes ( from ribo nucleic acid and "Greek soma ( meaning body") are complexes of RNA and Protein that After being produced in the nucleolus, ribosomes are exported to the cytoplasm where they translate mRNA.

History

A drawing of a cell nucleus published by Walther Flemming in 1882. Walther Flemming ( April 21[[ 843]] - August 4[[ 905]] was a German Biologist and the founder of Cytogenetics.

The nucleus was the first organelle to be discovered, and was first described by Franz Bauer in 1802. Franz Andreas Bauer (later Francis born 14 March 1758 in Feldsberg (now Valtice, Czech Republic) - died 11 December 1840 ^[1] It was later described in more detail by Scottish botanist Robert Brown in 1831 in a talk at the Linnean Society of London. Botany, plant science(s, phytology, or plant biology is a branch of Biology and is the scientific study of plant Life Robert Brown FRS ( 21 December, 1773 &ndash 10 June, 1858) was a Scottish scientist who is acknowledged as the leading botanist The Linnean Society of London is the World 's premier society for the study and dissemination of Taxonomy and natural history Brown was studying orchids 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). Matthias Jakob Schleiden ( April 5, 1804 - June 23, 1881) was a German Botanist and co-founder of the Cell theory 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. Franz Julius Ferdinand Meyen ( June 28, 1804 - September 2, 1840) was a German Physician and Botanist. 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"). Robert Remak ( July 26 1815 - August 29 1865) was a Polish / German embryologist physiologist and neurologist born in Rudolf Ludwig Karl Virchow ( 13 October 1821 &ndash 5 September 1902) was a German doctor, anthropologist, public health The function of the nucleus remained unclear. ^[3]

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. Oscar Hertwig ( April 21, 1849, Friedberg, Hesse - October 25, 1922, Berlin) was a German Zoologist For soil improvement see Fertilization (soil. Sea urchins are small globular spiny sea cat animals composing most of class Echinoidea. The term sperm is derived from the Greek word (σπέρμα sperma (meaning "seed" and refers to the male reproductive cells. An oocyte, ovocyte, or rarely ocyte, is a female Gametocyte or Germ cell involved in reproduction. 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"). Ernst Heinrich Philipp August Haeckel ( February 16, 1834 — August 9, 1919)also written von Haeckel, was an eminent German 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. Prehistoric amphibian Amphibians (class Amphibia such as Frogs Toads Salamanders Newts Gymnophiona, Sirens and Molluscs are animals belonging to the phylum Mollusca. There are around 250000 extant Species within the phylum with an estimated 70000 Eduard Strasburger produced the same results for plants (1884). Eduard Adolf Strasburger ( February 1, 1844, Warsaw &ndash May 19, 1912 Bonn) was a Polish - German 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. Friedrich Leopold August Weismann (Birth January 17, 1834 in Frankfurt am Main; Death 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. Mitosis is the process in which a Eukaryotic cell separates the Chromosomes in its Cell nucleus, into two identical sets in two daughter nuclei Mendelian inheritance (or Mendelian genetics or Mendelism) is a set of primary tenets relating to the transmission of hereditary characteristics from parent ^[3]

Structure

The nucleus is the largest cellular organelle in animals. In Cell biology, an organelle (pronunciation /ɔː(rgəˡnɛl/ is a specialized subunit within a cell that has a specific function and is usually separately enclosed ^[4] In mammalian cells, the average diameter typically varies from 11 to 22 micrometers (μm) and occupies about 10% of the total volume. Mammals ( class Mammalia) are a class of Vertebrate Animals characterized by the presence of Sweat glands, including sweat glands ^[5] The viscous liquid within it is called nucleoplasm, and is similar to the cytoplasm found outside the nucleus. Similar to the Cytoplasm of a cell, the nucleus contains nucleoplasm or nuclear sap. The cytoplasm is the contents of a cell that is enclosed within the Plasma membrane.

Nuclear envelope and pores

Main articles: Nuclear envelope and Nuclear pores

The eukaryotic cell nucleus. The nuclear envelope (NE(also known as the perinuclear envelope, nuclear membrane, nucleolemma or karyotheca) is a double lipid bilayer that Nuclear pores are large Protein complexes that cross the Nuclear envelope, which is the double membrane surrounding the eukaryotic Visible in this diagram are the ribosome-studded double membranes of the nuclear envelope, the DNA (complexed as chromatin), and the nucleolus. Ribosomes ( from ribo nucleic acid and "Greek soma ( meaning body") are complexes of RNA and Protein that The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer Deoxyribonucleic acid ( DNA) is a Nucleic acid that contains the genetic instructions used in the development and functioning of all known Chromatin is the complex basis of DNA and protein that makes up Chromosomes It is found inside the nuclei of eukaryotic cells, and within the The nucleolus (also called nucleole) is a structure found within the nucleus in which Ribosomal RNA is transcribed. Within the cell nucleus is a viscous liquid called nucleoplasm, similar to the cytoplasm found outside the nucleus. Similar to the Cytoplasm of a cell, the nucleus contains nucleoplasm or nuclear sap.

A cross section of a nuclear pore on the surface of the nuclear envelope (1). Nuclear pores are large Protein complexes that cross the Nuclear envelope, which is the double membrane surrounding the eukaryotic The nuclear envelope (NE(also known as the perinuclear envelope, nuclear membrane, nucleolemma or karyotheca) is a double lipid bilayer that Other diagram labels show (2) the outer ring, (3) spokes, (4) basket, and (5) filaments.

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 (NE(also known as the perinuclear envelope, nuclear membrane, nucleolemma or karyotheca) is a double lipid bilayer that The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer The nuclear envelope completely encloses the nucleus and separates the cell's genetic material from the surrounding cytoplasm, serving as a barrier to prevent macromolecules from diffusing freely between the nucleoplasm and the cytoplasm. The term macromolecule by definition implies "large Molecule " ^[6] The outer nuclear membrane is continuous with the membrane of the rough endoplasmic reticulum (RER), and is similarly studded with ribosomes. The endoplasmic reticulum (Greek endo = "within" (prefix plásma = "formed entity" Latin reticulum = "little net" or ER, is an Organelle Ribosomes ( from ribo nucleic acid and "Greek soma ( meaning body") are complexes of RNA and Protein that The space between the membranes is called the perinuclear space and is continuous with the RER lumen. A lumen (Lat lūmen, an opening or light (pl lumina is the inside space or lining of a tubular structure such as an artery or intestine

Nuclear pores, which provide aqueous channels through the envelope, are composed of multiple proteins, collectively referred to as nucleoporins. Nuclear pores are large Protein complexes that cross the Nuclear envelope, which is the double membrane surrounding the eukaryotic The pores are about 125 million daltons in molecular weight and consist of around 50 (in yeast) to 100 proteins (in vertebrates). The unified atomic mass unit ( u) or Dalton ( Da) or sometimes universal mass unit, is an unit of Mass used to express The molecular mass (abbreviated m of a substance, more commonly referred to as molecular weight and abbreviated as MW, is the Mass of one Yeasts are a growth form of eukaryotic Microorganisms classified in the kingdom Fungi, with about 1500 Species currently described Vertebrates are members of the Subphylum Vertebrata, Chordates with backbones or spinal columns The grouping sometimes includes ^[4] 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 acids and proteins, from inappropriately entering or exiting the nucleus. A nucleic acid is a Macromolecule composed of chains of monomeric Nucleotides In Biochemistry these Molecules carry Genetic information 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. ^[4]

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 karyopherins. Karyopherins are a group of Proteins involved in transporting molecules through the pores of the Nuclear envelope (the membrane around a cell'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 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. Steroid hormones are Steroids which act as Hormones Mammalian steroid hormones can be grouped into five groups by the receptors to which they bind Cortisol is a Corticosteroid Hormone produced by the Adrenal gland (in the Zona fasciculata of the Adrenal cortex) Aldosterone is a hormone that causes the tubules of the kidneys to retain sodium and water Cell signaling is part of a Complex system of Communication that governs basic cellular activities and coordinates cell actions In the field of Molecular biology, nuclear receptors are a class of Proteins found within the interior of cells that are responsible for sensing the presence of 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. In the field of Molecular biology, a transcription factor (sometimes called a sequence-specific DNA binding factor is a Protein that binds to specific sequences In Biochemistry, a ligand ( latin ligare = to bind is a substance that is able to bind to and form a complex with a Biomolecule Histone deacetylases (HDAC ( EC number 351 are a class of Enzymes that remove Acetyl groups from an ε-N-acetyl Lysine Amino acid ^[4]

Cytoskeleton

Main article: Nuclear lamina

In animal cells, two networks of intermediate filaments provide the nucleus with mechanical support: the nuclear lamina forms an organized meshwork on the internal face of the envelope, while less organized support is provided on the cytosolic face of the envelope. In Eukaryotic cells during Interphase, there is a bilayered Nuclear envelope surrounding the nucleus, which separates the Cytoplasm and its Intermediate filaments (IFs are cytoskeletal structures formed by members of a family of related proteins called Keratin. In Eukaryotic cells during Interphase, there is a bilayered Nuclear envelope surrounding the nucleus, which separates the Cytoplasm and its Both systems provide structural support for the nuclear envelope and anchoring sites for chromosomes and nuclear pores. ^[5]

The nuclear lamina is mostly composed of lamin proteins. Lamins are Fibrous proteins providing structural function and transcriptional regulation in the Cell nucleus. 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. ^[10][11] 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. A fluorescence microscope (colloquially synonymous with epifluorescent microscope) is a light Microscope used to study properties of organic or inorganic substances The actual function of the veil is not clear, although it is excluded from the nucleolus and is present during interphase. The nucleolus (also called nucleole) is a structure found within the nucleus in which Ribosomal RNA is transcribed. Interphase is the phase of the Cell cycle in which the cell spends the majority of its time and performs the majority of its purposes including preparation for Cell ^[13] The lamin structures that make up the veil bind chromatin and disrupting their structure inhibits transcription of protein-coding genes. Chromatin is the complex basis of DNA and protein that makes up Chromosomes It is found inside the nuclei of eukaryotic cells, and within the ^[14]

Like the components of other intermediate filaments, 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. Intermediate filaments (IFs are cytoskeletal structures formed by members of a family of related proteins called Keratin. A monomer (from Greek mono "one" and meros "part" is a small Molecule that may become chemically bonded to other A common motif in the Secondary structure of Proteins the alpha helix (α-helix is a right-handed coiled conformation resembling a spring, in which A dimer is a Chemical or Biological entity consisting of two subunits called Monomers which are held together by either Intramolecular forces For the coiled coil shape in general see Coil. A coiled coil is a Structural motif in Proteins in which 2-7 Two of these dimer structures then join side by side, in an antiparallel arrangement, to form a tetramer called a protofilament. In biochemistry two molecules are antiparallel if they run side-by-side in opposite directions A tetramer is a Protein with four subunits (tetrameric There are homo-tetramers (all Subunits are identical such as Glutathione S-transferase or 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. ^[5]

Mutations in lamin genes leading to defects in filament assembly are known as laminopathies. Laminopathies are a group of rare Genetic disorders caused by Mutations in genes encoding proteins of the Nuclear lamina. The most notable laminopathy is the family of diseases known as progeria, which causes the appearance of premature aging in its sufferers. Progeria is a condition of early aging which usually refers specifically to Hutchinson-Gilford Progeria syndrome. Ageing or aging (American English is the accumulation of changes in an organism The exact mechanism by which the associated biochemical changes give rise to the aged phenotype is not well understood. A phenotype is any observable characteristic of an Organism, such as its morphology, Development, biochemical or physiological properties ^[15]

Chromosomes

Main article: Chromosome

A mouse fibroblast nucleus in which DNA is stained blue. A chromosome is an organized structure of DNA and Protein that is found in cells. A fibroblast is a type of cell that synthesizes and maintains the Extracellular matrix of many Animal tissues Deoxyribonucleic acid ( DNA) is a Nucleic acid that contains the genetic instructions used in the development and functioning of all known The distinct chromosome territories of chromosome 2 (red) and chromosome 9 (green) are visible stained with fluorescent in situ hybridization. FISH ( Fluorescent In situ hybridization) is a cytogenetic technique that can be used to detect and localize the presence or absence

The cell nucleus contains the majority of the cell's genetic material, in the form of multiple linear DNA molecules organized into structures called chromosomes. Deoxyribonucleic acid ( DNA) is a Nucleic acid that contains the genetic instructions used in the development and functioning of all known A chromosome is an organized structure of DNA and Protein that is found in cells. 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 chromosomes familiar from a karyotype. The cell cycle, or cell-division cycle, is the series of events that take place in a eukaryotic cell leading to its replication Chromatin is the complex basis of DNA and protein that makes up Chromosomes It is found inside the nuclei of eukaryotic cells, and within the A chromosome is an organized structure of DNA and Protein that is found in cells. A karyotype is the characteristic Chromosome complement of a Eukaryote Species. A small fraction of the cell's genes are located instead in the mitochondria. In Cell biology, a mitochondrion (plural mitochondria) is a membrane-enclosed Organelle found in most eukaryotic cells.

There are two types of chromatin. Euchromatin is the less compact DNA form, and contains genes that are frequently expressed by the cell. Euchromatin is a lightly packed form of Chromatin that is rich in Gene concentration and is often (but not always under active transcription. Gene expression is the process by which inheritable information from a Gene, such as the DNA sequence, is made into a functional Gene product, such ^[16] The other type, heterochromatin, is the more compact form, and contains DNA that are infrequently transcribed. Heterochromatin is a tightly packed form of DNA Its major characteristic is that transcription is limited 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 telomeres and centromeres. A telomere is a region of repetitive DNA at the end of Chromosomes which protects the end of the chromosome from destruction A centromere is a region of DNA typically found near the middle of a Chromosome where two Sister chromatids come in contact ^[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 nucleosomes, have been associated with a number of autoimmune diseases, such as systemic lupus erythematosus. Nucleosomes form the fundamental repeating units of eukaryotic Chromatin, which is used to pack the large eukaryotic genomes into the nucleus while still ensuring Autoimmune diseases arise from an overactive Immune response of the body against substances and tissues normally present in the body Systemic lupus erythematosus ( SLE or lupus,) is a chronic autoimmune disease that can be fatal though with recent medical advances fatalities are becoming ^[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. Anti-nuclear antibodies ( ANA s also known as anti-nuclear factor or ANF) are antibodies directed against contents of the cell nucleus Multiple sclerosis (abbreviated MS also known as disseminated sclerosis or encephalomyelitis disseminata) is an autoimmune condition in which the ^[22] As in the case of progeria, the role played by the antibodies in inducing the symptoms of autoimmune diseases is not obvious.

Nucleolus

Main article: Nucleolus

An electron micrograph of a cell nucleus, showing the darkly stained nucleolus. The nucleolus (also called nucleole) is a structure found within the nucleus in which Ribosomal RNA is transcribed. A micrograph, microphotograph or photomicrograph is a Photograph or similar image taken through a Microscope or similar device to show The nucleolus (also called nucleole) is a structure found within the nucleus in which Ribosomal RNA is transcribed.

The nucleolus is a discrete densely-stained structure found in the nucleus. The nucleolus (also called nucleole) is a structure found within the nucleus in which Ribosomal RNA is transcribed. It is not surrounded by a membrane, and is sometimes called a suborganelle. It forms around tandem repeats of rDNA, DNA coding for ribosomal RNA (rRNA). Tandem is a Latin Adverb meaning "at length" or "finally" Ribosomal RNA ( rRNA) is the central component of the Ribosome, the protein manufacturing machinery of all living cells. These regions are called nucleolar organizer regions (NOR). The main roles of the nucleolus are to synthesize rRNA and assemble ribosomes. The structural cohesion of the nucleolus depends on its activity, as ribosomal assembly in the nucleolus results in the transient association of nucleolar components, facilitating further ribosomal assembly, and hence further association. This model is supported by observations that inactivation of rDNA results in intermingling of nucleolar structures. ^[23]

The first step in ribosomal assembly is transcription of the rDNA, by a protein called RNA polymerase I, forming a large pre-rRNA precursor. An impressive 50%+ of all RNA synthesis in a cell is accounted for by the transcription of rRNA This is cleaved into the subunits 5. 8S, 18S, and 28S rRNA. ^[24] The transcription, post-transcriptional processing, and assembly of rRNA occurs in the nucleolus, aided by small nucleolar RNA (snoRNA) molecules, some of which are derived from spliced introns from messenger RNAs encoding genes related to ribosomal function. Small nucleolar RNAs (snoRNAs are a class of small RNA molecules that guide chemical modifications ( Methylation or pseudouridylation of Ribosomal RNAs Introns, derived from the term "intragenic regions" and also called intervening sequence (IVS are DNA regions in a Gene that are not translated into Messenger ribonucleic acid ( mRNA) is a molecule of RNA encoding a chemical "blueprint" for a Protein product The assembled ribosomal subunits are the largest structures passed through the nuclear pores. ^[4]

When observed under the electron microscope, the nucleolus can be seen to consist of three distinguishable regions: the innermost fibrillar centers (FCs), surrounded by the dense fibrillar component (DFC), which in turn is bordered by the granular component (GC). An electron microscope is a type of Microscope that uses Electrons to illuminate a specimen and create an enlarged image Transcription of the rDNA occurs either in the FC or at the FC-DFC boundary, and therefore when rDNA transcription in the cell is increased more FCs are detected. Most of the cleavage and modification of rRNAs occurs in the DFC, while the latter steps involving protein assembly onto the ribosomal subunits occurs in the GC. ^[24]

Other subnuclear bodies

Besides the nucleolus, the nucleus contains a number of other non-membrane delineated bodies. These include Cajal bodies, Gemini of coiled bodies, polymorphic interphase karyosomal association (PIKA), promyelocytic leukaemia (PML) bodies, paraspeckles and splicing speckles. Paraspeckles are irregularly shaped sub- cellular compartments approximately 0 Although little is known about a number of these domains, they are significant in that they show that the nucleoplasm is not uniform mixture, but rather contains organized functional subdomains. ^[27]

Other subnuclear structures appear as part of abnormal disease processes. For example, the presence of small intranuclear rods have been reported in some cases of nemaline myopathy. Nemaline myopathy (also called rod myopathy or nemaline rod myopathy) is a Congenital, Hereditary neuromuscular disorder that causes muscle This condition typically results from mutations in actin, and the rods themselves consist of mutant actin as well as other cytoskeletal proteins. Actin is a globular roughly 42-kDa Protein found in all eukaryotic cells (except for Nematode sperm where it may be present at concentrations of ^[29]

Cajal bodies and gems

A nucleus typically contains between 1 and 10 compact structures called Cajal bodies or coiled bodies (CB), whose diameter measures between 0. Cajal bodies (CBs are spherical sub-organelles found in the nucleus of proliferative cells like Tumor cells or metabolically active cells like Neurons 2 µm and 2. 0 µm depending on the cell type and species. ^[25] When seen under an electron microscope, they resemble balls of tangled thread^[26] and are dense foci of distribution for the protein coilin. An electron microscope is a type of Microscope that uses Electrons to illuminate a specimen and create an enlarged image Coilin, also known as COIL, is a human Gene ^[30] CBs are involved in a number of different roles relating to RNA processing, specifically small nucleolar RNA (snoRNA) and small nuclear RNA (snRNA) maturation, and histone mRNA modification. Small nucleolar RNAs (snoRNAs are a class of small RNA molecules that guide chemical modifications ( Methylation or pseudouridylation of Ribosomal RNAs Small nuclear RNA (snRNA is a class of small RNA molecules that are found within the nucleus of eukaryotic cells ^[25]

Similar to Cajal bodies are Gemini of coiled bodies, or gems, whose name is derived from the Gemini constellation in reference to their close "twin" relationship with CBs. Gemini (ˈgɛmɪnaɪ Twins, symbol, Unicode ♊ is one of the Constellations of the Zodiac known as "the twins" Gems are similar in size and shape to CBs, and in fact are virtually indistinguishable under the microscope. ^[30] Unlike CBs, gems do not contain small nuclear ribonucleoproteins (snRNPs), but do contain a protein called survivor of motor neurons (SMN) whose function relates to snRNP biogenesis. snRNP s (pronounced "snurps" or small nuclear ribonucleoproteins, are particles that combine with Pre-mRNA and various proteins to form Spliceosomes Neurons (ˈnjuːɹɒn also known as neurones and nerve cells) are responsive cells in the Nervous system that process and transmit information Gems are believed to assist CBs in snRNP biogenesis,^[31] though it has also been suggested from microscopy evidence that CBs and gems are different manifestations of the same structure. ^[30]

PIKA and PTF domains

PIKA domains, or polymorphic interphase karyosomal associations, were first described in microscopy studies in 1991. Their function was and remains unclear, though they were not thought to be associated with active DNA replication, transcription, or RNA processing. ^[32] They have been found to often associate with discrete domains defined by dense localization of the transcription factor PTF, which promotes transcription of snRNA. In the field of Molecular biology, a transcription factor (sometimes called a sequence-specific DNA binding factor is a Protein that binds to specific sequences Small nuclear RNA (snRNA is a class of small RNA molecules that are found within the nucleus of eukaryotic cells ^[33]

PML bodies

Promyelocytic leukaemia bodies (PML bodies) are spherical bodies found scattered throughout the nucleoplasm, measuring around 0. 2–1. 0 µm. They are known by a number of other names, including nuclear domain 10 (ND10), Kremer bodies, and PML oncogenic domains. They are often seen in the nucleus in association with Cajal bodies and cleavage bodies. It has been suggested that they play a role in regulating transcription. ^[27]

Paraspeckles

Main article: Paraspeckle

Discovered by Fox et al. Paraspeckles are irregularly shaped sub- cellular compartments approximately 0 in 2002, paraspeckles are irregularly shaped compartments in the nucleus' interchromatin space. Paraspeckles are irregularly shaped sub- cellular compartments approximately 0 ^[34] First documented in HeLa cells, where there are generally 10–30 per nucleus,^[35] paraspeckles are now known to also exist in all human primary cells, transformed cell lines and tissue sections. ^[36] Their name is derived from their distribution in the nucleus; the "para" is short for parallel and the "speckles" refers to the splicing speckles to which they are always in close proximity. ^[35]

Paraspeckles are dynamic structures that are altered in response to changes in cellular metabolic activity. They are transcription dependent^[34] and in the absence of RNA Pol II transcription, the paraspeckle disappears and all of its associated protein components (PSP1, p54nrb, PSP2, CFI(m)68 and PSF) form a crescent shaped perinucleolar cap in the nucleolus. The nucleolus (also called nucleole) is a structure found within the nucleus in which Ribosomal RNA is transcribed. This phenomenon is demonstrated during the cell cycle. In the cell cycle, paraspeckles are present during interphase and during all of mitosis except for telophase. The cell cycle, or cell-division cycle, is the series of events that take place in a eukaryotic cell leading to its replication Interphase is the phase of the Cell cycle in which the cell spends the majority of its time and performs the majority of its purposes including preparation for Cell Mitosis is the process in which a Eukaryotic cell separates the Chromosomes in its Cell nucleus, into two identical sets in two daughter nuclei Telophase (sometimes spelled telephase) from the Ancient Greek "τελος" (end and "φασις" (stage is a stage in either Meiosis During telophase, when the two daughter nuclei are formed, there is no RNA Pol II transcription so the protein components instead form a perinucleolar cap. Ribonucleic acid ( RNA) is a Nucleic acid that consists of a long chain of Nucleotide units Transcription is the synthesis of RNA under the direction of DNA ^[36]

Splicing speckles

Sometimes referred to as interchromatin granule clusters, speckles are rich in splicing snRNPs and other splicing proteins necessary for pre-mRNA processing. Because of a cell's changing requirements, the composition and location of these bodies changes according to mRNA transcription and regulation via phosphorylation of specific proteins. Phosphorylation is the addition of a Phosphate (PO4 group to a Protein molecule or a small molecule ^[37]

Function

The main function of the cell nucleus is to control gene expression and mediate the replication of DNA during the cell cycle. The cell cycle, or cell-division cycle, is the series of events that take place in a eukaryotic cell leading to its replication The nucleus provides a site for genetic transcription that is segregated from the location of translation in the cytoplasm, allowing levels of gene regulation that are not available to prokaryotes. Transcription is the synthesis of RNA under the direction of DNA Translation is the first stage of Protein biosynthesis (part of the overall process of Gene expression) Gene modulation redirects here For information on therapeutic regulation of gene expression see Therapeutic gene modulation. The prokaryotes (proʊˈkærioʊts singular prokaryote /proʊˈkæriət/ are a group of Organisms that lack a Cell nucleus (= karyon or any other

Cell compartmentalization

The nuclear envelope allows the nucleus to control its contents, and separate them from the rest of the cytoplasm where necessary. This is important for controlling processes on either side of the nuclear membrane. In some cases where a cytoplasmic process needs to be restricted, a key participant is removed to the nucleus, where it interacts with transcription factors to downregulate the production of certain enzymes in the pathway. This regulatory mechanism occurs in the case of glycolysis, a cellular pathway for breaking down glucose to produce energy. See also Gluconeogenesis, which carries out a process wherein glucose is synthesized rather than catabolized Glucose (Glc a Monosaccharide (or simple Sugar) also known as grape sugar, is an important Carbohydrate in Biology. Hexokinase is an enzyme responsible for the first the step of glycolysis, forming glucose-6-phosphate from glucose. Variation across species Hexokinases have been found in every organism checked ranging from bacteria, Yeast, and Plants to humans and other Vertebrates Glucose 6-phosphate (also known as Robison ester) is Glucose sugar Phosphorylated on carbon 6 At high concentrations of fructose-6-phosphate, a molecule made later from glucose-6-phosphate, a regulator protein removes hexokinase to the nucleus,^[38] where it forms a transcriptional repressor complex with nuclear proteins to reduce the expression of genes involved in glycolysis. Fructose 6-phosphate (also known as the Neuberg ester) is Fructose sugar Phosphorylated on carbon 6 (ie ^[39]

In order to control which genes are being transcribed, the cell separates some transcription factor proteins responsible for regulating gene expression from physical access to the DNA until they are activated by other signaling pathways. In the field of Molecular biology, a transcription factor (sometimes called a sequence-specific DNA binding factor is a Protein that binds to specific sequences This prevents even low levels of inappropriate gene expression. For example in the case of NF-κB-controlled genes, which are involved in most inflammatory responses, transcription is induced in response to a signal pathway such as that initiated by the signaling molecule TNF-α, binds to a cell membrane receptor, resulting in the recruitment of signalling proteins, and eventually activating the transcription factor NF-κB. NF-κB ( nuclear factor-kappa B) is a protein complex that is a Transcription factor. Inflammation ( Latin, inflamatio, to set on fire is the complex biological response of vascular tissues to harmful stimuli such as Pathogens Cell signaling is part of a Complex system of Communication that governs basic cellular activities and coordinates cell actions Tumor necrosis factor ( TNF, cachexin or cachectin and formally known as Tumor necrosis factor-alpha) is a Cytokine involved in systemic A nuclear localisation signal on the NF-κB protein allows it to be transported through the nuclear pore and into the nucleus, where it stimulates the transcription of the target genes. A nuclear localizing sequence ( NLS) is an Amino acid sequence which acts like a 'tag' on the exposed surface of a Protein. ^[5]

The compartmentalization allows the cell to prevent translation of unspliced mRNA. ^[40] Eukaryotic mRNA contains introns that must be removed before being translated to produce functional proteins. Introns, derived from the term "intragenic regions" and also called intervening sequence (IVS are DNA regions in a Gene that are not translated into The splicing is done inside the nucleus before the mRNA can be accessed by ribosomes for translation. Without the nucleus ribosomes would translate newly transcribed (unprocessed) mRNA resulting in misformed and nonfunctional proteins.

Gene expression

Main article: Gene expression

A micrograph of ongoing gene transcription of ribosomal RNA illustrating the growing primary transcripts. Gene expression is the process by which inheritable information from a Gene, such as the DNA sequence, is made into a functional Gene product, such Transcription is the synthesis of RNA under the direction of DNA Ribosomal RNA ( rRNA) is the central component of the Ribosome, the protein manufacturing machinery of all living cells. A primary transcript is an RNA molecule that has not yet undergone any modification after its synthesis. "Begin" indicates the 3' end of the DNA, where new RNA synthesis begins; "end" indicates the 5' end, where the primary transcripts are almost complete. Directionality, in Molecular biology, refers to the end-to-end chemical orientation of a single strand of Nucleic acid. Directionality, in Molecular biology, refers to the end-to-end chemical orientation of a single strand of Nucleic acid.

Gene expression first involves transcription, in which DNA is used as a template to produce RNA. Transcription is the synthesis of RNA under the direction of DNA In the case of genes encoding proteins, that RNA produced from this process is messenger RNA (mRNA), which then needs to be translated by ribosomes to form a protein. Messenger ribonucleic acid ( mRNA) is a molecule of RNA encoding a chemical "blueprint" for a Protein product Translation is the first stage of Protein biosynthesis (part of the overall process of Gene expression) Ribosomes ( from ribo nucleic acid and "Greek soma ( meaning body") are complexes of RNA and Protein that As ribosomes are located outside the nucleus, mRNA produced needs to be exported. ^[41]

Since the nucleus is the site of transcription, it also contains a variety of proteins which either directly mediate transcription or are involved in regulating the process. These proteins include helicases that unwind the double-stranded DNA molecule to facilitate access to it, RNA polymerases that synthesize the growing RNA molecule, topoisomerases that change the amount of supercoiling in DNA, helping it wind and unwind, as well as a large variety of transcription factors that regulate expression. Helicases are a class of Enzymes vital to all living Organisms They are motor proteins that move directionally along a Nucleic acid phosphodiester backbone RNA polymerase ( RNAP or RNApol) is an Enzyme that produces RNA. Topoisomerases (type I, type II) are Isomerase Enzymes that act on the topology of DNA. In a "relaxed" double-helical segment of DNA, the two strands twist around the helical axis once every 10 In the field of Molecular biology, a transcription factor (sometimes called a sequence-specific DNA binding factor is a Protein that binds to specific sequences ^[42]

Processing of pre-mRNA

Main article: Post-transcriptional modification

Newly synthesized mRNA molecules are known as primary transcripts or pre-mRNA. Post-transcriptional modification is a process in Cell biology by which in Eukaryotic cells, primary transcript RNA is converted into mature A primary transcript is an RNA molecule that has not yet undergone any modification after its synthesis. They must undergo post-transcriptional modification in the nucleus before being exported to the cytoplasm; mRNA that appears in the nucleus without these modifications is degraded rather than used for protein translation. Post-transcriptional modification is a process in Cell biology by which in Eukaryotic cells, primary transcript RNA is converted into mature Translation is the first stage of Protein biosynthesis (part of the overall process of Gene expression) The three main modifications are 5' capping, 3' polyadenylation, and RNA splicing. The 5' cap is a specially altered Nucleotide end to the 5' end of Precursor messenger RNA and some other primary RNA transcripts as found in Eukaryotes Polyadenylation is the synthesis of a poly(A tail a stretch of RNA where all the bases are Adenines at the end of an RNA molecule In Molecular biology, splicing is a modification of an RNA after transcription, in which Introns are removed and Exons are joined While in the nucleus, pre-mRNA is associated with a variety of proteins in complexes known as heterogeneous ribonucleoprotein particles (hnRNPs). Heterogeneous ribonucleoprotein particles (hnRNPs are complexes of RNA and Protein present in the Cell nucleus during gene transcription and Addition of the 5' cap occurs co-transcriptionally and is the first step in post-transcriptional modification. The 3' poly-adenine tail is only added after transcription is complete. Adenine is a Purine with a variety of roles in Biochemistry including Cellular respiration, in the form of both the energy-rich Adenosine

RNA splicing, carried out by a complex called the spliceosome, is the process by which introns, or regions of DNA that do not code for protein, are removed from the pre-mRNA and the remaining exons connected to re-form a single continuous molecule. A spliceosome is a complex of specialized RNA and Protein subunits that removes Introns from a transcribed pre- mRNA ( HnRNA Introns, derived from the term "intragenic regions" and also called intervening sequence (IVS are DNA regions in a Gene that are not translated into An exon is a Nucleic acid sequence that is represented in the mature form of an RNA molecule after a portions of a precursor RNA Introns have been This process normally occurs after 5' capping and 3' polyadenylation but can begin before synthesis is complete in transcripts with many exons. ^[4] Many pre-mRNAs, including those encoding antibodies, can be spliced in multiple ways to produce different mature mRNAs that encode different protein sequences. Antibodies (also known as immunoglobulins, abbreviated Ig) are Gamma globulin Proteins that are found in Blood or other Bodily In Biochemistry, the primary structure of a biological molecule is the exact specification of its atomic composition and the chemical bonds connecting those atoms (including This process is known as alternative splicing, and allows production of a large variety of proteins from a limited amount of DNA. Alternative splicing is the RNA splicing variation mechanism in which the Exons of the primary gene transcript the Pre-mRNA, are separated and reconnected

Dynamics and regulation

Nuclear transport

Main article: Nuclear transport

Macromolecules, such as RNA and proteins, are actively transported across the nuclear membrane in a process called the Ran-GTP nuclear transport cycle. The entry and exit of large molecules from the Cell nucleus is tightly controlled by the Nuclear pore complexes (NPCs The term macromolecule by definition implies "large Molecule " Ribonucleic acid ( RNA) is a Nucleic acid that consists of a long chain of Nucleotide units Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl Active transport is the mediated process of moving particles across Biological membrane against the concentration gradient Ran GTPase is a Small GTPase that is involved in transport into and out of the Cell nucleus during Interphase and also involved in Mitosis Guanosine-5'-triphosphate ( GTP) is a Purine Nucleotide. One role is as substrate for the synthesis of RNA during transcription.

The entry and exit of large molecules from the nucleus is tightly controlled by the nuclear pore complexes. Although small molecules can enter the nucleus without regulation,^[43] macromolecules such as RNA and proteins require association karyopherins called importins to enter the nucleus and exportins to exit. Karyopherins are a group of Proteins involved in transporting molecules through the pores of the Nuclear envelope (the membrane around a cell's Importin is a type of Protein that moves other protein molecules into the nucleus by binding to a specific recognition sequence called the Nuclear localization Karyopherins are a group of Proteins involved in transporting molecules through the pores of the Nuclear envelope (the membrane around a cell's "Cargo" proteins that must be translocated from the cytoplasm to the nucleus contain short amino acid sequences known as nuclear localization signals which are bound by importins, while those transported from the nucleus to the cytoplasm carry nuclear export signals bound by exportins. A nuclear localizing sequence ( NLS) is an Amino acid sequence which acts like a 'tag' on the exposed surface of a Protein. A nuclear export signal (NES is a short Amino acid sequence of 5-6 hydrophobic residues in a Protein that targets it for export from the Cell nucleus The ability of importins and exportins to transport their cargo is regulated by GTPases, enzymes that hydrolyze the molecule guanosine triphosphate to release energy. GTPases (singular GTPase are a large family of Hydrolase Enzymes that can bind and hydrolyze guanosine triphosphate (GTP. Hydrolysis is a Chemical reaction during which one or more water molecules are split into hydrogen and hydroxide ions which may go on to participate in further reactions Guanosine-5'-triphosphate ( GTP) is a Purine Nucleotide. One role is as substrate for the synthesis of RNA during transcription. The key GTPase in nuclear transport is Ran, which can bind either GTP or GDP (guanosine diphosphate) depending on whether it is located in the nucleus or the cytoplasm. Ran GTPase is a Small GTPase that is involved in transport into and out of the Cell nucleus during Interphase and also involved in Mitosis Whereas importins depend on RanGTP to dissociate from their cargo, exportins require RanGTP in order to bind to their cargo. ^[9]

Nuclear import depends on the importin binding its cargo in the cytoplasm and carrying it through the nuclear pore into the nucleus. Inside the nucleus, RanGTP acts to separate the cargo from the importin, allowing the importin to exit the nucleus and be reused. Nuclear export is similar, as the exportin binds the cargo inside the nucleus in a process facilitated by RanGTP, exits through the nuclear pore, and separates from its cargo in the cytoplasm.

Specialized export proteins exist for translocation of mature mRNA and tRNA to the cytoplasm after post-transcriptional modification is complete. This quality-control mechanism is important due to the these molecules' central role in protein translation; mis-expression of a protein due to incomplete excision of exons or mis-incorporation of amino acids could have negative consequences for the cell; thus incompletely modified RNA that reaches the cytoplasm is degraded rather than used in translation. ^[4]

Assembly and disassembly

An image of a newt lung cell stained with fluorescent dyes during metaphase. A newt is an Amphibian of the Salamandridae family order Urodela or Caudata, found in North America, Europe, and lung is the essential Respiration organ in air-breathing Animals including most Tetrapods a few Fish and a few Snails The most primitive Staining is an auxiliary Technique used in Microscopy to enhance contrast in the microscopic image Fluorescence is a Luminescence that is mostly found as an A dye can generally be described as a Colored substance that has an affinity to the substrate to which it is being applied Metaphase from the Ancient Greek μετά (after and φάσις (stage is a stage of Mitosis in the eukaryotic Cell cycle in which The mitotic spindle can be seen, stained green, attached to the two sets of chromosomes, stained light blue. In Cell biology, the spindle apparatus (also called spindle fibers) is the structure that separate the Chromosomes into the daughter cells during A chromosome is an organized structure of DNA and Protein that is found in cells. All chromosomes but one are already at the metaphase plate.

During its lifetime a nucleus may be broken down, either in the process of cell division or as a consequence of apoptosis, a regulated form of cell death. Cell division is a process by which a cell, called the parent cell divides into two or more cells called daughter cells. During these events, the structural components of the nucleus—the envelope and lamina—are systematically degraded.

During the cell cycle the cell divides to form two cells. The cell cycle, or cell-division cycle, is the series of events that take place in a eukaryotic cell leading to its replication In order for this process to be possible, each of the new daughter cells must have a full set of genes, a process requiring replication of the chromosomes as well as segregation of the separate sets. This occurs by the replicated chromosomes, the sister chromatids, attaching to microtubules, which in turn are attached to different centrosomes. Sister chromatids are identical copies of a chromosome connected by a Centromere. Microtubules are one of the components of the Cytoskeleton. They have a diameter of 25 nm and length varying from 200 nanometers to 25 micrometers In Cell biology, the centrosome is an Organelle that serves as the main Microtubule organizing center (MTOC of the animal cell as well The sister chromatids can then be pulled to separate locations in the cell. However, in many cells the centrosome is located in the cytoplasm, outside the nucleus, the microtubules would be unable to attach to the chromatids in the presence of the nuclear envelope. ^[44] Therefore the early stages in the cell cycle, beginning in prophase and until around prometaphase, the nuclear membrane is dismantled. Prophase is a stage of Mitosis in which the Chromatin condenses into a highly ordered structure called a Chromosome Prometaphase is the phase of Mitosis following Prophase and preceding Metaphase, in eukaryotic Somatic cells The nuclear ^[12] Likewise, during the same period, the nuclear lamina is also disassembled, a process regulated by phosphorylation of the lamins. ^[45] Towards the end of the cell cycle, the nuclear membrane is reformed, and around the same time, the nuclear lamina are reassembled by dephosphorylating the lamins. ^[45]

Apoptosis is a controlled process in which the cell's structural components are destroyed, resulting in death of the cell. Changes associated with apoptosis directly affect the nucleus and its contents, for example in the condensation of chromatin and the disintegration of the nuclear envelope and lamina. The destruction of the lamin networks is controlled by specialized apoptotic proteases called caspases, which cleave the lamin proteins and thus degrade the nucleus' structural integrity. A protease is any Enzyme that conducts Proteolysis, that is begins protein Catabolism by Hydrolysis of the Peptide bonds that link Caspases, or c ysteine- asp artic acid prote ases, are a family of Cysteine proteases which play essential roles in Apoptosis (programmed Lamin cleavage is sometimes used as a laboratory indicator of caspase activity in assays for early apoptotic activity. An assay is a procedure where a property or concentration of an Analyte is measured ^[12] Cells that express mutant caspase-resistant lamins are deficient in nuclear changes related to apoptosis, suggesting that lamins play a role in initiating the events that lead to apoptotic degradation of the nucleus. ^[12] Inhibition of lamin assembly itself is an inducer of apoptosis. ^[46]

The nuclear envelope acts as a barrier that prevents both DNA and RNA viruses from entering the nucleus. Some viruses require access to proteins inside the nucleus in order to replicate and/or assemble. DNA viruses, such as herpesvirus replicate and assemble in the cell nucleus, and exit by budding through the inner nuclear membrane. The Herpesviridae are a large family of DNA viruses that cause diseases in animals including humans This process is accompanied by disassembly of the lamina on the nuclear face of the inner membrane. ^[12]

Anucleated and polynucleated cells

Human red blood cells, like those of other mammals, lack nuclei. This occurs as a normal part of the cells' development.

Although most cells have a single nucleus, some cell types have no nucleus, and others have many nuclei. This can be a normal process, as in the maturation of mammalian red blood cells, or an anomalous result of faulty cell division. Red blood cells are the most common type of Blood cell and the Vertebrate body's principal means of delivering Oxygen to the body tissues via the Blood

Anucleated cells contain no nucleus and are therefore incapable of dividing to produce daughter cells. The best-known anucleated cell is the mammalian red blood cell, or erythrocyte, which also lacks other organelles such as mitochondria and serves primarily as a transport vessel to ferry oxygen from the lungs to the body's tissues. In Cell biology, a mitochondrion (plural mitochondria) is a membrane-enclosed Organelle found in most eukaryotic cells. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the lung is the essential Respiration organ in air-breathing Animals including most Tetrapods a few Fish and a few Snails The most primitive Erythrocytes mature via erythropoiesis in the bone marrow, where they lose their nuclei, organelles, and ribosomes. Erythropoiesis is the process by which Red blood cells (erythrocytes are produced Bone marrow is the flexible tissue found in the hollow interior of Bones In adults marrow in large bones produces new Blood cells It constitutes 4% of The nucleus is expelled during the process of differentiation from an erythroblast to a reticulocyte, the immediate precursor of the mature erythrocyte. An erythroblast is a type of red blood cell which still retains a Cell nucleus. Reticulocytes are immature Red blood cells typically composing about 1% of the red cells in the human body ^[47] The presence of mutagens may induce the release of some immature "micronucleated" erythrocytes into the bloodstream. In Biology, a mutagen ( Latin, literally origin of change) is a physical or chemical agent that changes the genetic information (usually DNA) ^[48][49] Anucleated cells can also arise from flawed cell division in which one daughter lacks a nucleus and the other is binucleate.

Polynucleated cells contain multiple nuclei. Most Acantharean species of protozoa^[50] and some fungi in mycorrhizae^[51] have naturally polynucleated cells. The Acantharea are a small group of Radiolarian Protozoa, distinguished mainly by their skeletons Protozoa (in Greek πρῶτον proton "first" and ζῷα zoia "animals" are unicellular Eukaryotes (singular A fungus (ˈfʌŋgəs is a eukaryotic Organism that is a member of the kingdom Fungi (ˈfʌndʒaɪ A mycorrhiza (Greek for fungus roots coined by Frank 1885 typically seen in the plural forms mycorrhizae or mycorrhizas) is a symbiotic (occasionally In humans, skeletal muscle cells, called myocytes, become polynucleated during development; the resulting arrangement of nuclei near the periphery of the cells allows maximal intracellular space for myofibrils. Skeletal muscle is a type of Striated muscle, which usually attaches to tendons A myocyte (also known as a myocardial cell is the type of cell found in heart muscles. Myofibrils (obsolete term sarcostyles) are cylindrical Organelles found within Muscle cells They are bundles of actomyosin filaments ^[4] Multinucleated cells can also be abnormal in humans; for example, cells arising from the fusion of monocytes and macrophages, known as giant multinucleated cells, sometimes accompany inflammation^[52] and are also implicated in tumor formation. Monocyte is a type of Leukocyte, part of the Human body 's Immune system. Macrophages ( Greek: "big eaters" from makros "large" + phagein "eat" ( Mø) are cells within the tissues that ^[53]

Evolution

As the major defining characteristic of the eukaryotic cell, the nucleus' evolutionary origin has been the subject of much speculation. eVolution is the third Album by eLDee, it was due to be released in 2008 Four major theories have been proposed to explain the existence of the nucleus, although none have yet earned widespread support. ^[54]

The theory known as the "syntrophic model" proposes that a symbiotic relationship between the archaea and bacteria created the nucleus-containing eukaryotic cell. This article is about the biological phenomenon for other uses see Symbiosis (disambiguation The term symbiosis (from the Greek The Bacteria ( singular: bacterium) are a large group of unicellular Microorganisms Typically a few Micrometres in length bacteria have It is hypothesized that the symbiosis originated when ancient archaea, similar to modern methanogenic archaea, invaded and lived within bacteria similar to modern myxobacteria, eventually forming the early nucleus. Methanogenesis or biomethanation is the formation of Methane by Microbes known as Methanogens Organisms capable of producing methane have been The myxobacteria (" slime bacteria " are a group of bacteria that predominantly live in the soil This theory is analogous to the accepted theory for the origin of eukaryotic mitochondria and chloroplasts, which are thought to have developed from a similar endosymbiotic relationship between proto-eukaryotes and aerobic bacteria. In Cell biology, a mitochondrion (plural mitochondria) is a membrane-enclosed Organelle found in most eukaryotic cells. Chloroplasts are Organelles found in Plant cells and eukaryotic Algae that conduct Photosynthesis. ^[55] The archaeal origin of the nucleus is supported by observations that archaea and eukarya have similar genes for certain proteins, including histones. In Biology, histones are the chief Protein components of Chromatin. Observations that myxobacteria are motile, can form multicellular complexes, and possess kinases and G proteins similar to eukarya, support a bacterial origin for the eukaryotic cell. In Chemistry and Biochemistry, a kinase, alternatively known as a phosphotransferase, is a type of Enzyme that transfers Phosphate GTP chemical structurepng|thumb|180px| Guanosine triphosphate]] G proteins short for guanine nucleotide-binding proteins, are a family of Proteins involved ^[56]

A second model proposes that proto-eukaryotic cells evolved from bacteria without an endosymbiotic stage. This model is based on the existence of modern planctomycetes bacteria that possess a nuclear structure with primitive pores and other compartmentalized membrane structures. Planctomycetes are a Phylum of aquatic Bacteria and are found in field samples of brackish, and marine and fresh Water ^[57] A similar proposal states that a eukaryote-like cell, the chronocyte, evolved first and phagocytosed archaea and bacteria to generate the nucleus and the eukaryotic cell. Phagocytosis is the cellular process of engulfing solid particles by the Cell membrane to form an internal Phagosome, or "food vacuole ^[58]

The most controversial model, known as viral eukaryogenesis, posits that the membrane-bound nucleus, along with other eukaryotic features, originated from the infection of a prokaryote by a virus. Viral eukaryogenesis is the Hypothesis that the Cell nucleus of Eukaryotic life forms evolved from a large DNA virus in a form of endosymbiosis The suggestion is based on similarities between eukaryotes and viruses such as linear DNA strands, mRNA capping, and tight binding to proteins (analogizing histones to viral envelopes). In Biology, histones are the chief Protein components of Chromatin. Many Viruses (eg influenza and many animal viruses have viral envelopes covering their protein Capsids The envelopes are typically derived from portions of One version of the proposal suggests that the nucleus evolved in concert with phagocytosis to form an early cellular "predator". Phagocytosis is the cellular process of engulfing solid particles by the Cell membrane to form an internal Phagosome, or "food vacuole ^[59] Another variant proposes that eukaryotes originated from early archaea infected by poxviruses, on the basis of observed similarity between the DNA polymerases in modern poxviruses and eukaryotes. Poxviruses (members of the family Poxviridae) are viruses that can as a family infect both Vertebrate and Invertebrate animals A DNA Polymerase is an Enzyme that assists in DNA replication. ^[60][61] It has been suggested that the unresolved question of the evolution of sex could be related to the viral eukaryogenesis hypothesis. ^[62]

Finally, a very recent proposal suggests that traditional variants of the endosymbiont theory are insufficiently powerful to explain the origin of the eukaryotic nucleus. This model, termed the exomembrane hypothesis, suggests that the nucleus instead originated from a single ancestral cell that evolved a second exterior cell membrane; the interior membrane enclosing the original cell then became the nuclear membrane and evolved increasingly elaborate pore structures for passage of internally synthesized cellular components such as ribosomal subunits. Ribosomes ( from ribo nucleic acid and "Greek soma ( meaning body") are complexes of RNA and Protein that ^[63]

References

Further reading

• Goldman, Robert D. ; Yosef Gruenbaum, Robert D. Moir, Dale K. Shumaker and Timothy P. Spann (2002). "Nuclear lamins: building blocks of nuclear architecture". Genes & Dev. (16): 533–547. doi:10.1101/gad.960502. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.  

A review article about nuclear lamins, explaining their structure and various roles

• Görlich, Dirk; Ulrike Kutay (1999). "Transport between the cell nucleus and the cytoplasm". Ann. Rev. Cell Dev. Biol. (15): 607–660. PMID 10611974.  

A review article about nuclear transport, explains the principles of the mechanism, and the various transport pathways

• Lamond, Angus I. ; William C. Earnshaw (24 APRIL 1998). "Structure and Function in the Nucleus". Science 280: 547–553. PMID 9554838.  

A review article about the nucleus, explaining the structure of chromosomes within the organelle, and describing the nucleolus and other subnuclear bodies

• Pennisi E. (2004). "Evolutionary biology. The birth of the nucleus". Science 305 (5685): 766–768. doi:10.1126/science.305.5685.766. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 15297641.  

A review article about the evolution of the nucleus, explaining a number of different theories

• Pollard, Thomas D. ; William C. Earnshaw (2004). Cell Biology. Philadelphia: Saunders. ISBN 0-7216-3360-9.  

A university level textbook focusing on cell biology. Contains information on nucleus structure and function, including nuclear transport, and subnuclear domains

External links

• cellnucleus.com Website covering structure and function of the nucleus from the Department of Oncology at the University of Alberta.
• The Nuclear Protein Database Information on nuclear components.
• The Nucleus Collection in the Image & Video Library of The American Society for Cell Biology contains peer-reviewed still images and video clips that illustrate the nucleus.
• Nuclear Envelope and Nuclear Import Section from Landmark Papers in Cell Biology, Joseph G. Gall, J. Richard McIntosh, eds. , contains digitized commentaries and links to seminal research papers on the nucleus. Published online in the Image & Video Library of The American Society for Cell Biology
• Cytoplasmic patterns generated by human antibodies

Gallery of nucleus images

Comparison of human and chimpanzee chromosomes. Chimpanzee (often shortened to chimp) is the common name for the two extant Species of Apes in the Genus Pan.

Mouse chromosome territories in different cell types.

24 chromosome territories in human cells.