HeLa cells stained for 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 nucleus is dividing.

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. nuclei; from Latin nucleus or nuculeus, 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 proteins, such as histones, to form chromosomes. The genes 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. Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... Dividing HeLa cells as seen by electron microscopy for other meanings, see also the disambiguation page Hela A HeLa cell (also Hela or hela cell) is an immortal cell line used in medical research. ... The structure of part of a DNA double helix Deoxyribonucleic acid, or DNA, is a nucleic acid molecule that contains the genetic instructions used in the development and functioning of all known living organisms. ... For other uses, see Blue (disambiguation). ... Transmission image of HeLa cells, with overlay of Hoechst 33258 staining (blue). ... This article does not cite any references or sources. ... Mitosis divides genetic information during cell division. ... Image File history File links Biological_cell. ... Image File history File links Biological_cell. ... Schematic of typical animal cell, showing subcellular components. ... The nucleolus is contained within the cell nucleus. ... Figure 1: Ribosome structure indicating small subunit (A) and large subunit (B). ... In cell biology, a vesicle is a relatively small and enclosed compartment, separated from the cytosol by at least one lipid bilayer. ... The endoplasmic reticulum or ER is an organelle found in all eukaryotic cells that is an interconnected network of tubules, vesicles and cisternae that is responsible for several specialized functions: Protein translation, folding, and transport of proteins to be used in the cell membrane (e. ... Micrograph of Golgi apparatus, visible as a stack of semicircular black rings near the bottom. ... The eukaryotic cytoskeleton. ... The endoplasmic reticulum or ER (endoplasmic means within the cytoplasm, reticulum means little net) is an organelle found in all eukaryotic cells. ... Electron micrograph of a mitochondrion showing its mitochondrial matrix and membranes In cell biology, a mitochondrion (plural mitochondria) is a membrane-enclosed organelle that is found in most eukaryotic cells. ... Schematic of typical animal cell, showing subcellular components. ... Organelles. ... Organelles labeled at upper left. ... A centriole is a barrel shaped organelle[1] found in most eukaryotic cells, though absent in higher plants and fungi. ... Cell biology (also called cellular biology or formerly cytology, from the Greek kytos, container) is an academic discipline that studies cells. ... For other uses, see Latin (disambiguation). ... Schematic of typical animal cell, showing subcellular components. ... Kingdoms Animalia - Animals Fungi Plantae - Plants Chromalveolata Protista Alternative phylogeny Unikonta Opisthokonta Metazoa Choanozoa Eumycota Amoebozoa Bikonta Apusozoa Cabozoa Rhizaria Excavata Corticata Archaeplastida Chromalveolata Animals, plants, fungi, and protists are eukaryotes (IPA: ), organisms whose cells are organized into complex structures by internal membranes and a cytoskeleton. ... Drawing of the structure of cork as it appeared under the microscope to Robert Hooke from Micrographia which is the origin of the word cell being used to describe the smallest unit of a living organism Cells in culture, stained for keratin (red) and DNA (green) The cell is the... This article is about the general scientific term. ... The structure of part of a DNA double helix Deoxyribonucleic acid, or DNA, is a nucleic acid molecule that contains the genetic instructions used in the development and functioning of all known living organisms. ... A representation of the 3D structure of myoglobin, showing coloured alpha helices. ... Schematic representation of the assembly of the core histones into the nucleosome. ... Figure 1: A representation of a condensed eukaryotic chromosome, as seen during cell division. ... For other uses, see Gene (disambiguation). ... In biology the genome of an organism is the whole hereditary information of an organism that is encoded in the DNA (or, for some viruses, RNA). ... Gene expression, or simply expression, is the process by which the inheritable information which comprises a gene, such as the DNA sequence, is made manifest as a physical and biologically functional gene product, such as protein or RNA. Several steps in the gene expression process may be modulated, including the...

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 pores 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 ions. 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. The nuclear envelope (also known as the perinuclear envelope, nuclear membrane, nucleolemma or karyotheca) is the double membrane of the nucleus that encloses genetic material in eukaryotic cells. ... Organelles. ... The nuclear lamina is the dense, fibrillar network composed of intermediate filaments made of lamin that lines the inner surface of the nuclear envelope. ... The eukaryotic cytoskeleton. ... Nuclear pore. ... This article is about the electrically charged particle. ... Macromolecules, such as RNA and proteins, are actively transported across the nuclear membrane in a process called the Ran-GTP nuclear transport cycle. ...

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 ribosomes. After being produced in the nucleolus, ribosomes are exported to the cytoplasm where they translate mRNA. Ribonucleic acid or RNA is a nucleic acid polymer consisting of nucleotide monomers that plays several important roles in the processes that translate genetic information from deoxyribonucleic acid (DNA) into protein products; RNA acts as a messenger between DNA and the protein synthesis complexes known as ribosomes, forms vital portions... The nucleolus is contained within the cell nucleus. ... Figure 1: Ribosome structure indicating small subunit (A) and large subunit (B). ...

[edit] History

A drawing of a cell nucleus published by Walther Flemming in 1882.

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 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). 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] Image File history File linksMetadata Flemming1882Tafel1Fig14. ... Image File history File linksMetadata Flemming1882Tafel1Fig14. ... Illustrations of cells with chromosomes and mitosis, from the book Zell-substanz, Kern und Zelltheilung, 1882 Walther Flemming (born April 21, 1843 in Sachsenberg, Germany; died August 4, 1905 in Kiel) was a founder of the study of cytogenetics. ... Botany is the scientific study of plant life. ... Robert Brown (1773–1858) Robert Brown (December 21, 1773–June 10, 1858) is acknowledged as the leading British botanist to collect in Australia during the first half of the 19th century. ... The Linnean Society of London is the worlds premier society for the study and dissemination of taxonomy. ... Orchid re-directs here; for alternate uses see Orchid (disambiguation) Genera Over 800 See List of Orchidaceae genera. ... Matthias Jakob Schleiden (April 5, 1804 - June 23, 1881) was a German botanist and co-founder of the cell theory. ... Franz Julius Ferdinand Meyen (June 28, 1804 - September 2, 1840) was a German physician and botanist. ... Robert Remak (July 26, 1815 - August 29, 1865) was a German embryologist, physiologist, and neurologist, he is best known for naming the three layers of the germ layer. ... [[ Rudolf Ludwig Karl Virchow (born October 13, 1821, in Schivelbein (Pomerania); died September 5, 1902, in Berlin) was a German doctor, anthropologist, public health activist, pathologist, prehistorian, biologist and politician. ...

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.^[3] Oskar Hertwig (April 21, 1849, Friedberg, Hessen - October 25, 1922, Berlin) was a German zoologist. ... Categories: Biology stubs ... Subclasses Euechinoidea Superorder Atelostomata Order Cassiduloida Order Spatangoida (heart urchins) Superorder Diadematacea Order Diadematoida Order Echinothurioida Order Pedinoida Superorder Echinacea Order Arbacioida Order Echinoida Order Phymosomatoida Order Salenioida Order Temnopleuroida Superorder Gnathostomata Order Clypeasteroida (sand dollars) Order Holectypoida Perischoechinoidea Order Cidaroida (pencil urchins) Sea urchins are small spiny sea creatures... For other uses, see Sperm (disambiguation). ... An oocyte or ovocyte is a female gametocyte or germ cell involved in reproduction. ... Ernst Haeckel. ... In biology, phylogenetics (Greek: phylon = tribe, race and genetikos = relative to birth, from genesis = birth) is the study of evolutionary relatedness among various groups of organisms (e. ... For other uses, see Amphibian (disambiguation). ... Classes Caudofoveata Aplacophora Polyplacophora - Chitons Monoplacophora Bivalvia - Bivalves Scaphopoda - Tusk shells Gastropoda - Snails and Slugs Cephalopoda - Squids, Octopuses, etc. ... Eduard Adolf Strasburger (February 1, 1844, Warsaw - May 19, 1912, Bonn) was one of the most famous German botanists of the 19th century. ... August Weismann Friedrich Leopold August Weismann (b. ... Mitosis divides genetic information during cell division. ... Mendelian inheritance (or Mendelian genetics or Mendelism) is a set of primary tenets relating to the transmission of hereditary characteristics from parent organisms to their children; it underlies much of genetics. ...

[edit] Structure

The nucleus is the largest cellular organelle in animals.^[4] In mammalian 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. Schematic of typical animal cell, showing subcellular components. ... Subclasses & Infraclasses Subclass †Allotheria* Subclass Prototheria Subclass Theria Infraclass †Trituberculata Infraclass Metatheria Infraclass Eutheria Mammals (class Mammalia) are warm-blooded, vertebrate animals characterized by the presence of sweat glands, including those that produce milk, and by the presence of: hair, three middle ear bones used in hearing, and a neocortex... Similar to the cytoplasm of a cell, the nucleus contains nucleoplasm or nuclear sap. ... Organelles. ...

[edit] 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 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 macromolecules 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. The nuclear envelope (also known as the perinuclear envelope, nuclear membrane, nucleolemma or karyotheca) is the double membrane of the nucleus that encloses genetic material in eukaryotic cells. ... Nuclear pore. ... Image File history File links Diagram_human_cell_nucleus. ... Image File history File links Diagram_human_cell_nucleus. ... Figure 1: Ribosome structure indicating small subunit (A) and large subunit (B). ... Look up cell membrane in Wiktionary, the free dictionary. ... The structure of part of a DNA double helix Deoxyribonucleic acid, or DNA, is a nucleic acid molecule that contains the genetic instructions used in the development and functioning of all known living organisms. ... Chromatin is the complex of DNA and protein found inside the nuclei of eukaryotic cells. ... The nucleolus is contained within the cell nucleus. ... Similar to the cytoplasm of a cell, the nucleus contains nucleoplasm or nuclear sap. ... Image File history File links NuclearPore_crop. ... Image File history File links NuclearPore_crop. ... Nuclear pore. ... The nuclear envelope (also known as the perinuclear envelope, nuclear membrane, nucleolemma or karyotheca) is the double membrane of the nucleus that encloses genetic material in eukaryotic cells. ... The nuclear envelope (also known as the perinuclear envelope, nuclear membrane, nucleolemma or karyotheca) is the double membrane of the nucleus that encloses genetic material in eukaryotic cells. ... Look up cell membrane in Wiktionary, the free dictionary. ... Illustration of a polypeptide macromolecule Structure of a polyphenylene dendrimer macromolecule reported by Müllen and coworkers in Chem. ... The endoplasmic reticulum or ER (endoplasmic means within the cytoplasm, reticulum means little net) is an organelle found in all eukaryotic cells. ... Figure 1: Ribosome structure indicating small subunit (A) and large subunit (B). ... artery anatomy, showing lumen The lumen (pl. ...

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 vertebrates).^[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. 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] Nuclear pore. ... The unified atomic mass unit (u), or dalton (Da), is a small unit of mass used to express atomic and molecular masses. ... The molecular mass of a substance (less accurately called molecular weight and abbreviated as MW) is the mass of one molecule of that substance, relative to the unified atomic mass unit u (equal to 1/12 the mass of one atom of carbon-12). ... Typical divisions Ascomycota (sac fungi) Saccharomycotina (true yeasts) Taphrinomycotina Schizosaccharomycetes (fission yeasts) Basidiomycota (club fungi) Urediniomycetes Sporidiales Yeasts are a growth form of eukaryotic microorganisms classified in the kingdom Fungi, with approximately 1,500 species described. ... This article does not cite any references or sources. ... Look up nucleic acid in Wiktionary, the free dictionary. ...

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. 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. 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.^[4] Karyopherins are a group of proteins involved in transporting molecules through the nuclear pores of the nuclear envelope. ... Steroid hormones are steroids which act as hormones. ... Cortisol is a corticosteroid hormone produced by the adrenal cortex (in the adrenal gland). ... Aldosterone is a steroid hormone (mineralocorticoid family) produced by the outer-section (zona glomerulosa) of the adrenal cortex in the adrenal gland to regulate sodium and potassium balance in the blood. ... Cell signaling is part of a complex system of communication that governs basic cellular activities and coordinates cell actions. ... Nuclear receptors are a class of intracellular receptors which function as ligand activated transcription factors which up or down regulate the expression of genes. ... In molecular biology, a transcription factor is a protein that binds DNA at a specific promoter or enhancer region or site, where it regulates transcription. ... It has been suggested that this article or section be merged with ligand. ... Histone deacetylases (HDAC) are a class of enzymes that remove acetyl groups from an ε-N-acetyl lysine amino acid on a histone. ...

[edit] 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. Both systems provide structural support for the nuclear envelope and anchoring sites for chromosomes and nuclear pores.^[5] The nuclear lamina is the dense, fibrillar network composed of intermediate filaments made of lamin that lines the inner surface of the nuclear envelope. ... Intermediate filaments are one component of the cytoskeleton - important structural components of living cells. ... The nuclear lamina is the dense, fibrillar network composed of intermediate filaments made of lamin that lines the inner surface of the nuclear envelope. ...

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.^[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. 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] A lamin is a fibrous protein that gives the cell nucleus its shape. ... Microscopy is any technique for producing visible images of structures or details too small to otherwise be seen by the human eye. ... The nucleolus is contained within the cell nucleus. ... This article does not cite any references or sources. ... Chromatin is the complex of DNA and protein found inside the nuclei of eukaryotic cells. ...

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. 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.^[5] // Intermediate filaments (IFs) are important structural proteins which are located both in the cytoplasm and the nucleus. ... In chemistry, a monomer (from Greek mono one and meros part) is a small molecule that may become chemically bonded to other monomers to form a polymer. ... Side view of an α-helix of alanine residues in atomic detail. ... Sucrose, or common table sugar, is composed of glucose and fructose. ... A coiled coil is a structural motif found in many proteins. ... Two antiparallel molecules run side-by-side in opposite directions. ... A tetramer is a protein with four subunits (tetrameric). ...

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] Normal nuclear lamina (a and b) and mutant nuclear lamina (c and d) from a patient with HGPS, visualized by immunofluorescence - note the irregular and bumpy shape of the laminopathic nuclei[1] Laminopathies are a group of rare genetic disorders caused by mutations in genes encoding proteins of the nuclear... The term Progeria narrowly refers to Hutchinson-Gilford Progeria syndrome, but the term is also used more generally to describe any of the so-called accelerated aging diseases. The word progeria is derived from the Greek for prematurely old. Because the accelerated aging diseases display different aspects of aging, but... Ageing or aging is the process of getting older. ... Individuals in the mollusk species Donax variabilis show diverse coloration and patterning in their phenotypes. ...

[edit] Chromosomes

Main article: Chromosome

A mouse fibroblast 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 chromosomes. 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. A small fraction of the cell's genes are located instead in the mitochondria. Figure 1: A representation of a condensed eukaryotic chromosome, as seen during cell division. ... Image File history File links MouseChromosomeTerritoriesBMC_Cell_Biol6-44Fig2e. ... Image File history File links MouseChromosomeTerritoriesBMC_Cell_Biol6-44Fig2e. ... NIH/3T3 Fibroblasts A fibroblast is a type of cell that synthesizes and maintains the extracellular matrix of many animal tissues. ... The structure of part of a DNA double helix Deoxyribonucleic acid, or DNA, is a nucleic acid molecule that contains the genetic instructions used in the development and functioning of all known living organisms. ... A metaphase cell positive for the bcr/abl rearrangement using FISH. The chromosomes can be seen in blue. ... The structure of part of a DNA double helix Deoxyribonucleic acid, or DNA, is a nucleic acid molecule that contains the genetic instructions used in the development and functioning of all known living organisms. ... Figure 1: A representation of a condensed eukaryotic chromosome, as seen during cell division. ... 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 of DNA and protein found inside the nuclei of eukaryotic cells. ... Figure 1: A representation of a condensed eukaryotic chromosome, as seen during cell division. ... Karyogram of human male using Giemsa staining. ... In cell biology, a mitochondrion is an organelle found in the cells of most eukaryotes. ...

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 telomeres and centromeres.^[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] 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, or simply expression, is the process by which the inheritable information which comprises a gene, such as the DNA sequence, is made manifest as a physical and biologically functional gene product, such as protein or RNA. Several steps in the gene expression process may be modulated, including the... For differently-colored eyes, see Heterochromia. ... A telomere is a region of highly repetitive DNA at the end of a chromosome that functions as a disposable buffer. ... The centromere is a region of chromosomes with a special sequence and structure. ...

Antibodies to certain types of chromatin organization, particularly nucleosomes, have been associated with a number of autoimmune diseases, 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. A nucleosome is a unit made of DNA and histones. ... Autoimmune diseases arise from an overactive immune response of the body against substances and tissues normally present in the body. ... Anti-nuclear antibodies (ANAs, also known as anti-nuclear factor or ANF) are detected in a large group of autoimmune disorders. ...

[edit] Nucleolus

Main article: Nucleolus

An electron micrograph of a cell nucleus, showing the darkly stained nucleolus.

The nucleolus is a discrete densely-stained structure found in the nucleus. 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). 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 nucleolus is contained within the cell nucleus. ... Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... An electron micrograph is a micrograph made with an electron microscope. ... The nucleolus is contained within the cell nucleus. ... The nucleolus is contained within the cell nucleus. ... Look up Tandem in Wiktionary, the free dictionary. ... Ribosomal RNA (rRNA), a type of RNA synthesized in the nucleolus by RNA Pol I, is the central component of the ribosome, the protein manufacturing machinery of all living cells. ...

The first step in ribosomal assembly is transcription of the rDNA, by a protein called RNA polymerase I, forming a large pre-rRNA precursor. 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. The assembled ribosomal subunits are the largest structures passed through the nuclear pores.^[4] RNA polymerase I (also called Pol I) transcribes DNA to synthesize rRNA (Ribosomal RNA). ... A non-coding RNA (ncRNA) is any RNA molecule that functions without being translated into a protein. ... Diagram of the location of introns and exons within a gene. ... The life cycle of an mRNA in a eukaryotic cell. ...

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). 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] An electron microscope is a type of microscope that uses electrons to illuminate and create an image of a specimen. ...

[edit] 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. 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] Discovered by Fox et al. ...

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. This condition typically results from mutations in actin, and the rods themselves consist of mutant actin as well as other cytoskeletal proteins.^[29] Nemaline myopathy (NM; also called rod myopathy or nemaline rod myopathy) is a group of congenital, hereditary neuromuscular disorders that cause muscle weakness, generally nonprogressive, of varying severity. ... G-Actin (PDB code: 1j6z). ...

[edit] 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 0.2 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.^[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.^[25] Cajal bodies are spherical structures found in the nucleus of proliferative cells like tumor cells, or metabolically active cells like neurons. ... An electron microscope is a type of microscope that uses electrons to illuminate and create an image of a specimen. ... Coilin protein is one the main molecular components of Cajal bodies (CBs). ... A non-coding RNA (ncRNA) is any RNA molecule that is not translated into a protein. ... A non-coding RNA (ncRNA) is any RNA molecule that functions without being translated into a protein. ...

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. Gems are similar in size and shape to CBs, and in fact are virtually indistinguishable under the microscope.^[30] Unlike CBs, gems don't contain small nuclear ribonucleoproteins (snRNPs), but do contain a protein called survivor of motor neurons (SMN) whose function relates to snRNP biogenesis. 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] Gemini (IPA: , Latin: , symbol , ) is one of the constellations of the zodiac known as the twins. It is part of the winter sky, lying between Taurus to the west and the dim Cancer to the east, with Auriga and the near-invisible Lynx to the north and Monoceros and Canis... SnRNPs (pronounced snurps), or small nuclear ribonucleoproteins, are particles which combine with pre-mRNA and various proteins to form spliceosomes (a type of large molecular complex). ... Neurons (also called nerve cells) are the primary cells of the nervous system. ...

[edit] 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.^[33] In molecular biology, a transcription factor is a protein that binds DNA at a specific promoter or enhancer region or site, where it regulates transcription. ... Ribonucleic acid (RNA) is a nucleic acid consisting of a string of covalently-bound nucleotides. ...

[edit] 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]

[edit] Paraspeckles

Main article: Paraspeckle

Discovered by Fox et al. in 2002, paraspeckles are irregularly shaped compartments in the nucleus' interchromatin space.^[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] Discovered by Fox et al. ... Discovered by Fox et al. ...

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. 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. 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.^[36] The nucleolus is contained within the cell nucleus. ... The cell cycle, or cell-division cycle, is the series of events that take place in a eukaryotic cell leading to its replication. ... This article does not cite any references or sources. ... Mitosis divides genetic information during cell division. ... A cell during telophase that has almost completed cytokinesis. ... Ribonucleic acid or RNA is a nucleic acid polymer consisting of nucleotide monomers that plays several important roles in the processes that translate genetic information from deoxyribonucleic acid (DNA) into protein products; RNA acts as a messenger between DNA and the protein synthesis complexes known as ribosomes, forms vital portions... A micrograph of ongoing gene transcription of ribosomal RNA illustrating the growing primary transcripts. ...

[edit] 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.^[37] A phosphorylated serine residue Phosphorylation is the addition of a phosphate (PO4) group to a protein or a small molecule or the introduction of a phosphate group into an organic molecule. ...

[edit] Function

The main function of the cell nucleus is to control gene expression and mediate the replication of DNA during the cell cycle. 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. The cell cycle, or cell-division cycle, is the series of events that take place in a eukaryotic cell leading to its replication. ... A micrograph of ongoing gene transcription of ribosomal RNA illustrating the growing primary transcripts. ... Translation is the second process of protein biosynthesis (part of the overall process of gene expression). ... Gene regulation is the general term for cellular control of protein synthesis at the DNA-RNA transcription step. ... Prokaryotic bacteria cell structure Prokaryotes (IPA: //) are a group of organisms that lack a cell nucleus (= karyon), or any other membrane-bound organelles. ...

[edit] 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. Hexokinase is an enzyme responsible for the first the step of glycolysis, forming glucose-6-phosphate from glucose. 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.^[39] The word glycolysis is derived from Greek γλυκύς (sweet) and λύσις (rupture). ... Glucose (Glc), a monosaccharide (or simple sugar), is an important carbohydrate in biology. ... A hexokinase is an enzyme that phosphorylates a six-carbon sugar, a hexose, to a hexose phosphate. ... Glucose 6-phosphate is glucose sugar phosphorylated on carbon 6. ... Fructosephosphates are sugar phosphates based upon fructose. ...

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. 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. 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.^[5] In molecular biology, a transcription factor is a protein that binds DNA at a specific promoter or enhancer region or site, where it regulates transcription. ... NF-κB, or Nuclear Factor kappa B, is a nuclear transcription factor found in all cell types and is involved in cellular responses to stimuli such as stress, cytokines, free radicals, ultraviolet irradiation, and bacterial or viral antigens. ... An abscess on the skin, showing the redness and swelling characteristic of inflammation. ... Cell signaling is part of a complex system of communication that governs basic cellular activities and coordinates cell actions. ... In medicine, tumor necrosis factor alpha (TNFα, cachexin or cachectin) is an important cytokine involved in systemic inflammation and the acute phase response. ... A nuclear localizing sequence (NLS) is an amino acid sequence which acts like a tag on the exposed surface of a protein. ...

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. 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. Diagram of the location of introns and exons within a gene. ...

[edit] Gene expression

Main article: Gene expression

A micrograph of ongoing gene transcription of ribosomal RNA illustrating the growing primary transcripts. "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.

Gene expression first involves transcription, in which DNA is used as a template to produce RNA. 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. As ribosomes are located outside the nucleus, mRNA produced needs to be exported. ^{[citation needed]} Gene expression, or simply expression, is the process by which the inheritable information which comprises a gene, such as the DNA sequence, is made manifest as a physical and biologically functional gene product, such as protein or RNA. Several steps in the gene expression process may be modulated, including the... Image File history File links Download high-resolution version (1039x1250, 533 KB) Micrograph of gene transcription in progress, original author identified as Dr. Hans-Heinrich Trepte. ... Image File history File links Download high-resolution version (1039x1250, 533 KB) Micrograph of gene transcription in progress, original author identified as Dr. Hans-Heinrich Trepte. ... In genetics, transcription is the first of the two-step protein biosynthesis process. ... Ribosomal RNA (rRNA), a type of RNA synthesized in the nucleolus by RNA Pol I, is the central component of the ribosome, the protein manufacturing machinery of all living cells. ... Primary transcript is an RNA molecule that has not yet undergone any modification after its synthesis. ... In molecular biology, the 5 end and the 3 end (pronounced 5-prime and 3-prime) are respectively the leading and tail ends of a strand of nucleic acid. ... In molecular biology, the 5 end and the 3 end (pronounced 5-prime and 3-prime) are respectively the leading and tail ends of a strand of nucleic acid. ... A micrograph of ongoing gene transcription of ribosomal RNA illustrating the growing primary transcripts. ... The life cycle of an mRNA in a eukaryotic cell. ... Translation is the second process of protein biosynthesis (part of the overall process of gene expression). ... Figure 1: Ribosome structure indicating small subunit (A) and large subunit (B). ...

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.^{[citation needed]} This article or section does not cite its references or sources. ... This article does not cite any references or sources. ... Topoisomerase I solves the problem caused by tension generated by winding/unwinding of DNA. It wraps around DNA and makes a cut permitting the helix to spin. ... It has been suggested that this article or section be merged with Superhelix. ... In molecular biology, a transcription factor is a protein that binds DNA at a specific promoter or enhancer region or site, where it regulates transcription. ...

[edit] Processing of pre-mRNA

Main article: Post-transcriptional modification

Newly synthesized mRNA molecules are known as primary transcripts or pre-mRNA. 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. The three main modifications are 5' capping, 3' polyadenylation, and RNA splicing. While in the nucleus, pre-mRNA is associated with a variety of proteins in complexes known as heterogeneous ribonucleoprotein particles (hnRNPs). Addition of the 5' cap occurs co-transcriptionally and is the first step in post-translational modification. The 3' poly-adenine tail is only added after transcription is complete. To meet Wikipedias quality standards, this article or section may require cleanup. ... Primary transcript is an RNA molecule that has not yet undergone any modification after its synthesis. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... Translation is the second process of protein biosynthesis (part of the overall process of gene expression). ... The 5 cap is a specially altered dinucleotide end to the 5 end of preliminary messenger RNA as found in eukaryotes. ... Polyadenylation is the covalent linkage of a polyadenylyl moiety to a messenger RNA (mRNA) molecule. ... RNA splicing is the excision of introns from RNA during the formation of mRNA and the removal of introns from mRNA precursors and the reattachment or annealing of exons. ... Heterogeneous ribonucleoprotein particles (hnRNPs) are complexes of RNA and protein present in the cell nucleus during gene transcription and subsequent post-transcriptional modification of the newly synthesized RNA. Key protein components include protein K and polypyrimidine-tract binding protein (PTB), which is regulated by phosphorylation catalyzed by protein kinase A... For the programming language Adenine, see Adenine (programming language). ...

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. 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. This process is known as alternative splicing, and allows production of a large variety of proteins from a limited amount of DNA.
A spliceosome is a complex of RNA and many protein subunits called snRNPs, that removes the non-coding introns from unprocessed mRNA. Spliceosomes are unique to eukaryotic mRNA as the mRNA of prokaryotes lack introns. ... Diagram of the location of introns and exons within a gene. ... An exon is any region of DNA within a gene, that is transcribed to the final messenger RNA (mRNA) molecule, rather than being spliced out from the transcribed RNA molecule. ... Each antibody binds to a specific antigen; an interaction similar to a lock and key. ... A protein primary structure is a chain of amino acids. ... Various modes of alternative splicing Alternative splicing is the process that occurs in eukaryotes in which the splicing process of a pre-mRNA transcribed from one gene can lead to different mature mRNA molecules and therefore to different proteins. ...

[edit] Dynamics and regulation

[edit] Nuclear transport

Main article: Nuclear transport