 = Cell nucleus = 
 
 In cell biology , the nucleus ( <unk> nuclei ; from Latin nucleus or <unk> , meaning <unk> ) is a membrane @-@ enclosed organelle found in eukaryotic cells . <unk> usually have a single nucleus , but a few cell types , such as mammalian red blood cells , have no nuclei , and a few others have many . 
 Cell nuclei contain 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 are the cell 's nuclear genome and are structured in such a way to promote cell function . The nucleus maintains the integrity of genes and controls the activities of the cell by regulating gene expression — the nucleus is , therefore , the control center of the cell . The main structures making up the nucleus are the nuclear envelope , a double membrane that encloses the entire organelle and <unk> its contents from the cellular cytoplasm , and the nuclear matrix ( which includes the nuclear lamina ) , a network within the nucleus that adds mechanical support , much like the cytoskeleton , which supports the cell as a whole . 
 Because the nuclear membrane is <unk> to large molecules , nuclear pores are required that regulate nuclear transport of molecules across the envelope . The pores cross both nuclear membranes , providing a channel through which larger molecules must be actively transported by carrier proteins while allowing free movement of small molecules and ions . Movement of large molecules such as proteins and RNA through the pores is required for both gene expression and the maintenance of chromosomes . Although the interior of the nucleus does not contain any membrane @-@ bound sub compartments , its contents are not uniform , and a number of sub @-@ nuclear bodies exist , made up of unique proteins , RNA molecules , and particular parts of the chromosomes . The best @-@ known of these is the nucleolus , which is mainly involved in the assembly of ribosomes . After being produced in the nucleolus , ribosomes are exported to the cytoplasm where they translate mRNA . 
 
 = = History = = 
 
 The nucleus was the first organelle to be discovered . What is most likely the oldest preserved drawing dates back to the early <unk> <unk> van <unk> ( 1632 – 1723 ) . He observed a " <unk> " , the nucleus , in the red blood cells of salmon . Unlike mammalian red blood cells , those of other vertebrates still contain nuclei . 
 The nucleus was also described by Franz Bauer in 1804 and in more detail in 1831 by Scottish botanist Robert Brown in a talk at the <unk> Society of London . Brown was studying orchids under microscope when he observed an <unk> area , which he called the " <unk> " or " nucleus " , in the cells of the flower 's outer layer . 
 He did not suggest a potential function . In 1838 , <unk> <unk> proposed that the nucleus plays a role in generating cells , thus he introduced the name " <unk> " ( cell builder ) . He believed that he had observed new cells assembling around " <unk> " . Franz <unk> was a strong opponent of this view , having already described cells <unk> by division and believing that many cells would have no nuclei . The idea that cells can be generated de <unk> , by the " <unk> " or otherwise , contradicted work by Robert <unk> ( 1852 ) and Rudolf Virchow ( 1855 ) who decisively <unk> the new paradigm that cells are generated solely by cells ( " <unk> <unk> e <unk> " ) . The function of the nucleus remained unclear . 
 Between 1877 and 1878 , Oscar <unk> published several studies on the fertilization of sea <unk> eggs , showing that the nucleus of the sperm enters the <unk> 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 <unk> 's theory that the complete phylogeny of a species would be repeated during embryonic development , including generation of the first nucleated cell from a " <unk> " , a <unk> mass of primordial <unk> ( " <unk> " ) . Therefore , the necessity of the sperm nucleus for fertilization was discussed for quite some time . However , <unk> confirmed his observation in other animal groups , including amphibians and molluscs . Eduard <unk> produced the same results for plants in 1884 . This paved the way to assign the nucleus an important role in heredity . In 1873 , August <unk> postulated the equivalence of the maternal and paternal <unk> cells for heredity . The function of the nucleus as carrier of genetic information became clear only later , after mitosis was discovered and the <unk> rules were rediscovered at the beginning of the 20th century ; the chromosome theory of heredity was therefore developed . 
 
 = = Structures = = 
 
 The nucleus is the largest cellular organelle in animal cells . In mammalian cells , the average diameter of the nucleus is approximately 6 micrometres ( µm ) , which occupies about 10 % of the total cell volume . The <unk> liquid within it is called nucleoplasm , and is similar in composition to the <unk> found outside the nucleus . It appears as a dense , roughly spherical or irregular organelle . 
 
 = = = Nuclear envelope and pores = = = 
 
 The nuclear envelope , otherwise known as nuclear membrane , consists of two cellular membranes , an inner and an outer membrane , arranged parallel to one another and separated by 10 to 50 <unk> ( 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 <unk> freely between the nucleoplasm and the cytoplasm . The outer nuclear membrane is continuous with the membrane of the rough <unk> <unk> ( <unk> ) , and is similarly studded with ribosomes . The space between the membranes is called the <unk> space and is continuous with the <unk> <unk> . 
 Nuclear pores , which provide <unk> channels through the envelope , are composed of multiple proteins , collectively referred to as <unk> . The pores are about 125 million <unk> in molecular weight and consist of around 50 ( in yeast ) to several hundred proteins ( in vertebrates ) . The pores are 100 nm in total diameter ; however , the gap through which molecules freely diffuse is only about 9 nm wide , due to the presence of regulatory systems within the center of the pore . This size selectively allows the passage of small water @-@ soluble molecules while preventing larger molecules , such as nucleic acids and larger proteins , from <unk> 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 , each of which contains an <unk> @-@ <unk> ring @-@ shaped structure at a position where the inner and outer membranes fuse . Attached to the ring is a structure called the nuclear basket that extends into the nucleoplasm , and a series of <unk> extensions that reach into the cytoplasm . Both structures serve to mediate binding to nuclear transport proteins . 
 Most proteins , ribosomal subunits , and some <unk> are transported through the pore complexes in a process mediated by a family of transport factors known as karyopherins . Those karyopherins that mediate movement into the nucleus are also called importins , whereas those that mediate movement out of the nucleus are called exportins . Most karyopherins interact directly with their cargo , although some use <unk> proteins . <unk> hormones such as <unk> and <unk> , as well as other small <unk> @-@ soluble molecules involved in <unk> signaling , can diffuse through the cell membrane and into the cytoplasm , where they bind nuclear receptor proteins that are <unk> into the nucleus . There they serve as transcription factors when bound to their ligand ; in the absence of ligand , many such receptors function as <unk> <unk> that <unk> gene expression . 
 
 = = = Nuclear lamina = = = 
 
 In animal cells , two networks of intermediate filaments provide the nucleus with mechanical support : The nuclear lamina forms an organized <unk> on the internal face of the envelope , while less organized support is provided on the <unk> face of the envelope . Both systems provide structural support for the nuclear envelope and anchoring sites for chromosomes and nuclear pores . 
 The nuclear lamina is composed mostly of lamin proteins . Like all proteins , lamins are synthesized in the cytoplasm and later transported to the nucleus interior , where they are assembled before being incorporated into the existing network of nuclear lamina . <unk> found on the <unk> face of the membrane , such as <unk> and <unk> , bind to the cytoskeleton to provide structural support . <unk> are also found inside the nucleoplasm where they form another regular structure , known as the <unk> veil , 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 . <unk> structures that make up the veil , such as <unk> , bind chromatin and disrupting their structure inhibits transcription of protein @-@ coding genes . 
 Like the components of other intermediate filaments , the lamin <unk> 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 <unk> arrangement , to form a <unk> called a <unk> . Eight of these <unk> form a lateral arrangement that is twisted to form a <unk> filament . These filaments can be assembled or <unk> in a dynamic manner , meaning that changes in the length of the filament depend on the competing rates of filament addition and removal . 
 <unk> in lamin genes leading to defects in filament assembly cause a group of rare genetic disorders known as <unk> . The most notable <unk> is the family of diseases known as <unk> , which causes the appearance of premature aging in its <unk> . The exact mechanism by which the associated biochemical changes give rise to the aged phenotype is not well understood . 
 
 = = = <unk> = = = 
 
 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 . Each human cell contains roughly two meters of DNA . 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 <unk> . A small fraction of the cell 's genes are located instead in the mitochondria . 
 There are two types of chromatin . <unk> is the less compact DNA form , and contains genes that are frequently expressed by the cell . The other type , heterochromatin , is the more compact form , and contains DNA that is infrequently transcribed . This structure is further categorized into <unk> heterochromatin , consisting of genes that are organized as heterochromatin only in certain cell types or at certain stages of development , and <unk> heterochromatin that consists of chromosome structural components such as <unk> and <unk> . During interphase the chromatin <unk> itself into discrete individual patches , called chromosome territories . Active genes , which are generally found in the <unk> region of the chromosome , tend to be located towards the chromosome 's territory boundary . 
 Antibodies to certain types of chromatin organization , in particular , <unk> , have been associated with a number of <unk> diseases , such as systemic lupus <unk> . These are known as anti @-@ nuclear antibodies ( ANA ) and have also been observed in concert with multiple <unk> as part of general immune system <unk> . As in the case of <unk> , the role played by the antibodies in inducing the symptoms of <unk> diseases is not obvious . 
 
 = = = <unk> = = = 
 
 The nucleolus is a discrete densely stained structure found in the nucleus . It is not surrounded by a membrane , and is sometimes called a <unk> . It forms around tandem repeats of rDNA , DNA coding for ribosomal RNA ( rRNA ) . These regions are called nucleolar organizer regions ( <unk> ) . 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 <unk> of nucleolar structures . 
 In the first step of ribosome assembly , a protein called RNA polymerase I transcribes rDNA , which forms a large pre @-@ rRNA precursor . This is <unk> into the subunits <unk> , <unk> , and <unk> rRNA . The transcription , post @-@ transcriptional processing , and assembly of rRNA occurs in the nucleolus , aided by small nucleolar RNA ( <unk> ) molecules , some of which are derived from <unk> introns from messenger <unk> encoding genes related to ribosomal function . The assembled ribosomal subunits are the largest structures passed through the nuclear pores . 
 When observed under the electron microscope , the nucleolus can be seen to consist of three distinguishable regions : the innermost <unk> centers ( <unk> ) , surrounded by the dense <unk> component ( DFC ) , which in turn is bordered by the <unk> component ( <unk> ) . <unk> 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 <unk> are detected . Most of the cleavage and modification of <unk> occurs in the DFC , while the latter steps involving protein assembly onto the ribosomal subunits occur in the <unk> . 
 
 = = = 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 , <unk> interphase <unk> association ( <unk> ) , 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 a uniform mixture , but rather contains organized functional <unk> . 
 Other subnuclear structures appear as part of abnormal disease processes . For example , the presence of small <unk> rods has been reported in some cases of <unk> <unk> . This condition typically results from mutations in actin , and the rods themselves consist of mutant actin as well as other <unk> proteins . 
 
 = = = = 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 . When seen under an electron microscope , they resemble balls of tangled thread and are dense <unk> of distribution for the protein coilin . CBs are involved in a number of different roles relating to RNA processing , specifically small nucleolar RNA ( <unk> ) and small nuclear RNA ( <unk> ) maturation , and <unk> mRNA modification . 
 Similar to Cajal bodies are Gemini of Cajal 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 . Unlike CBs , gems do not contain small nuclear <unk> ( <unk> ) , but do contain a protein called survival of motor <unk> ( SMN ) whose function relates to <unk> <unk> . Gems are believed to assist CBs in <unk> <unk> , though it has also been suggested from microscopy evidence that CBs and gems are different manifestations of the same structure . Later <unk> studies have shown gems to be twins of Cajal bodies with the difference being in the coilin component ; Cajal bodies are SMN positive and coilin positive , and gems are SMN positive and coilin negative . 
 
 = = = = <unk> and <unk> domains = = = = 
 
 <unk> domains , or <unk> interphase <unk> associations , were first described in microscopy studies in 1991 . Their function remains unclear , though they were not thought to be associated with active DNA replication , transcription , or RNA processing . They have been found to often associate with discrete domains defined by dense localization of the transcription factor <unk> , which promotes transcription of small nuclear RNA ( <unk> ) . 
 
 = = = = PML bodies = = = = 
 
 <unk> leukaemia bodies ( PML bodies ) are spherical bodies found scattered throughout the nucleoplasm , measuring around 0 @.@ 1 – 1 @.@ 0 µm . They are known by a number of other names , including nuclear domain 10 ( <unk> ) , <unk> bodies , and PML <unk> domains . PML bodies are named after one of their major components , the promyelocytic leukemia protein ( PML ) . They are often seen in the nucleus in association with Cajal bodies and cleavage bodies . PML bodies belong to the nuclear matrix , an ill @-@ defined super @-@ structure of the nucleus proposed to anchor and regulate many nuclear functions , including DNA replication , transcription , or epigenetic <unk> . The PML protein is the key organizer of these domains that recruits an ever @-@ growing number of proteins , whose only common known feature to date is their ability to be <unk> . Yet , <unk> / - mice ( which have their PML gene deleted ) cannot assemble nuclear bodies , develop normally and live well , demonstrating that PML bodies are <unk> for most basic biological functions . 
 
 = = = = <unk> speckles = = = = 
 
 <unk> are subnuclear structures that are enriched in pre @-@ messenger RNA splicing factors and are located in the interchromatin regions of the nucleoplasm of mammalian cells . At the fluorescence @-@ microscope level they appear as irregular , <unk> structures , which vary in size and shape , and when examined by electron microscopy they are seen as clusters of interchromatin granules . <unk> are dynamic structures , and both their protein and RNA @-@ protein components can cycle continuously between speckles and other nuclear locations , including active transcription sites . Studies on the composition , structure and behaviour of speckles have provided a model for understanding the functional compartmentalization of the nucleus and the organization of the gene @-@ expression machinery splicing <unk> 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 <unk> of specific proteins . The splicing speckles are also known as nuclear speckles ( nuclear <unk> ) , splicing factor compartments ( SF compartments ) , interchromatin <unk> clusters ( <unk> ) , B snurposomes . B snurposomes are found in the amphibian <unk> nuclei and in <unk> <unk> embryos . B snurposomes appear alone or attached to the Cajal bodies in the electron <unk> of the amphibian nuclei . <unk> function as storage sites for the splicing factors . 
 
 = = = = <unk> = = = = 
 
 Discovered by Fox et al. in 2002 , paraspeckles are irregularly shaped compartments in the nucleus ' interchromatin space . First documented in <unk> cells , where there are generally 10 – 30 per nucleus , paraspeckles are now known to also exist in all human primary cells , transformed cell lines , and tissue sections . 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 . 
 <unk> are dynamic structures that are altered in response to changes in cellular metabolic activity . They are transcription dependent and in the absence of RNA <unk> II transcription , the <unk> disappears and all of its associated protein components ( <unk> , <unk> , <unk> , <unk> ( m ) 68 , and <unk> ) form a crescent shaped <unk> 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 <unk> . During <unk> , when the two daughter nuclei are formed , there is no RNA <unk> II transcription so the protein components instead form a <unk> cap . 
 
 = = = = <unk> fibrils = = = = 
 
 <unk> fibrils are visible only under electron microscope . They are located next to the <unk> active chromatin and are hypothesized to be the sites of active pre @-@ mRNA processing . 
 
 = = Function = = 
 
 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 main function of the cell nucleus is to control gene expression and mediate the replication of DNA during the cell cycle . 
 The nucleus is an organelle found in eukaryotic cells . Inside its fully enclosed nuclear membrane , it contains the majority of the cell 's genetic material . This material is organized as DNA molecules , along with a variety of proteins , to form chromosomes . 
 
 = = = 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 most cases where a <unk> process needs to be restricted , a key participant is removed to the nucleus , where it interacts with transcription factors to <unk> 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 . <unk> is an enzyme responsible for the first the step of glycolysis , forming glucose @-@ 6 @-@ phosphate from glucose . At high concentrations of <unk> @-@ 6 @-@ phosphate , a molecule made later from glucose @-@ 6 @-@ phosphate , a regulator protein removes <unk> to the nucleus , where it forms a transcriptional <unk> complex with nuclear proteins to reduce the expression of genes involved in glycolysis . 
 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 <unk> @-@ α , binds to a cell membrane receptor , resulting in the recruitment of <unk> proteins , and eventually activating the transcription factor NF @-@ κB . A nuclear <unk> 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 . 
 The compartmentalization allows the cell to prevent translation of <unk> mRNA . <unk> 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 ( <unk> ) mRNA , resulting in <unk> and <unk> proteins . 
 
 = = = Gene expression = = = 
 
 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 . 
 Since the nucleus is the site of transcription , it also contains a variety of proteins that either directly mediate transcription or are involved in regulating the process . These proteins include <unk> , which <unk> the double @-@ stranded DNA molecule to facilitate access to it , RNA <unk> , which synthesize the growing RNA molecule , <unk> , which change the amount of <unk> in DNA , helping it wind and <unk> , as well as a large variety of transcription factors that regulate expression . 
 
 = = = <unk> of pre @-@ mRNA = = = 
 
 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 cytoplasm without these modifications is degraded rather than used for protein translation . The three main modifications are 5 ' capping , 3 ' <unk> , and RNA splicing . While in the nucleus , pre @-@ mRNA is associated with a variety of proteins in complexes known as heterogeneous <unk> particles ( <unk> ) . <unk> of the 5 ' cap occurs co @-@ <unk> and is the first step in post @-@ transcriptional modification . The 3 ' <unk> @-@ <unk> tail is only added after transcription is complete . 
 RNA splicing , carried out by a complex called the <unk> , 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 ' <unk> but can begin before synthesis is complete in transcripts with many exons . Many pre @-@ <unk> , including those encoding antibodies , can be <unk> in multiple ways to produce different mature <unk> 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 . 
 
 = = <unk> and regulation = = 
 
 
 = = = Nuclear transport = = = 
 
 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 , macromolecules such as RNA and proteins require association karyopherins called importins to enter the nucleus and exportins to exit . " <unk> " proteins that must be <unk> 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 . The ability of importins and exportins to transport their cargo is regulated by <unk> , enzymes that <unk> the molecule <unk> <unk> to release energy . The key <unk> in nuclear transport is <unk> , which can bind either <unk> or GDP ( <unk> <unk> ) , depending on whether it is located in the nucleus or the cytoplasm . Whereas importins depend on RanGTP to <unk> from their cargo , exportins require RanGTP in order to bind to their cargo . 
 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 <unk> 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 . 
 <unk> 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 these molecules ' central role in protein translation . <unk> @-@ 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 . 
 
 = = = Assembly and disassembly = = = 
 
 During its lifetime , a nucleus may be broken down or destroyed , either in the process of cell division or as a consequence of apoptosis ( the process of programmed cell death ) . During these events , the structural components of the nucleus — the envelope and lamina — can be systematically degraded . In most cells , the disassembly of the nuclear envelope marks the end of the <unk> of mitosis . However , this disassembly of the nucleus is not a universal feature of mitosis and does not occur in all cells . Some <unk> eukaryotes ( e.g. , <unk> ) undergo so @-@ called closed mitosis , in which the nuclear envelope remains intact . In closed mitosis , the daughter chromosomes migrate to opposite poles of the nucleus , which then divides in two . The cells of higher eukaryotes , however , usually undergo open mitosis , which is characterized by breakdown of the nuclear envelope . The daughter chromosomes then migrate to opposite poles of the <unk> spindle , and new nuclei <unk> around them . 
 At a certain point during the cell cycle in open mitosis , the cell divides to form two cells . 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 , <unk> to microtubules , which in turn are attached to different centrosomes . The sister chromatids can then be pulled to separate locations in the cell . In many cells , the <unk> 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 . Therefore , the early stages in the cell cycle , beginning in <unk> and until around <unk> , the nuclear membrane is dismantled . Likewise , during the same period , the nuclear lamina is also <unk> , a process regulated by <unk> of the lamins by protein <unk> such as the <unk> protein kinase . Towards the end of the cell cycle , the nuclear membrane is reformed , and around the same time , the nuclear lamina are reassembled by <unk> the lamins . 
 However , in <unk> , the nuclear envelope remains intact , the centrosomes are located in the cytoplasm , and the microtubules come in contact with chromosomes , whose <unk> regions are incorporated into the nuclear envelope ( the so @-@ called closed mitosis with <unk> spindle ) . In many other <unk> ( e.g. , <unk> , <unk> ) and fungi , the centrosomes are <unk> , and their nuclear envelope also does not <unk> during cell division . 
 <unk> 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 <unk> called <unk> , which <unk> the lamin proteins and , thus , <unk> the nucleus ' structural integrity . <unk> cleavage is sometimes used as a laboratory indicator of <unk> activity in assays for early apoptotic activity . <unk> that express mutant <unk> @-@ 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 . <unk> of lamin assembly itself is an <unk> of apoptosis . 
 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 <unk> replicate and assemble in the cell nucleus , and exit by budding through the inner nuclear membrane . This process is accompanied by disassembly of the lamina on the nuclear face of the inner membrane . 
 
 = = = Disease @-@ related dynamics = = = 
 
 Initially , it has been suspected that <unk> in general and <unk> in particular do not enter the nucleus . Now there is a body of evidence that under <unk> conditions ( e.g. lupus <unk> ) <unk> can enter the nucleus . 
 
 = = <unk> per cell = = 
 
 Most eukaryotic cell types usually have a single nucleus , but some have no nuclei , while others have several . This can result from normal development , as in the maturation of mammalian red blood cells , or from faulty cell division . 
 
 = = = <unk> cells = = = 
 
 An <unk> cell contains no nucleus and is , therefore , incapable of dividing to produce daughter cells . The best @-@ known <unk> cell is the mammalian red blood cell , or <unk> , 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 . <unk> mature through <unk> in the bone marrow , where they lose their nuclei , organelles , and ribosomes . The nucleus is expelled during the process of differentiation from an <unk> to a <unk> , which is the immediate precursor of the mature <unk> . The presence of <unk> may induce the release of some immature " <unk> " <unk> into the bloodstream . <unk> cells can also arise from flawed cell division in which one daughter lacks a nucleus and the other has two nuclei . 
 In flowering plants , this condition occurs in <unk> tube elements . 
 
 = = = Multinucleated cells = = = 
 
 Multinucleated cells contain multiple nuclei . Most <unk> species of <unk> and some fungi in <unk> have naturally multinucleated cells . Other examples include the intestinal parasites in the genus <unk> , which have two nuclei per cell . In humans , skeletal muscle cells , called <unk> and <unk> , become multinucleated during development ; the resulting arrangement of nuclei near the periphery of the cells allows <unk> intracellular space for <unk> . Multinucleated and <unk> cells can also be abnormal in humans ; for example , cells arising from the fusion of <unk> and macrophages , known as giant multinucleated cells , sometimes accompany inflammation and are also implicated in tumor formation . 
 A number of <unk> are known to have two nuclei . Unlike other multinucleated cells these nuclei contain two distinct lineages of DNA : one from the <unk> and the other from a symbiotic <unk> . <unk> the <unk> and the <unk> of the <unk> remain functional . 
 
 = = Evolution = = 
 
 As the major defining characteristic of the eukaryotic cell , the nucleus ' evolutionary origin has been the subject of much speculation . Four major hypotheses have been proposed to explain the existence of the nucleus , although none have yet earned widespread support . 
 The first model known as the " <unk> model " proposes that a symbiotic relationship between the archaea and bacteria created the nucleus @-@ containing eukaryotic cell . ( <unk> of the <unk> and <unk> domain have no cell nucleus . ) It is hypothesized that the symbiosis originated when ancient archaea , similar to modern <unk> archaea , invaded and lived within bacteria similar to modern <unk> , eventually forming the early nucleus . This theory is analogous to the accepted theory for the origin of eukaryotic mitochondria and <unk> , which are thought to have developed from a similar <unk> relationship between proto @-@ eukaryotes and <unk> bacteria . The <unk> origin of the nucleus is supported by observations that archaea and <unk> have similar genes for certain proteins , including histones . Observations that <unk> are <unk> , can form multicellular complexes , and possess <unk> and G proteins similar to <unk> , support a bacterial origin for the eukaryotic cell . 
 A second model proposes that proto @-@ eukaryotic cells evolved from bacteria without an <unk> stage . This model is based on the existence of modern <unk> bacteria that possess a nuclear structure with primitive pores and other <unk> membrane structures . A similar proposal states that a <unk> @-@ like cell , the <unk> , evolved first and <unk> archaea and bacteria to generate the nucleus and the eukaryotic cell . 
 The most controversial model , known as viral <unk> , posits that the membrane @-@ bound nucleus , along with other eukaryotic features , originated from the infection of a <unk> by a virus . The suggestion is based on similarities between eukaryotes and viruses such as linear DNA strands , mRNA capping , and tight binding to proteins ( <unk> histones to viral <unk> ) . One version of the proposal suggests that the nucleus evolved in concert with <unk> to form an early cellular " predator " . Another variant proposes that eukaryotes originated from early archaea infected by <unk> , on the basis of observed similarity between the DNA <unk> in modern <unk> and eukaryotes . It has been suggested that the unresolved question of the evolution of sex could be related to the viral <unk> hypothesis . 
 A more recent proposal , the <unk> 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 . 
 
 = = Gallery = = 
 
 
 