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Because the nuclear membrane is impermeable to large molecules , nuclear pores are required that regulate nuclear transport of molecules across the envelope . The pores cross both nuclear membranes , providing a channel through which larger molecules must be actively transported by carrier proteins while allowing free... |
= = History = = |
The nucleus was the first organelle to be discovered . What is most likely the oldest preserved drawing dates back to the early microscopist Antonie van Leeuwenhoek ( 1632 – 1723 ) . He observed a " lumen " , the nucleus , in the red blood cells of salmon . Unlike mammalian red blood cells , those of other vertebrates... |
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 Linnean Society of London . Brown was studying orchids under microscope when he observed an opaque area , which he called the " areola " or " nucleus " , in the cells of the flower 's out... |
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 ,... |
Between 1877 and 1878 , Oscar Hertwig published several studies on the fertilization of sea urchin eggs , showing that the nucleus of the sperm enters the oocyte and fuses with its nucleus . This was the first time it was suggested that an individual develops from a ( single ) nucleated cell . This was in contradictio... |
= = 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 viscous liquid within it is called nucleoplasm , and is similar in composition to the cytosol found o... |
= = = 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 nanometres ( nm ) . The nuclear envelope completely encloses the nucleus and separates the cell 's genetic material from the surr... |
Nuclear pores , which provide aqueous channels through the envelope , are composed of multiple proteins , collectively referred to as nucleoporins . The pores are about 125 million daltons in molecular weight and consist of around 50 ( in yeast ) to several hundred proteins ( in vertebrates ) . The pores are 100 nm in... |
Most proteins , ribosomal subunits , and some DNAs are transported through the pore complexes in a process mediated by a family of transport factors known as karyopherins . Those karyopherins that mediate movement into the nucleus are also called importins , whereas those that mediate movement out of the nucleus are c... |
= = = 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 n... |
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 . Lamins found on the cytosolic face of the membrane , suc... |
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 calle... |
Mutations in lamin genes leading to defects in filament assembly cause a group of rare genetic disorders 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 bioc... |
= = = Chromosomes = = = |
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... |
There are two types of chromatin . Euchromatin 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 facultative heterochr... |
Antibodies to certain types of chromatin organization , in particular , nucleosomes , have been associated with a number of autoimmune diseases , such as systemic lupus erythematosus . These are known as anti @-@ nuclear antibodies ( ANA ) and have also been observed in concert with multiple sclerosis as part of gener... |
= = = 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 th... |
In the first step of ribosome assembly , a protein called RNA polymerase I transcribes rDNA , which forms a large pre @-@ rRNA precursor . This is cleaved into the subunits 5.8S , 18S , and 28S rRNA . The transcription , post @-@ transcriptional processing , and assembly of rRNA occurs in the nucleolus , aided by smal... |
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 t... |
= = = 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 know... |
Other subnuclear structures appear as part of abnormal disease processes . For example , the presence of small intranuclear rods has 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 cytoskeleta... |
= = = = 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 foci of distribu... |
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... |
= = = = RAFA and PTF domains = = = = |
RAFA domains , or polymorphic interphase karyosomal 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 ... |
= = = = PML bodies = = = = |
Promyelocytic 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 ( ND10 ) , Kremer bodies , and PML oncogenic domains . PML bodies are named after one of their ... |
= = = = Splicing speckles = = = = |
Speckles 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 , punctate structures , which vary in size and shape , and when examined... |
= = = = Paraspeckles = = = = |
Discovered by Fox et al. in 2002 , paraspeckles are irregularly shaped compartments in the nucleus ' interchromatin space . First documented in HeLa 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... |
Paraspeckles are dynamic structures that are altered in response to changes in cellular metabolic activity . They are transcription dependent 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... |
= = = = Perichromatin fibrils = = = = |
Perichromatin fibrils are visible only under electron microscope . They are located next to the transcriptionally 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 ... |
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 cytoplasmic process needs to be restricted , a key participant is removed to th... |
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 , ... |
The compartmentalization allows the cell to prevent translation of unspliced mRNA . 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 ,... |
= = = 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 , m... |
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 helicases , which unwind the double @-@ stranded DNA molecule to facilitate access to it , RNA polymerases , which synthe... |
= = = Processing 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 t... |
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 ' po... |
= = Dynamics 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 . " 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 exportin binds th... |
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 these molecules ' central role in protein translation . Mis @-@ expression of a protein due to incomplete excisio... |
= = = Assembly and disassembly = = = |
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