In cell biology , the NUCLEUS (pl. nuclei; from Latin nucleus or nuculeus, meaning kernel or seed) is a membrane -enclosed organelle found in eukaryotic cells . Eukaryotes 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 . Human skeletal muscle cells have more than one nucleus, as do eukaryotes like fungi.
Cell nuclei contain most of the cell's genetic material , organized
as multiple long linear
Because the nuclear membrane is impermeable to large molecules,
nuclear pores are required to 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
* 1 History
* 2 Structures
* 2.5 Other subnuclear bodies
* 2.5.1 Cajal bodies and gems * 2.5.2 RAFA and PTF domains * 2.5.3 PML bodies * 2.5.4 Splicing speckles * 2.5.5 Paraspeckles * 2.5.6 Perichromatin fibrils
* 3 Function
* 4 Dynamics and regulation
* 4.1 Nuclear transport * 4.2 Assembly and disassembly * 4.3 Disease-related dynamics
* 5 Nuclei per cell
* 5.1 Anucleated cells * 5.2 Multinucleated cells
Oldest known depiction of cells and their nuclei by Antonie van Leeuwenhoek , 1719 Drawing of a Chironomus salivary gland cell published by Walther Flemming in 1882. The nucleus contains Polytene chromosomes .
The nucleus was the first organelle to be discovered. What is most likely the oldest preserved drawing dates back to the early microscopist Antonie van Leeuwenhoek (1632–1723). He observed a "lumen", the nucleus, in the red blood cells of salmon . 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 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 outer layer.
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.
Between 1877 and 1878,
Oscar Hertwig published several studies on the
fertilization of sea urchin eggs, showing that the nucleus of the
sperm enters the oocyte and fuses with its nucleus. This was the first
time it was suggested that an individual develops from a (single)
nucleated cell. This was in contradiction to
Ernst Haeckel 's theory
that the complete phylogeny of a species would be repeated during
embryonic development, including generation of the first nucleated
cell from a "monerula", a structureless mass of primordial mucus
("Urschleim"). Therefore, the necessity of the sperm nucleus for
fertilization was discussed for quite some time. However, Hertwig
confirmed his observation in other animal groups, including amphibians
and molluscs .
Eduard Strasburger produced the same results for plants
in 1884. This paved the way to assign the nucleus an important role in
heredity. In 1873,
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 (or
karyolymph), and is similar in composition to the cytosol found
outside the nucleus. It appears as a dense, roughly spherical or
irregular organelle. The composition by dry weight of the nucleus is
NUCLEAR ENVELOPE AND PORES
The eukaryotic cell nucleus. Visible in this diagram are the ribosome -studded double membranes of the nuclear envelope, the DNA (complexed as chromatin ), and the nucleolus . Within the cell nucleus is a viscous liquid called nucleoplasm , similar to the cytoplasm found outside the nucleus. A cross section of a nuclear pore on the surface of the nuclear envelope (1). Other diagram labels show (2) the outer ring, (3) spokes, (4) basket, and (5) filaments.
The nuclear envelope , otherwise known as nuclear membrane, consists of two cellular membranes , an inner and an outer membrane, arranged parallel to one another and separated by 10 to 50 nanometres (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. 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 .
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 total diameter; however, the gap through which molecules freely diffuse is only about 9 nm wide, due to the presence of regulatory systems within the center of the pore. This size selectively allows the passage of small water-soluble molecules while preventing larger molecules, such as nucleic acids and larger proteins, from inappropriately entering or exiting the nucleus. These large molecules must be actively transported into the nucleus instead. The nucleus of a typical mammalian cell will have about 3000 to 4000 pores throughout its envelope, each of which contains an eightfold-symmetric 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 filamentous extensions that reach into the cytoplasm. Both structures serve to mediate binding to nuclear transport proteins.
Most proteins, ribosomal subunits, and some DNAs are transported through the pore complexes in a process mediated by a family of transport factors known as karyopherins . Those karyopherins that mediate movement into the nucleus are also called importins, whereas those that mediate movement out of the nucleus are called exportins. Most karyopherins interact directly with their cargo, although some use adaptor proteins . 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 a ligand, many such receptors function as histone deacetylases that repress gene expression.
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.
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, such as emerin and nesprin , bind to the cytoskeleton to provide structural support. Lamins are also found inside the nucleoplasm where they form another regular structure, known as the nucleoplasmic 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 . Lamin structures that make up the veil, such as LEM3 , bind chromatin and disrupting their structure inhibits transcription of protein-coding genes.
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.
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 biochemical changes give rise to the aged phenotype is not well understood.
The cell nucleus contains the majority of the cell's genetic material
in the form of multiple linear
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
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 general immune system dysfunction. As in the case of progeria , the role played by the antibodies in inducing the symptoms of autoimmune diseases is not obvious.
Main article: Nucleolus An electron micrograph of a cell nucleus, showing the darkly stained nucleolus 3D rendering of nucleus with location of 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 r
In the first step of ribosome assembly, a protein called RNA
polymerase I transcribes rDNA, which forms a large pre-r
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 r
OTHER SUBNUCLEAR BODIES
SUBNUCLEAR STRUCTURE SIZES STRUCTURE NAME STRUCTURE DIAMETER
Cajal bodies 0.2–2.0 µm
PIKA 5 µm
PML bodies 0.2–1.0 µm
Paraspeckles 0.2–1.0 µm
Speckles 20–25 nm
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 a uniform mixture, but rather contains organized functional subdomains.
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 cytoskeletal 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 foci of distribution for the protein
coilin . CBs are involved in a number of different roles relating to
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 ribonucleoproteins (snRNPs), but do contain a protein called survival of motor neuron (SMN) whose function relates to snRNP biogenesis. Gems are believed to assist CBs in snRNP biogenesis, though it has also been suggested from microscopy evidence that CBs and gems are different manifestations of the same structure. Later ultrastructural 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.
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
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 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 silencing. 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 SUMOylated . Yet, pml-/- mice (which have their PML gene deleted) cannot assemble nuclear bodies, develop normally and live well, demonstrating that PML bodies are dispensable for most basic biological functions.
Speckles are subnuclear structures that are enriched in pre-messenger
Main article: Paraspeckle
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. 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.
Paraspeckles are dynamic structures that are altered in response to
changes in cellular metabolic activity. They are transcription
dependent and in the absence of
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-m
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
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
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 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, where it forms a transcriptional repressor 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
The compartmentalization allows the cell to prevent translation of
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
PROCESSING OF PRE-MRNA
Main article: Post-transcriptional modification
Newly synthesized m
DYNAMICS AND REGULATION
Main article: Nuclear transport Macromolecules , such as RNA and proteins , are actively transported across the nuclear membrane in a process called the Ran -GTP nuclear transport cycle.
The entry and exit of large molecules from the nucleus is tightly
controlled by the nuclear pore complexes. Although small molecules can
enter the nucleus without regulation, macromolecules such as
Nuclear import depends on the importin binding its cargo in the cytoplasm and carrying it through the nuclear pore into the nucleus. Inside the nucleus, RanGTP acts to separate the cargo from the importin, allowing the importin to exit the nucleus and be reused. Nuclear export is similar, as the exportin binds the cargo inside the nucleus in a process facilitated by RanGTP, exits through the nuclear pore, and separates from its cargo in the cytoplasm.
Specialized export proteins exist for translocation of mature mRNA
ASSEMBLY AND DISASSEMBLY
An image of a newt lung cell stained with fluorescent dyes during metaphase . The mitotic spindle can be seen, stained green, attached to the two sets of chromosomes , stained light blue. All chromosomes but one are already at the metaphase plate.
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 prophase of mitosis . However, this disassembly of the nucleus is not a universal feature of mitosis and does not occur in all cells. Some unicellular eukaryotes (e.g., yeasts) 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 mitotic spindle, and new nuclei reassemble 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 , attaching 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 centrosome is located in the cytoplasm, outside the nucleus; the microtubules would be unable to attach to the chromatids in the presence of the nuclear envelope. Therefore, the early stages in the cell cycle, beginning in prophase and until around prometaphase , the nuclear membrane is dismantled. Likewise, during the same period, the nuclear lamina is also disassembled, a process regulated by phosphorylation of the lamins by protein kinases such as the CDC2 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 dephosphorylating the lamins.
However, in dinoflagellates , the nuclear envelope remains intact, the centrosomes are located in the cytoplasm, and the microtubules come in contact with chromosomes, whose centromeric regions are incorporated into the nuclear envelope (the so-called closed mitosis with extranuclear spindle). In many other protists (e.g., ciliates , sporozoans ) and fungi, the centrosomes are intranuclear, and their nuclear envelope also does not disassemble during cell division.
Apoptosis is a controlled process in which the cell's structural components are destroyed, resulting in death of the cell. Changes associated with apoptosis directly affect the nucleus and its contents, for example, in the condensation of chromatin and the disintegration of the nuclear envelope and lamina. The destruction of the lamin networks is controlled by specialized apoptotic proteases called caspases , which cleave the lamin proteins and, thus, degrade the nucleus' structural integrity. Lamin cleavage is sometimes used as a laboratory indicator of caspase activity in assays for early apoptotic activity. Cells that express mutant caspase-resistant lamins are deficient in nuclear changes related to apoptosis, suggesting that lamins play a role in initiating the events that lead to apoptotic degradation of the nucleus. Inhibition of lamin assembly itself is an inducer of apoptosis.
The nuclear envelope acts as a barrier that prevents both
Initially, it has been suspected that immunoglobulins in general and autoantibodies in particular do not enter the nucleus. Now there is a body of evidence that under pathological conditions (e.g. lupus erythematosus ) IgG can enter the nucleus.
NUCLEI 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.
Human red blood cells, like those of other mammals, lack nuclei. This occurs as a normal part of the cells' development.
An anucleated cell contains no nucleus and is, therefore, incapable of dividing to produce daughter cells. The best-known anucleated cell is the mammalian red blood cell, or erythrocyte , which also lacks other organelles such as mitochondria , and serves primarily as a transport vessel to ferry oxygen from the lungs to the body's tissues. Erythrocytes mature through erythropoiesis in the bone marrow , where they lose their nuclei, organelles, and ribosomes. The nucleus is expelled during the process of differentiation from an erythroblast to a reticulocyte , which is the immediate precursor of the mature erythrocyte. The presence of mutagens may induce the release of some immature "micronucleated" erythrocytes into the bloodstream. Anucleated 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 sieve tube elements .
Multinucleated cells contain multiple nuclei. Most acantharean species of protozoa and some fungi in mycorrhizae have naturally multinucleated cells. Other examples include the intestinal parasites in the genus Giardia , which have two nuclei per cell. In humans, skeletal muscle cells, called myocytes and syncytium , become multinucleated during development; the resulting arrangement of nuclei near the periphery of the cells allows maximal intracellular space for myofibrils . Multinucleated and binucleated cells can also be abnormal in humans; for example, cells arising from the fusion of monocytes and macrophages , known as giant multinucleated cells , sometimes accompany inflammation and are also implicated in tumor formation.
A number of dinoflagellates are known to have two nuclei. Unlike other multinucleated cells these nuclei contain two distinct lineages of DNA: one from the dinoflagellate and the other from a symbiotic diatom . The mitochondria and the plastids of the diatom somehow remain functional.
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 "syntrophic model" proposes that a
symbiotic relationship between the archaea and bacteria created the
nucleus-containing eukaryotic cell. (Organisms of the
A second model proposes that proto-eukaryotic cells evolved from bacteria without an endosymbiotic stage. This model is based on the existence of modern planctomycetes bacteria that possess a nuclear structure with primitive pores and other compartmentalized membrane structures. A similar proposal states that a eukaryote-like cell, the chronocyte , evolved first and phagocytosed archaea and bacteria to generate the nucleus and the eukaryotic cell.
The most controversial model, known as viral eukaryogenesis , posits
that the membrane-bound nucleus, along with other eukaryotic features,
originated from the infection of a prokaryote by a virus. The
suggestion is based on similarities between eukaryotes and viruses
such as linear
A more recent proposal, the exomembrane hypothesis, suggests that the nucleus instead originated from a single ancestral cell that evolved a second exterior cell membrane; the interior membrane enclosing the original cell then became the nuclear membrane and evolved increasingly elaborate pore structures for passage of internally synthesized cellular components such as ribosomal subunits.
Comparison of human and chimpanzee chromosomes. *
Mouse chromosome territories in different cell types. *
24 chromosome territories in human cells.
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Library resources about CELL NUCLEUS -------------------------
* Online books * Resources in your library * Resources in other libraries
* Goldman, Robert D.; Gruenbaum, Y; Moir, RD; Shumaker, DK; Spann, TP (2002). "Nuclear lamins: building blocks of nuclear architecture". Genes & Dev. 16 (5): 533–547. PMID 11877373 . doi :10.1101/gad.960502 .
A review article about nuclear lamins, explaining their structure and various roles
* Görlich, Dirk; Kutay, U (1999). "Transport between the cell nucleus and the cytoplasm". Annu. Rev. Cell Dev. Biol. 15: 607–660. PMID 10611974 . doi :10.1146/annurev.cellbio.15.1.607 .
A review article about nuclear transport, explains the principles of the mechanism, and the various transport pathways
* Lamond, Angus I.; Earnshaw, WC (1998-04-24). "Structure and Function in the Nucleus". Science. 280 (5363): 547–553. PMID 9554838 . doi :10.1126/science.280.5363.547 .
A review article about the nucleus, explaining the structure of chromosomes within the organelle, and describing the nucleolus and other subnuclear bodies
* Pennisi E. (2004). "Evolutionary biology. The birth of the nucleus". Science. 305 (5685): 766–768. PMID 15297641 . doi :10.1126/science.305.5685.766 .
A review article about the evolution of the nucleus, explaining a number of different theories
* Pollard, Thomas D.; William C. Earnshaw (2004). Cell Biology. Philadelphia: Saunders. ISBN 0-7216-3360-9 .
A university level textbook focusing on cell biology. Contains information on nucleus structure and function, including nuclear transport, and subnuclear domains
Wikimedia Commons has media related to CELL NUCLEUS .
* MBInfo - The Nucleus * cellnucleus.com Website covering structure and function of the nucleus from the Department of Oncology at the