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.
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 isolates 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 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
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.
Nuclear envelope and pores
2.2 Nuclear lamina
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
2.5.6 Perichromatin fibrils
3.1 Cell compartmentalization
3.3 Processing of pre-mRNA
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
7 See also
9 Further reading
10 External links
Oldest known depiction of cells and their nuclei by Antonie van
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
Antonie van Leeuwenhoek
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
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
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,
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 therefore developed.
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
Protein 11%, Residual Protein
Nuclear envelope and pores
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, 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
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
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.
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 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. 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 karyotype. A small fraction of the cell's genes are located
instead in the mitochondria.
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,
DNA that is 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. During interphase the chromatin organizes itself
into discrete individual patches, called chromosome
territories. Active genes, which are generally found in the
euchromatic region of the chromosome, tend to be located towards the
chromosome's territory boundary.
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
Main article: Nucleolus
An electron micrograph of a cell nucleus, showing the darkly stained
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 rDNA,
DNA coding for
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
DNA results in intermingling of nucleolar structures.
In the first step of ribosome assembly, a protein called RNA
polymerase I transcribes rDNA, which forms a large pre-r
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 small nucleolar
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.
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
DNA occurs either in the FC or at the FC-DFC
boundary, and, therefore, when r
DNA 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 occur in the
Other subnuclear bodies
Subnuclear structure sizes
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
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
RNA processing, specifically small nucleolar
and small nuclear
RNA (snRNA) maturation, and histone mRNA
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
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 domains defined by dense
localization of the transcription factor PTF, which promotes
transcription of small nuclear
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,
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
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 by electron microscopy they are seen
as clusters of interchromatin granules. Speckles 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
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. The splicing speckles are also known as nuclear speckles
(nuclear specks), splicing factor compartments (SF compartments),
interchromatin granule clusters (IGCs), B snurposomes. B
snurposomes are found in the amphibian oocyte nuclei and in Drosophila
melanogaster embryos. B snurposomes appear alone or attached to the
Cajal bodies in the electron micrographs of the amphibian nuclei.
IGCs function as storage sites for the splicing factors.
Main article: Paraspeckle
Discovered by Fox et al. in 2002, paraspeckles are irregularly shaped
compartments in the nucleus' interchromatin space. First
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
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
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
Clastosomes are small nuclear bodies (0.2-0.5 µm) described as
donut-shaped due to the peripheral capsule around these bodies.
This name is derived from the Greek klastos, broken and soma,
body. Clastosomes are not typically present in normal cells,
making them hard to detect. They form under high proteolysis
conditions within the nucleus and degrade once there is a decrease in
activity or if cells are treated with proteasome inhibitors.
The scarcity of clastosomes in cells indicates that they are not
required for proteasome function. Osmotic stress has also been
shown to cause the formation of clastosomes. These nuclear bodies
contain catalytic and regulatory sub-units of the proteasome and its
substrates, indicating that clastosomes are sites for degrading
The Clastosome would be an important addition to the ‘Cell
Nucleus’ Wiki page because it plays a role in cell function. With
further research, we may discover that this nuclear body has a more
influential job in cell function than what is already known.
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
along with a variety of proteins, to form chromosomes.
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
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.
The compartmentalization allows the cell to prevent translation of
RNA contains introns that must be
removed before being translated to produce functional proteins. The
splicing is done inside the nucleus before the m
RNA can be accessed by
ribosomes for translation. Without the nucleus, ribosomes would
translate newly transcribed (unprocessed) mRNA, resulting in malformed
and nonfunctional proteins.
A micrograph of ongoing gene transcription of ribosomal RNA
illustrating the growing primary transcripts. "Begin" indicates the 5'
end of the DNA, where new
RNA synthesis begins; "end" indicates the 3'
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,
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
RNA produced needs to be
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
RNA polymerases, which bind to the
DNA promoter to synthesize the
RNA molecule, topoisomerases, which change the amount of
supercoiling in DNA, helping it wind and unwind, as well as a large
variety of transcription factors that regulate expression.
Processing of pre-mRNA
Main article: Post-transcriptional modification
Newly synthesized m
RNA molecules are known as primary transcripts or
pre-mRNA. They must undergo post-transcriptional modification in the
nucleus before being exported to the cytoplasm; m
RNA that appears in
the cytoplasm 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-transcriptional modification. The 3' poly-adenine tail is only
added after transcription is complete.
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-m
RNA 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. 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
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
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" proteins that must be
translocated from the cytoplasm to the nucleus contain short amino
acid sequences known as nuclear localization signals, which are bound
by importins, while those transported from the nucleus to the
cytoplasm carry nuclear export signals bound by exportins. The ability
of importins and exportins to transport their cargo is regulated by
GTPases, enzymes that hydrolyze the molecule guanosine triphosphate to
release energy. The key
GTPase in nuclear transport is Ran, which can
bind either GTP or GDP (guanosine diphosphate), depending on whether
it is located in the nucleus or the cytoplasm. Whereas importins
depend on RanGTP to dissociate 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 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 m
RNA 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 excision of exons or mis-incorporation of
amino acids could have negative consequences for the cell; thus,
RNA that reaches the cytoplasm is degraded
rather than used in translation.
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
DNA and RNA
viruses from entering the nucleus. Some viruses require access to
proteins inside the nucleus in order to replicate and/or assemble. DNA
viruses, such as herpesvirus replicate and assemble in the cell
nucleus, and exit by budding through the inner nuclear membrane. This
process is accompanied by disassembly of the lamina on the nuclear
face of the inner membrane.
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
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
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
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
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
Bacteria domain have no cell nucleus.) It is hypothesized that the
symbiosis originated when ancient archaea, similar to modern
methanogenic archaea, invaded and lived within bacteria similar to
modern myxobacteria, eventually forming the early nucleus. This theory
is analogous to the accepted theory for the origin of eukaryotic
mitochondria and chloroplasts, which are thought to have developed
from a similar endosymbiotic relationship between proto-eukaryotes and
aerobic bacteria. The archaeal origin of the nucleus is supported
by observations that archaea and eukarya have similar genes for
certain proteins, including histones. Observations that myxobacteria
are motile, can form multicellular complexes, and possess kinases and
G proteins similar to eukarya, support a bacterial origin for 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
DNA strands, m
RNA capping, and tight binding to
proteins (analogizing histones to viral envelopes). One version of the
proposal suggests that the nucleus evolved in concert with
phagocytosis to form an early cellular "predator". Another variant
proposes that eukaryotes originated from early archaea infected by
poxviruses, on the basis of observed similarity between the DNA
polymerases in modern poxviruses and eukaryotes. It has been
suggested that the unresolved question of the evolution of sex could
be related to the viral eukaryogenesis hypothesis.
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.
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Library resources about
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. doi:10.1101/gad.960502.
A review article about nuclear lamins, explaining their structure and
Görlich, Dirk; Kutay, U (1999). "Transport between the cell nucleus
and the cytoplasm". Annu. Rev. Cell Dev. Biol. 15: 607–660.
doi:10.1146/annurev.cellbio.15.1.607. PMID 10611974.
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" (PDF). Science. 280 (5363): 547–553.
doi:10.1126/science.280.5363.547. PMID 9554838.
A review article about the nucleus, explaining the structure of
chromosomes within the organelle, and describing the nucleolus and
other subnuclear bodies
Pennisi E. (2004). "Evolutionary biology. The birth of the nucleus".
Science. 305 (5685): 766–768. doi:10.1126/science.305.5685.766.
A 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 University of Alberta.
http://npd.hgu.mrc.ac.uk/user/?page=compartment The Nuclear Protein
Database] Information on nuclear components.
The Nucleus Collection in the Image & Video Library of The
American Society for Cell Biology contains peer-reviewed still images
and video clips that illustrate the nucleus.
Nuclear Envelope and Nuclear Import Section from Landmark Papers in
Cell Biology, Joseph G. Gall, J. Richard McIntosh, eds., contains
digitized commentaries and links to seminal research papers on the
nucleus. Published online in the Image & Video Library of The
American Society for Cell Biology
Cytoplasmic patterns generated by human antibodies
Structures of the cell / organelles
Spindle pole body
Structures of the cell nucleus / nuclear protein
Cajal (coiled) body
Dot (PML body)
Transition nuclear protein:
Nuclear matrix (Nucleoskeleton)
see also transcription factors and intracellular receptors
see also nucleus diseases