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The nucleus (pl. nuclei; latin nucleus or nuculeus,
meaning kernel) is a membrane-enclosed organelle
found in eukaryotic cell.
It contains most of the cell's genetic material.
Genetic material is organized as multiple long
linear DNA molecules in complex with proteins,
histones to form chromosomes.
The genes within these chromosomes are the cell's
nuclear genome.
The function of the nucleus is to maintain the
integrity of these genes and to control the activities
of the cell
Nucleus regulates gene expression therefore, it is a
control center of the cell.
The genetic material is surrounded by the nuclear
membrane known as nuclear envelop
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The first organelle discovered in the cell was nucleus.
Antonie van Leeuwenhoek (1632 – 1723). "Lumen", the nucleus,
in RBCs of salmon
Franz Bauer (1804) described nucleus in more detail.
Robert Brown (Scottish) (1831) described it in a talk at meeting
of the Linnean Society of London. He was studying orchids and
observed an opaque area, which he called the areola or nucleus,
in the cells of the flower's outer layer.
Matthias Schleiden (1838), suggested that nucleus plays a role in
generating cells, and called it "Cytoblast" (cell builder).
Franz Meven was a strong opponent of this view, having already
described cells multiplying by division and believing that many
cells would have no nuclei.
Robert Remak (1852) and Rudolf Virchow (1855) decisively
propagated the new paradigm that cells are generated solely by
cells. The function of the nucleus remained unclear by then.
Oscar Hertwig (1877- 1878) 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.
 The
function of the nucleus as carrier of
genetic information became clear 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
 Site of transcription of DNA into mRNA, rRNA
and tRNA
 Site of protein-rRNA assembly into ribosomal
subunits
The nucleus is the largest cellular organelle in
eukaryotic cell.
 The average diameter of the nucleus is
approximately 6 μm, which occupies about
10% of the total cell volume.
 The
viscous liquid within it is called
nucleoplasm.
 It appears as a dense, roughly spherical
organelle
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 Nucleus
is surrounded by double unit
membrane called nuclear envelope
 Nuclear envelope is perforated by nuclear
pores.
 Pores have a channel made of transport
proteins for transfer of molecules like
proteins and RNA inside actively and small
molecules like ions out side
 Sub-nuclear bodies exist, made up of
unique proteins and RNA molecules
the nucleolus
 Nucleolus is a site of production of
ribosome that are exported to the
cytoplasm where they act as sites of mRNA
translation into proteins
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DNA- protein complex is known chromatin which can be
seen in side the nucleus as dense granular material
Chromatin are of two types Euchromatin that is less
compact DNA form, and contains genes that are
frequently expressed by the cell
The other type heterochromatin the more compact
form, and contains DNA that is infrequently transcribed
Heterochromatin is further categorized into facultative
heterochromatin consisting of genes that are organized
only in certain cell types or at certain stages of
development
Constitutive heterochromatin consists of chromosome
structural
components
such
as
telomers and
centromeres
During interphase the chromatin organizes itself into
discrete individual patches called chromosomes
Active genes are towards the boundary of chromosome
The nuclear envelope, consists of two cellular
membranes, an inner and an outer membrane,
arranged parallel to one another and separated
by 10-50 nm perinuclear space
 Serves 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 also studded
with ribosomes
 The space between the membranes is called the
perinuclear space and is continuous with the
lumen of RER
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Nuclear pores provide aqueous channels through
the envelope, are composed of multiple proteins
known as nucleoporins.
 The pores are 25 million Da in MW and consist of
around 50 (Yeast) to several hundred proteins
(vertebrates).
 The pores are 100 nm ф; the gap through which
molecules freely diffuse is 9 nm wide. This size
selectively allows the passage of small watersoluble molecules while preventing larger
molecules, such as nucleic acids and larger
proteins
 These
large molecules must be actively
transported into the nucleus instead.
 The nucleus of a typical mammalian cell have
about 3000 to 4000 pores
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Pore 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
karyoporines
Those karyoporines that mediate movement into the
nucleus are also called importins, whereas those that
mediate movement out of the nucleus are called
exportins
 Steroid
hormones and other small lipidsoluble molecules involved in intercellular
signalling, can diffuse through the cell
membrane and into the cytoplasm, where
they bind with nuclear receptor proteins
that are transported into the nucleus.
 There
they serve as transcriptional
factors when bound to their receptors or
function as histone deacetylases that
repress gene expression
A network of protein filaments provide the
nucleus with mechanical support are known as
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
of lamin proteins which are synthesized in the
cytoplasm and later transported into the nucleus
 Nuclear matrix (frame work of protein fibers)
and lamina may help synthesize the genetic
material efficiently
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 In
non dividing nucleus a darkly stained
structure is nucleolus (pl; nucleoli)
 There may be one or more in single nucleus
 Consists of granules and fibers of chromatin
 Site of rRNA synthesis and assembling the
imported proteins with rRNA to make
ribosomal large and small subunits
 These subunits then exit the nucleus into the
cytoplasm and assemble into ribosome the
site of protein synthesis
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When observed with TEM, the nucleolus can be
seen to consist of three distinguishable regions
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The innermost fibrillar centers (FCs), surrounded
by the dense fibrillar component (DFC), which in
turn is bordered by the granular component (GC)
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Transcription of the rDNA occurs either in the FC
or at the FC-DFC boundary, and, therefore, when
rDNA transcription in the cell is increased, more
FCs are detected.
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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 granular component
 DNA
is organized into discrete units during
cell division called chromosomes that carry
the genetic information
 Chromosome consists of one long molecule of
DNA associated with many proteins
 Proteins allow the DNA to coil around to
reduce its length to fit into the nucleus
 In dividing cells chromatin condensed to form
chromosomes
 Their # is species specific: Human (46), fruit
fly (8) diploid number and 23 and 4 haploid
number respectively
 Chromosome
consists of two chromatids
having one long (q) and other short (p) arm
 Two arms are joined at a region known as
centromere
 The
ends of chromosome are known
telomeres
CHROMOSOME
Site of DNA replication
 Site of mRNA synthesis
 Site of tRNA synthesis
 Site of rRNA synthesis
 Site of assembly of ribosomal subunits
 Nucleus directs protein synthesis by synthesizing
mRNA per instructions of DNA
 mRNA is transported to cytoplasm via nuclear
pores
 In the cytoplasm mRNA attaches with the
ribosomes and translate mRNA’s message into
primary structure of specific proteins via
translation
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During its lifetime, a nucleus may be broken down,
either in the process of cell division or as
apoptosis (process of programmed cell death)
During these events the nuclear 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
unicellular eukaryotes (yeasts) undergo so-called closed
mitosis in which the nuclear envelope remains intact. In
close mitosis, the daughter chromosomes migrate to
opposite poles of the nucleus, which then divides in
two.
The cells of higher eukaryotes undergo open mitosis in
which nuclear envelope breaks down. The daughter
chromosomes then migrate to opposite poles of the
mitotic spindle, and new nuclei reassemble around
them.