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• The cell which we have already described is a typical
cell in which the nucleus and cytoplasm with its
organized are found.
• Such cell which contains a true nucleus is referred
to as eukaryotic cell (Gr.: Karyon = nucleus),
• But if the nuclear envelope is lacking, and thus, the
nuclear substance is in direct contact with the rest
of the protoplasm the cell is called =
prokaryotic cell (e.g., bacteria, some algae and most
viruses).
What is the nucleus?
• The nucleus is the brain of
eukaryotic cells.
• It is only present in eukaryotic cells
(which are eukaryotic because they
have a nucleus) and there is only
one of these organelles in each cell.
• It is a membrane bound structure
that contains the cell's hereditary
information and controls the cell's
growth and reproduction.
The nucleus has three primary responsibilities:
• execution and regulation of cytoplasmic events through its
gene products, storage of information in the form of the
DNA of genes, and retrieval of information from these genes
in the form of RNA.
• The nucleus is the site for three important processes:
• replication of chromatin (DNA + protein), transcription of
genes into RNA,
• and processing of those RNAs into mature forms, many of
which are exported to the cytosol for translation.
• The cell nucleus is a remarkable organelle because it
forms the package for our genes and their controlling
factors. It functions to:
1. Store genes on chromosomes.
2. Organize genes into chromosomes to allow cell
division.
3. Transport regulatory factors & gene products via
nuclear pores
4. Produce messages ( messenger Ribonucleic acid or
mRNA) that code for proteins .
5. Produce ribosomes in the nucleolus .
6. Organize the uncoiling of DNA to replicate key genes.
7. The nucleus also controls the synthesis of proteins in
1)
Nuclear envelope. ( erop raelcuN (2(3)Nucleolus.( 4) Chromatin .(5) (
Nucleus(6 ( .Nucleoplasm.
2)
3)
The whole structure is surrounded by cytoplasm .
Similar to the cytoplasm of a cell, the nucleus contains nucleoplasm - a
highly viscous liquid containing the chromosomes and nucleoli.
Chromosones contain information encoded in DNA attached to proteins
called histones and are usually arranged in to a dense network called
chromatin .
Nucleoli are granular structures which make ribonucleic DNA (rDNA)
and assemble it with proteins .
4)
5)
• The nucleus is the most prominent organelle in
the cell, taking up about 10% of the total
volume of the cell.
• Usually it is round and is the largest organelle
in the cell. The nucleus varies in diameter from
10 to 20 micrometres
• The cell nucleus is a remarkable organelle
because it forms the package for our genes and
their controlling factors
• The nucleus is surrounded by a double
membrane (each with the typical unit
membrane structure) forming the
nuclear
envelope
,.
(nuclear
membrane), which is similar to the
cell membrane, about 30 nm wide.
•
This membrane separates the contents
of the nucleus from the cytoplasm (cyto).
• The envelope helps to maintain the
shape of the nucleus and assists in
regulating the flow of molecules into
and out of the nucleus through nuclear
pores.
The nuclear envelope is supported from the exterior by the
intermediate filament network and microtubules,
• while the nuclear lamina, which is located adjacent to
• the inner surface of the nuclear envelope, provides internal
support.
•
The space between the outer and inner membranes is also
continuous with rough endoplasmic reticulum space.
•
It can fill with newly synthesized proteins just as the
rough endoplasmic reticulum does.
1.
The inner and outer membrane fuse at
regular spaces, forming nuclear pores,
and which is continuous with the
endoplasmic reticulum.
2.
The nuclear pore complexes are the
ports through which certain proteins
enter and RNAs exit the nucleus.
• nuclear pores allow specific
materials to pass in and out
of the nucleus, just like
proteins
in
the
cell
membrane regulate the
movement of molecules in
and out of the cell itself.
• These pores permit the
movement
of
selected
molecules, like mRNA, from
the nucleus to the rest of the
cell.
• The protoplasm of the cell-body is usually
known as cytoplasm or cytosome, while that
of the nucleus is referred to as
karyoplasms.
“Karyoplasmic ratio”
• It has been found that in any species there is a relationship between
the size of nucleus and the size of the cytoplasm, this is known as the
“Karyoplasmic ratio” This has been first originated
protozoa and then extended to other animals.
• The karyoplasmic ration has considerable importance in the life of
the cell, and is probably one of the factors which brings about celldivision
• The relation usually holds in cells which are homologous (i.e., of
similar origin and function) as shown in sea-urchin larvae either
from fertilized or unfertilized eggs. However, under different
conditions this ration may vary.
The interphase nucleus
• The nucleus in the interphase (the phase between two
successive divisions) contains the chromatin substance;
some of the larger flakes of chromatin are called
chromocentres, karyosomes or false nucleoli because they
are morphologically similar to some nucleoli.
• Chromatin substance is distributed throughout the nuclear
sap (nucleoplasm).
• In addition, there are one or more spherical bodies, the
nucleoli which differ from the karyosomes in some staining
properties and in chemical composition.
The nucleus also contains the nucleolus,
Each cell nucleus usually contains at least
one nucleolus which helps to synthesize
ribosomes.
It is very densely concentrated and appears
as a dark, almost perfectly circular dot in
the middle of the unstained nuclear region.
The nucleolus contains ribosomes, RNA,
DNA, and proteins. The nucleolus has some
of the ribosomes that synthesize proteins
(others are in the endoplasmic reticulum).
The mRNA content in this region is very
high: it's being produced in large amounts to
facilitate the protein-making activity .
Copious amounts of RNA and proteins can
be found in the nucleolus as well.
r RNA is produced in the nucleolus of the
cell and travels to the cytoplasm through the
pores of the nuclear envelope.
The
nucleolus
• The nucleolus contains nucleolar organizers which are parts of
chromosomes with the genes for ribosome synthesis on them. A number
of chromosomes get together and transcribe ribosomal RNA at this site.
• The above figure shows electron micrograph of a nucleolus with the
nuclear organizing region. The nuclear organizing (NO) regions are seen
as circular areas (pale) surrounded by a rim of electron dense filaments.
These filaments collectively are called the pars fibrosa (PF).
• This is formed from newly transcribed ribosomal RNA.
• Nucleoli increase in number and enlarge when the cell is stimulated to
produce proteins. This is a sign that the cell has been stimulated or is
actively involved in protein synthesis. Nucleoli disappear during cell
division and then reform at the chromosomal nucleolar organizing
centers .
The chromatin
• the chromosomes are organized into long structures called
chromatin and not into individual chromosomes as we
typically think.
• The chromatin (meaning "colored substance") contains
DNA and proteins formed into packets of code called
chromosomes. When the cell divides, the chromosomes fold
up on themselves, getting wider.
• Chromatin substance is distributed throughout the nuclear
sap (nucleoplasm).
• Chromatin has been divided into:
• euchromatin and
• heterochromatin.
•
Euchromatin and Heterochromatin
• Several different chemical techniques are used to
identify certain chromosomal regions by staining then
so that they form chromosomal bands.
• For example, darker bands are generally found near
the centromeres or on the ends (telomeres) of the
chromosome, while other regions do not stain as
strongly.
• The position of the dark-staining are heterochromatic
region or heterochromatin.
• Light staining are euchromatic region or euchromatin.
Eurochromatin / Heterochromatin
Light staining
Euchromatin
active genes
dark-staining
Heterochromatin
inactive genes
• most transcription occurs in regions
of euchromatin
• near to the centromers or on the ends
(telomeres) of the chromosome,
• (i) Constitutive: remains permanently in
the heterochromatic stage
• (ii) Facultative.consists of euchromatin
and characteristics of heterochromatin
during some phase of development.
The chromatin
The chromatin substance;
Heterochromatin was defined
as a structure that does not
alter in its condensation
throughout the cell cycle ,
Heterochromatin is localized
principally on the periphery of
the nucleus
Euchromatin is decondensed
during interphase.
and is localized in the interior
of the nucleoplasm.
Euchromatin and Heterochromatin
• Light staining are euchromatic region or euchromatin.
• Experiments of pulse labeling with 3H-uridine and
autoradiography showed that most transcription occurs
in regions of euchromatin and the nucleolus
•
•
dark-staining
are
heterochromatic
region
or
heterochromatin. Heterchromatin: A region of the
chromatin that does not uncoil after mitosis.
In general, heterochromatic regions are sites of inactive
genes; however some transcribed genes have been
located in regions of heterchromatin.
Heterochromatin
• Heterochromatin is classified into two groups:
• (i) Constitutive and
(ii) Facultative.
• Constitutive heterochromatin remains permanently in
the heterochromatic stage, i.e., it does not revert to
the euchromatic stage.
• In contrast, facultative heterochromatin consists of
euchromatin that takes on the staining and
compactness characteristics of heterochromatin
during some phase of development.
•
In mammalian cells, heterochromatin appears as
darkly staining regions of the nucleus, often
associated with the nuclear envelope
• Heterochromatin is classified into two groups:
• (i) Constitutive and (ii) Facultative.
•
• Constitutive
heterochromatin
remains
permanently in the heterochromatic stage, i.e., it
does not revert to the euchromatic stage.
• In contrast, facultative heterochromatin
consists of euchromatin that takes on the
staining and compactness characteristics of
heterochromatin during some phase of
development.
The chromatin substance;
constitutive
heterochromatin,
containing few genes and
formed principally of repetitive
sequences located in large
regions with centromeres and
telomeres.
facultative
heterochromatin
composed of transcriptionally
active regions that can adopt the
structural
and
functional
characteristics
of
heterochromatin, such as the
inactive X chromosome of
mammals.
Chromatin
• A)Heterochromatin
• B) Euchromatin
• This is the condensed form of • Euchromatin
chromatin organization.
threadlike, delicate.
is
• It is seen as dense patches of • the
presence
of
chromatin. Sometimes it lines the
euchromatin
is
nuclear membrane, however, it is
significant because the
broken by clear areas at the pores
regions of DNA to be
so that transport is allowed.
transcribed
or
duplicated must uncoil
before the genetic code
• Abundant heterochromatin is seen
can be read .
in resting, or reserve cells such as
small lymphocytes (memory cells)
waiting for exposure to a foreign • It is most abundant in
antigen.
active,
transcribing
cells .
• Heterochromatin is considered
transcriptionally inactive .
Proteins associated with Chromatin
• The complexes between eukaryotic DNA and
proteins are called Chromatin, which typically
contains about twice as much protein as DNA.
• The major proteins of chromatin are the histones –
small proteins containing a high proportion of basic
aminoacids (arginine and lysine) that facilitate
binding negatively charged DNA molecule .
• There are 5 major types of histones: H1, H2A, H2B,
H3, and H4 – which are very similar among different
sp of eukaryotes.
The major histone proteins:
Histone
Mol. Wt
H1
H2A
H2B
H3
H4
22,500
13,960
13,774
15,273
11,236
No. of
Amino acid
244
129
125
135
102
Percentage
Lys + Arg
30.8
20.2
22.4
22.9
24.5
The DNA double helix is bound to proteins called histones. The histones
have positively charged (basic) amino acids to bind the negatively charged (acidic)
DNA. Here is an SDS gel of histone proteins, separated by size
• In addition, chromatin contains an approximately equal
mass of a wide variety of non-histone chromosomal
proteins.
• There are more than a thousand different types of these
proteins, which are involved in a range of activities,
including DNA replication and gene expression.
• The DNA of prokaryotes is similarly associated with
proteins, some of which presumably function as histones
do, packing the DNA within the bacterial cell.
Histones, however are unique feature of eukaryotic cells and
are responsible for distinct structural organization of
eukaryotic chromatin
Nucleosome,
• The fundamental unit of chromatin, termed the
nucleosome, is composed of DNA and histone
proteins. This structure provides the first level of
compaction of DNA into the nucleus.
The nucleosome is the fundamental unit of
chromatin. It is composed of:
a core particle and
a linker region (or internucleosomal region)
that joins adjacent core particles .
• The the nucleosome, was described by Roger Kornberg in 1974.
• Two types of experiments led to Kornberg’s proposal of the
nucleosome model.
• First, partial digestion of chromatin with micrococcal nuclease
(an enzyme that degrades DNA) was found to yield DNA fragments
approximately 200 base pairs long.
• In contrast, a similar digestion of naked DNA (not associated with
protein) yielded a continuous smear randomly sized fragments.
• These results suggest that the binding of proteins to DNA in
chromatin protects the regions of DNA from
from nuclease digestion, so that enzyme can
attack DNA only at sites separated by
approximately 200 base pairs.
• Second: Electron microscopy revealed that chromatin
fibers have a beaded appearance, with the beads spaced
at intervals of approximately 200 base pairs.
• Thus, both nuclease digestion and the electron
microscopic studies suggest that chromatin is composed
of repeating 200 base pair unit, which were called
nucleosome.
• Detailed analysis of these nucleosome core particles has
shown that they contain 146 base pairs of DNA
wrapped 1.75 times around a histone core consisting
of two molecules each of H2A, H2B, H3, and H4 (the core
histones).
• One molecule of the fifth histone H1, is bound to the DNA as
it enters and exists each nucleosome core particle. This
forms a chromatin subunit known as chromatosome.
• The length of the linker region, however, varies between
species and cell type. It is within this region that the variable
linker histones are incorporated. Therefore, the total length of
DNA in the nucleosome can vary with species from 160 to 241
base pairs.
IV- General steps in chromatin assembly
• The assembly of DNA into chromatin involves a range of events,
beginning with the formation of the basic unit, the nucleosome,
and ultimately giving rise to a complex organization of specific
domains within the nucleus.
• The first step is the deposition onto the DNA of a tetramer of
newly synthesized (H3-H4)2 to form a sub-nucleosomal particle,
which is followed by the addition of two H2A-H2B dimers. This
produces a nucleosomal core particle consisting of 146 base
pairs of DNA wound around the histone octamer.
• This core particle and the linker DNA together form the
nucleosome. Newly synthesized histones are specifically
modified (e.g.the acetylation of histone H4).
•
• The next step is the maturation step that requires ATP to
establish regular spacing of the nucleosome cores to form the
nucleofilament. During this step the newly incorporated histones
are de-acetylated.
•
• During the maturation step, incorporation of linker histones,
non-histone chromatin associated proteins, called HMG
(High Mobility Group), and other specific DNA-binding
factors help to space and fold the nucleofilament. Therefore
the early steps in assembly can have a great impact on the
final characteristics of chromatin in specific nuclear
domains.
The end