The nucleus is the largest organelle in eukaryotic cells, taking up about 10% of the cell volume. It houses most of the cell's DNA and controls its metabolic and hereditary activities. The nucleus is enclosed by a double membrane with nuclear pores that regulate transport between the nucleus and cytoplasm. Inside the nucleus are nucleoplasm, nucleolus, and chromatin containing the cell's chromosomes. The nuclear envelope, nuclear lamina, and nuclear pores play important roles in controlling nuclear structure and transport.

1. NUCLEUS
SUBMITTED TO:
Dr. ASHIMA PATHAK
SUBMITTED BY:
MUGDHA SHARMA
2084044
MSc. BIOTECHNOLOGY
1ST SEM

2. CONTENT
INTRODUCTION
HISTORY
OCCURRENCE
POSITION
MORPHOLOGY
ULTRASTUCTURE + FUNCTIONS
REFERENCES

3. INTRODUCTION
o NUCLEUS IS THE LARGEST ORGANELLE INSIDE THE CELL TAKING UP ABOUT TENTH OF
THE ENTIRE CELL VOLUME (10% OF CELL VOLUME), ALSO KNOWN AS THE HEART OF
THE CELL.
o THIS MAKES IT ONE OF THE EASIEST ORGANELLES TO IDENTIFY UNDER MICROSCOPE.
o A SYNONYMOUS TERM FOR THIS ORGANELLE IS THE GREEK WORD "KARYON".
o NUCLEUS SERVES AS THE MAIN DISTINGUISHING FEATURE OF EUKARYOTIC
CELLS i,e., THIS IS THE TRUE NUCLEUS AS OPPOSED TO THE NUCLEAR
REGION, PROKARYON OR NUCLEIOD OF THE PROKARYOTIC CELLS.
o IT IS HERE THAT ALMOST ALL THE CELL'S DNA IS CONFINED, REPLICATED AND
TRANSCRIBED.
o THE NUCLEUS THUS CONTROLS DIFFERENT METABOLIC AS WELL AS HEREDITARY
ACTIVITIES OF THE CELL.

4. VINCENT ALLFREY (1968) GAVE A STATEMENT THAT COMPLETELY
QUALIFIES THE CENTRAL POSITION OF THE NUCLEUS IN THE
AFFAIRS OF AN EUKARYOTIC CEL;
“The cell nucleus, central and commanding, is
essential for the biosynthetic events that
characterize cell type and cell fraction; it is a vault
of genetic information encoding the past history
and future prospects of the cell, an organelle
submerged and deceptively serene in its sea of
turbulent cytoplasm, a firm and purposeful guide,
a barometer exquisitely sensitive to the changing
demands of the organism and its environment.
This is our subject — to be examined in terms of
its ultrastructure, composition and function.”
This Photo by Unknown author is licensed under CC
BY.

5. HISTORY
This Photo by Unknown author is licensed under CC
DISCOVERED AND
NAMED NUCLEUS IN 1833
IN THE PLANT CELLS
AND WERE QUICKLY
RECOGNIZED AS A
CONSTANT FEATURE OF
ALL ANIMAL AND PLAN
CELLS
ROBERT BROWN
This Photo by Unknown author is
M J SCHLEIDEN
DESCRIBED
NUCLEOLI IN 1838
ALTHOUGH IT WAS
FIRST NOTED BY
FOTANA.
This Photo by Unknown author is licensed under CC
W. FLEMMING
COINED THE TERM
CHROMATIN FOR
CHROMOSOMAL
MESHWORK IN 1879
BOWMAN
COINED THE TERM
NUCLEOLUS IN 1840

6. HISTORY
This Photo by Unknown author is
STRASBURGER
INTRODUCED THE
TERM
CYTOPLASM
&
NUCLEUS
IN 1882
This Photo by Unknown author is licensed under CC
JOACHIM HAMMERLING
ESTABLISHED
THE ROLE OF NUCLEUS
IN HEREDITARY BY THE
GRAFTING EXPERIMENT
IN 1953
ULTRASTRUCTURE OF
NUCLEAR ENVELOPE,
PORE COMPLEXES AND
NUCLEAR LAMINA
KIRCHNER et al.
(1977)
SCHATTEN AND
THOMAN (1978)

7. HAMMERLING'S EXPERIMENT
ON ACETABULARIA
• In a classical series of experiments, spanning between 1934 and
1954, on the unicellular alga Acetabularia
• Hammerling demonstrated by means of interspecific nuclear
transplants, that morphological features, notably the shape of cap,
were determined by the nucleus.
• He also showed that even after removal of the nucleus, the cell was
able to continue morphogenesis for a time and proposed that the
cytoplasm contained a store of morphogenetic material that had
been produced by the nucleus.

8. The body of an alga Acetabularia is about six centimeters long and is differentiated into
a foot, a stalk and a cap.
The cap has a characteristic shape for each species and is easily regenerated if removed.
The single nucleus is situated in the rhizoid portion.
§ Acetabularia crenulata has a cap, with about 31 rays, the tips of which
are pointed, but Acetabularia mediterranea has about 81 rays with
rounded tips.
§ If the cap, stalk or even the nucleated portion of the rhizoid is removed,
the remaining portion of the alga has the capacity to regenerate into a
whole plant.
§ The enucleated part loses the regeneration capacity after a
few decapitations, but the nucleated portion always maintains
this ability.
§ When the stalk of one species is grafted on to the nucleated rhizoid
of the other, an intermediate type of cap is formed.
§ On decapitation, a second cap develops which resembles the cap
of species which provides the nucleus When the nuclei of both
the species are present in the same cytoplasm, an intermediate type
of cap develops.
§ Such experiments have clearly established that
"the nucleus is the storehouse for and the control tower of, all hereditary
information."

9. OCCURRENCE
EUKARYOTIC CELL
 The nucleus is found in all the eukaryotic
cells of the plants and animals.
 Certain eukaryotic cells such as the mature
sieve tubes of higher plants and
mammaliam erythrocytes contain no
nucleus.( In such cells nuclei are present
during the early stages of development ).
 Since mature mammalian red blood cells
are without any nuclei, they are called red
blood “corpuscles” rather than cells.

10. OCCURRENCE
PROKARYOTIC CELL
 The prokaryotic cells of the bacteria do not
have true nucleus.
 Therefore, the single, circular and large DNA
molecule remains in direct contact with the
cytoplasm.
 And hence called the nucleoid.
This Photo by Unknown author is licensed under CC
BY-SA.

11. POSITION
• The position or location of the nucleus in a cell is usually the
characteristic of the cell type and it is often variable.
• The location of the nucleus is normally in the centre of the cell,
but it can also be found in peripheral locations.
• But its position may change from time to time according to the
metabolic states of the cell.
• For example,
• SECRETORY CELLS have their nucleus situated in the basal part.
• SKELETAL MUSCLE CELLS have their nuclei close to the plasma
membrane.
• In EMBRYONIC CELLS the nucleus generally occupies the
geometric centre of the cell but as the cells start to differentiate
and the rate of the metabolic activities increases, the
displacement in the position of the nucleus takes place.

12. MORPHOLOGY
NUMBER
SHAPE
SIZE

13. On the basis of number
Usually the cells contain single nucleus but the number of the nucleus may vary from
cell to cell.
According to the number of the nuclei following types of cells have been recognised :
Mononucleate
Cells
Contains single
nuclei
Polynucleate cells
Contains many
nuclei (3 to 100)
Binucleate cells
Contains 2 nuclei
Plant and Animal Cells Cells of certain
Paramecium and Cells of
Cartilage and Liver
Osteoblast, Striated Muscle
Fibes, and Siphonal algae
Vaucharia

14. On the basis of shape
The shape of the nucleus is related with the shape of the cell, but certain nuclei are
almost irregular in shape.
 Spheroid nuclei: The spheroid, cuboid or polyhedral cells contain the spheroid nuclei
 Ellipsoid nuclei: The nuclei of the cylindrical, prismatic or fusiform cells are ellipsoid
in shape
 Discoidal nuclei: The cells of the squamous epithelium contain the discoidal nuclei
Irregular nuclei: The leukocytes, glandular cells of some insects and spermatozoa
contain the irregular shaped nuclei

15. On the basis of size
• The nucleus occupies about 10 per cent of the total cell volume.
• Nuclei vary in size from about 3 µm to 25 µm in diameter, depending on cell type and contain
diploid set of chromosomes.
• The size of the nucleus is directly proportional to that of the cytoplasm.
• R. Hertwig gave the formula for the deduction of the size of the nucleus of a particular
cell: NP is the nucleoprotein index
Vn is the volume of the nucleus
Vc is the volume of the cell.
• The size of the nucleus is related with the number of the chromosomes or ploidy.
• The size of the nucleus of a cell depends on the volume of the cell, amount of the DNA and
proteins and metabolic phase of the cell.
NP = Vn
Vc - Vn

16. ULTRASTRUCTURE OF
NUCLEUS
The nucleus is composed of:
1. Nuclear Envelope
2. Nuclear Pore
3. Nuleoplasm
4. Nucleolus
5. Chromosome

17. NUCLEAR ENVELOPE
• The nuclear envelope encloses the DNA
and defines the nuclear compartment of
interphase and prophase nuclei.
• It is formed from two concentric unit
membranes + space, each 5–10 nm thick.
1. Inner nuclear membrane
2. Outer nuclear membrane
3. Perinuclear space

18. 1. INNER NUCLEAR MEMBRANE:
• Contains specific proteins that act as binding sites for the
supporting fibrous sheath of intermediate filaments (IF)
• Which is called NUCLEAR LAMINA, also called fibrous
lamina, nuclear cortex and lamina densa.
• Nuclear lamina has contact with the chromatin (or chromosomes)
and nuclear RNAs.
2. OUTER NUCLEAR MEMBRANE:
• Closely resembles the membrane of the endoplasmic reticulum, that
is continuous with it
• It is also surrounded by less organized intermediate filaments
• The outer surface of outer nuclear membrane is generally studded
with ribosomes engaged in protein synthesis, like the membrane of
RER.
3. PERINUCLEAR SPACE:
• The proteins made on these ribosomes are transported into space
between the inner and outer nuclear membrane.
• The perinuclear space is a 10 to 50 nm wide fluid-filled
compartment
• It is continuous with the ER lumen and may contain fibres,
crystalline deposits, lipid droplets or electron dense material.
NUCLEAR
MEMBRANE

19. FUNCTION OF NUCLEAR
MEMBRANE
• The critical function of the nuclear membranes is
to act as a barrier that separates the contents of
the nucleus from the cytoplasm.
• Like other cell membranes, each nuclear
membrane is a phospholipid bilayer permeable
only to small nonpolar molecules.
• Other molecules are unable to diffuse through
the bilayer.

20. NUCLEAR LAMINA
• The nuclear lamina is a protein meshwork which is 50 to 80 nm thick (DeRobertis and De
Robertis, Jr., 1987) or 10 to 20 nm thick (Alberts et al., 1987).
• It lines the inside surface of the inner nuclear membrane, except the areas of nucleopores,
and consists of a square lattice of intermediate filaments.
• In mammals, these intermediate filaments are of three types :
• Lamins A, B and C
• having M.W. 74,000, 72,000 and 62,000 Daltons, respectively.
• The lamins form dimers that have a rod-like domain and two globular heads at one end.
• Like other intermediate filament proteins, the laminas associate with each other to form
higher order structures although the extent and polarity of this association is thought to
differ from that of other intermediate filaments.

21. MODEL OF LAMIN ASSEMBLY
• Association of lamins with each other occurs to form higher order structure called
NUCLEAR LAMINA.
• This association with the inner nuclear membrane is facilitated by the post translational
addition of lipids in particular, PRENYLATION OF C-TERMINAL CYSTEINE RESIDUES.
• The first stage of this association is the interaction of two lamins to form a dimer in which
the a-helical regions of two polypeptide chains are wound around each other in a structure
called a coiled coil and form dimer.
• These lamin dimers then associate with each other to form the nuclear lamina.

22. • The nuclear lamina is a very dynamic structure.
• In mammalian cells undergoing mitosis, the transient phosphorylation of several serine
residues on the lamins causes the lamina to reversibly disassemble into tetramers of hypo
phosphorylated lamin A and lamin C and membrane associated lamin B.
• As a result, lamin A and C become entirely soluble during mitosis, and at telophase they
become dephosphorylated again and polymerize around chromatin.
• Lamin B seems to remain associated with membrane vesicles during mitosis, and these
vesicles in turn remain as a distinct subset of membrane components from which nuclear
envelope is reassembled at telophase.
• Inside an interphase nucleus, chromatin binds strongly to the inner part of the nuclear
lamina which is believed to interfere with chromosome condensation.
• In fact, during meiotic chromosome condensation, the nuclear lamina completely
disappears by the pachytene stage of prophase and reappears later during diplotene in
oocytes, but does not reappear at all in spermatocytes.

23. • The Lamins bind to specific inner nuclear membrane proteins such as emerin and the
lamin B receptor, mediating their attachment to the nuclear envelope and localizing
and organizing them within the nucleus
• The nuclear lamina also binds to chromatin through histones H2A and H2B as well as
other chromatin proteins.
• Lamins also extend in a loose meshwork throughout the interior of the nucleus
• Many nuclear proteins that function in DNA synthesis, transcription, or chromatin
modification are known to bind lamins, although the significance of these interactions
is only beginning to be understood.
The inner nuclear membrane
contains several integral proteins,
such as emerin and the lamin B
receptor (LBR) that interact with
nuclear lamins. The lamins also
interact with chromatin.

24. NUCLEAR PORE COMPLEXES
• The nuclear pore complexes are the only channels through
which small polar molecules, ions, and macromolecules
(proteins and RNAs) can travel between the nucleus and the
cytoplasm.
• The nuclear pore responsible for the selective traffic of
proteins and RNAs between the nucleus and the cytoplasm.
• The nuclear pore complex is an extremely large structure
about 30 times the size of a ribosome with a diameter of about
120 nm.
• ln vertebrates, the nuclear pore complex is composed of 30-50
different pore proteins called nucleoporins, most of which
are present in multiple copies.
This Photo by Unknown author is licensed under CC
BY-SA.

25. STRUCTURE OF NUCLEAR
PORE COMPLEX
• Pore complex as a whole is wheel like.
• Visualization of nuclear pore complexes by electron
microscopy reveals that the NPC appear to consist of eight
structural subunits surrounding a central channel via which
molecules pass.
• Nuclear pore complex consists of an assembly of eight spokes
arranged around a central channel therefore we can say it has
its octagonal symmetry.
• The spokes are connected to rings at the nuclear and
cytoplasmic surfaces, and the spoke-ring assembly is anchored
within the nuclear envelope at sites of fusion between the
inner and outer nuclear membranes.
• Protein filaments extend from both the cytoplasmic and
nuclear rings, forming a distinct basketlike structure on the
nuclear side.
• Two parallel rings, outlining the rim of the wheel, each consist
of the eight subunits seen in electron micrographs.
• Proteins extending from the rim into the perinuclear space
may help anchor the pore complex to the envelope called as
anchor protein .
• Fibers also extend from the rings into the cytosol and
nucleoplasm, with those on the nucleoplasm side forming a
basket.

26. Functions of nuclear pore complex
 Control the traffic of molecules between the nucleus and the cytoplasm, the
nuclear pore complex plays a fundamental role in the physiology of all
eukaryotic cells.
 RNAs synthesized in the nucleus are exported to the cytoplasm where they
function in protein synthesis.
 Conversely, proteins required for nuclear functions e.g., transcription factors are
transported to the nucleus from their sites of synthesis in the cytoplasm.
 Many proteins shuttle continuously between the nucleus and the cytoplasm.
 Depending on their size, molecules can travel through the nuclear pore complex
by one of two different mechanisms.

27. TRANSPORT OF MOLECULE ACROSS THE NUCLEAR PORE
COMPLEX
• Nuclear Pore Complexes are the only channels
through which small polar molecules, ions,
and macromolecules can travel through the
nucleus and the cytoplasm.
• Depending on their size, molecules can travel
through the nuclear pore complexes by one of
the two different mechanisms:
1. PASSIVE TRANSPORT
2. ACTIVE TRANSPORT
(energy dependent)
• The NPC forms the conduit for the exchange of
information between the nucleus and
cytoplasm
• Important molecules transported across the
NPC are Proteins required for DNA replication
and RNA

28. Macromolecular Transport
into and out of the Nucleus
• Transport of proteins are significant as they
are responsible for all aspects of genome
structure and function
• Important proteins transported are histones,
DNA polymerases, RNA polymerases,
transcription factors, splicing factors etc.
• Proteins are targeted to the nucleus by
specific amino acid sequences called nuclear
localization signals (NLS)
• The signal are recognized by transport
receptors and they direct the transport of
the proteins through the nuclear pore
complex.

29. NUCLEAR LOCALISATION SIGNAL
of T antigen
• NLS was 1st time mapped in detail and characterized by Alan Smith et al.,
1984.
• The viral Simian virus 40 (SV40) T antigen protein was used as a model for
studies of nuclear localization in animal cells.
• A virus-encoded protein T antigen is a 94-kd protein that is required for SV40
DNA replication and is normally localized to the nucleus of SV40-infected
cells.

30.  Nuclear localization signal for T antigen is seven-amino-acid
sequence
Pro-Lys-Lys-Lys-Arg-LysVal
responsible for the transport
 It was first identified by the finding that mutation of a single
lysine residue prevents nuclear import, resulting in the
accumulation of T antigen in the cytoplasm.
 Nuclear localization signals have since been identified in
many other proteins.
 Some of these sequences, like that of T antigen, are short
stretches rich in basic amino acid residues
(Lysine and Arginine )
Nuclear localization signal of
nucleoplasm in is bipartite,
consisting of a Lys-Arg
followed by a LysLys- Lys-Lys
sequence located ten amino
acids farther downstream
sequence
 However, many times amino acids that form the nuclear
localization signal are close together but not immediately
adjacent to each other.
 For example, the nuclear localization signal of Nucleoplasrnin
(a protein involved in chromatin assembly) consists of two
parts: a Lys-Arg pair followed by four lysins located ten
amino acids farther downstream

31.  Both the Lys-Arg and Lys-LysLys- Lys sequences of nucleoplasm are
required for nuclear targeting, but the ten amino acids between these
sequences can be mutated without affecting nuclear localization.
 When the NLS sequence is composed of two separated elements, it is called
bipartite like nuclear localization signal of nucleoplasrnin.
 Similar bipartite motifs appear to function as the localization signals of
many nuclear proteins and they are more common than T antigen NLS.
 Structures of other nuclear localization signals vary considerably.
 NLS consist of the basic amino acid residues-are often termed the basic or
"classical" nuclear localization signal.
 Proteins which are exported from nucleus have nuclear export signal (NES)

32. Recognition of nuclear
localization signal
 NLS are recognized by proteins that function as nuclear transport receptors.
 Most of the Nuclear transport receptors are members of the karyopherin protein family.
 Karyopherins are a group of proteins involved in transporting molecules between the
cytoplasm and the nucleus of a eukaryotic cell.
 They are of two types:
 Importins
 Exportins
 Importins, transport macromolecules to the nucleus from the cytoplasm.
 Exportins, which transport macromolecules from the nucleus to the cytoplasm.
 Some importins (Kap, β l) act with an adapter karyopherin (Kapa) a heterodimer to import
proteins containing the basic nuclear localization signal.
 Some karyopherins act as importins for one protein and exportins for another.

33. Regulation of movement of
macromolecules/proteins
 Movement of macromolecules through nuclear pore is regulated by a protein called Ran
 Ran also known as GTP-binding nuclear protein encoded by the RAN gene
 Activity of this Ran protein is regulated by GTP binding and hydrolysis.
 Ran is involved in transport into and out of the cell nucleus during interphase and also
involved in mitosis.
 It is a member of the Ras superfamily.

34. Ran
GTP- Binding nuclear protein
• Enzymes that stimulate GTP hydrolysis to GDP are
localized to the cytoplasmic side of the nuclear
envelope, whereas enzymes that stimulate the
exchange of GDP for GTP are localized to the
nuclear side
• There is high concentration of Ran/GTP in the
nucleus Ran GDP in cytoplasm
• This high concentration of Ran/GTP in the nucleus
determines the directionality of nuclear transport of
cargo proteins.
• The import and export cycles are both driven by a
concentration gradient of Ran-GTP that is
maintained by the presence of a GEF (guanine-
nucleotide exchange factor) in the nucleus and a
GAP (GTPase activating protein) in the cytosol.

35. Transport Through the Nuclear Pore Complex
Proteins made in the cytosol and destined for use in
the nucleus contain (NLS) that targets them as
“cargo” for transport through the NPC
An NLS-containing cargo protein binds to importin
Importin-cargo complex is then transported through
the nuclear pore complex
Nuclear Ran-GTP binds to importin, triggering the
release of the cargo protein in the nucleus
Ran-GTP-importin complex transported back to the
cytosol, where the hydrolysis of GTP to GDP
accompanied by the release of importin.

36. Transport Through the Nuclear Pore Complex
Ran-GTP binds to exportin in the nucleus
Promotes the binding of exportin to its
cargo a protein-RNA complex formed
The exportin-cargo-Ran-GTP complex is
exported through the nuclear pore to the
cytosol
Exportin and its cargo are released from
Ran, accompanied by hydrolysis of GTP to
GDP
Exportin then moves back into the
nucleus, where it can repeat the export
cycle

37. Regulation of nuclear import of
Transcription Factors
• The transcription factor NF-KB is maintained as an inactive
complex with IKB, which masks its nuclear localization
sequence (NLS) in the cytoplasm.
• In response to appropriate extracellular signals, IKB is
phosphorylated and degraded by proteolysis, allowing the
import of NF-KB to the nucleus.
• The yeast transcription factor Pho4 is maintained in the
cytoplasm by phosphorylation in the vicinity of its nuclear
localization sequence.
• Regulated dephosphorylation exposes the NLS and allows
Pho4 to be transported to the nucleus.

38. Transport of RNAs
• RNAs are transported across the nuclear envelope as ribonucleoprotein complexes (RNPs)
• Ribosomal RNAs are first associated with both ribosomal proteins and specific RNA processing proteins in
the nucleolus, and nascent 60S and 40S ribosomal subunits are then transported to the cytoplasm
• Their export from the nucleus is mediated by nuclear export signals present on proteins within the subunit
complex
• Pre-mRNAs and mRNAs are associated with a set of at least 20 proteins throughout their processing in the
nucleus and eventual transport to the cytoplasm, which is mediated by the mRNA exporter complex after its
recruitment to the processed mRNA
• tRNAs are exported from the nucleus by exportin-t, which binds directly to the tRNAs
• In contrast to mRNAs, tRNAs, and rRNAs, which function in the cytoplasm, many small RNAs (snRNAs
and snoRNAs) function within the nucleus as components of the RNA processing machinery
• Perhaps surprisingly, snRNAs are initially transported from the nucleus to the cytoplasm, where they
associate with proteins to form functional snRNPs and then return to the nucleus
• Transport receptor proteins that bind to the 5' caps of snRNAs appear to be involved in the export of the
snRNAs to the cytoplasm.
• In contrast, sequences present on the snRNP proteins are responsible for the transport of snRNPs from the
cytoplasm to the nucleus.

39. Transport of snRNAs between nucleus and
cytoplasm
 Small nuclear RNAs (snRNAs) are
initially exported from the nucleus to
the cytoplasm where they associate
with proteins to form snRNPs
(Ribonucleoprotein complex).
 The assembled snRNPs are then
transported back to the nucleus.

40. NUCLEOPLASM
• The space between the nuclear envelope and the nucleolus is filled by a
transparent, semi-solid, granular and slightly acidophilic ground substance
or the matrix known as the nuclear sap or nucleoplasm or karyolymph.
• The nuclear components such as the chromatin threads and the nucleolus
remain suspended in the nucleoplasm.
• The nucleoplasm has a complex chemical composition.
• It is composed of mainly the nucleoproteins but it also contains other
inorganic and organic substances, viz., nucleic acids, proteins, enzymes and
minerals.

41. NUCLEOPLASM
COMPONENTS
NUCLEIC ACIDS
(DNA,RNA)
PROTEINS
(BASIC AND
ACIDIC
PROTEINS)
ENZYMES
(DNA & RNA POLY,
NUCLEOSIDE
PHOSPHORYLASE)
LIPIDS
MINERAL
S
(P, K, Ca,
Na, Mg)
1. Nucleic acids. The most common nucleic acids of the nucleoplasm are the DNA and RNA. Both may
occur in the macromolecular state or in the form of their monomer nucleotides.
2. Proteins. The nucleoplasm contains many types of complex proteins. The nucleoproteins can be
categorized into following two types :
(i)Basic proteins. The proteins which take basic stain are known as the basic proteins. The most important
basic proteins of the nucleus are nucleoprotamines and the nucleohistones.
(ii) Non-histone or Acidic proteins. The acidic proteins either occur in the nucleoplasm or in the chromatin.
The most abundant acidic proteins of the euchromatin (a type of chromatin) are the phosphoproteins.

42. 3. Enzymes. The nucleoplasm contains many enzymes which are necessary for the synthesis
of the DNA and RNA. are the DNA polymerase, RNA polymerase, NAD synthetase, nucleoside
triphosphatase, adenosine diaminase, nucleoside phosphorylase, guanase, aldolase, enolase, 3-
phosphoglyceraldehyde dehydrogenase and pyruvate kinase. The nucleoplasm also contains
certain cofactors and coenzymes such as ATP and acetyl CoA.
4. Lipids. According to Stoneburg (1937) and Dounce (1955), the nucleoplasm contains small
lipid content.
5. Minerals. The nucleoplasm also contains several inorganic compounds such as phosphorus,
potassium, sodium, calcium and magnesium. The chromatin comparatively contains large amount
of these minerals than the nucleoplasm.

43. NUCLEOLUS
• The most prominent substructure within the
nucleus is the nucleolus.
• Frits Zernike in 1953 1st saw nucleolus.
• It is the site of rRNA transcription and processing,
and of ribosome assembly.
• The nucleolus is a ribosome production factory,
designed to fulfill the need for large-scale
production of rRNAs and assembly of the ribosomal
subunits.
• Recent evidence suggests that nucleoli also have a
more general role in RNA modification and that
several types of RNA move in and out of the
nucleolus at specific stages during their processing.

44. Organization of the Nucleolus
and the Ribosomal RNA Genes
 The nucleolus, which is not surrounded by a membrane, is organized
around the chromosomal regions that contain the genes for the 5.8S,
18S, and 28S rRNAs
 Eukaryotic ribosome's contain four types of RNA, designated the 5S,
5.8S, 18S, and 28S rRNAs
 The 5.8S, 18S, and 28S rRNAs are transcribed as a single unit within the
nucleolus by RNA polymerase I, yielding a 45S ribosomal precursor
RNA
 The 45S pre-rRNA is processed to the 18S rRNA of the 40S (small)
ribosomal subunit and to the 5.8S and 28S rRNAs of the 60S (large)
ribosomal subunit
 Transcription of the 5S rRNA, which is also found in the 60S ribosomal
subunit, takes place outside of the nucleolus and is catalyzed by RNA
polymerase III
 To meet the need for transcription of large numbers of rRNA
molecules, all cells contain multiple copies of the rRNA genes

45.  The human genome, for example, contains about 200 copies of the gene that encodes the 5.8S,
18S, and 28S rRNAs and approximately 2000 copies of the gene that encodes 5S rRNA.
 The genes for 5.8S, 18S, and 28S rRNAs are clustered in tandem arrays on five different human
chromosomes (chromosomes 13, 14, 15, 21, and 22); the 5S rRNA genes are present in a single
tandem array on chromosome 1.
 Morphologically, nucleoli consist of three distinguishable regions:
1. Fibrillar center, 2. Dense fibrillar component 3. Granular component
 These different regions are thought to represent the sites of progressive stages of rRNA
transcription, processing, and ribosome assembly.
 The rRNA genes are located in the fibrillar centers, with transcription occurring primarily at the
boundary of the fibrillar centers and dense fibrillar component.
 Processing of the pre-rRNA is initiated in the dense fibrillar component and continues in the
granular component, where the rRNA is assembled with ribosomal proteins to form nearly
completed preribosomal subunits, ready for export to the cytoplasm.

46. Transcription and
Processing of rRNA
 Transcription is the process of making an RNA copy of a gene sequence. This copy,
called a messenger RNA (mRNA) molecule, leaves the cell nucleus and enters the
cytoplasm, where it directs the synthesis of the protein, which it encodes.
 The processing of pre-rRNA requires the action of both proteins and RNAs that are
localized to the nucleolus. Nucleoli contain a large number (about 200) of small
nucleolar RNAs (snoRNAs) that function in pre-rRNA processing.
 The snoRNAs are complexed with proteins, forming snoRNPs.
 Individual snoRNPs consist of single snoRNAs associated with eight to ten proteins.
 The snoRNPs then assemble on the pre-rRNA to form processing complexes.

47.  snoRNAs are responsible for the cleavages of pre-rRNA
into 18S, 5.8S, and 28S products.
 Most abundant nucleolar snoRNA is U3, is required for
the initial cleavage of pre-rRNA within the 5′ external
transcribed spacer sequence.
 Similarly, U8 snoRNA is responsible for cleavage of
prerRNA to 5.8S and 28S rRNAs, and U22 snoRNA is
responsible for cleavage of pre-rRNA to 18S rRNA
Role of snoRNAs in base
modification of pre-rRNA.
 Majority of snoRNAs, however, function to direct the specific base
modifications of prerRNA, including the methylation of specific ribose
residues and the formation of pseudouridines.
 Most of the snoRNAs contain short sequences complementary to 18S or
28S rRNA
 These regions of complementarity include the sites of base modification
in the rRNA.
 By base pairing with specific regions of the prerRNA, the snoRNAs act
as guide RNAs that target the enzymes responsible for ribose
methylation or pseudouridylation to the correct site on the pre-rRNA
molecule.

48. Ribosome Assembly
 The formation of ribosomes involves the assembly of the ribosomal precursor RNA with
both ribosomal proteins and 5S rRNA .
 The genes that encode ribosomal proteins are transcribed outside of the nucleolus by
RNA polymerase II, yielding mRNAs that are translated on cytoplasmic ribosomes then
transported f to the nucleolus, where they are assembled with rRNAs to form
preribosomal particles.
 Although the genes for 5S rRNA are also transcribed outside of the nucleolus, by RNA
polymerase III, and are assembled into preribosomal particles.
 The association of ribosomal proteins with rRNA begins while the pre-rRNA is still being
synthesized, and more than half of the ribosomal proteins are complexed with the pre-
rRNA prior to its cleavage.
 The remaining ribosomal proteins and the 5S rRNA are incorporated into preribosomal
particles as cleavage of the pre-rRNA proceeds.
 Most of the preribosomal particles in the nucleolus represent precursors to the large
subunit. The final stages of ribosome maturation follow the export of preribosomal
particles to the cytoplasm, forming the active 40S and 60S subunits of eukaryotic
ribosome.

50. CHROMATIN FIBRES
• The nucleoplasm contains many thread-like, coiled
and much elongated structures which take readily
the basic stains such as the basic fuchsin.
• These thread-like structures are known as the
chromatin (Gr., chrome=colour) substance or
chromatin fibres.
• Such chromatin fibres are observed only in the
interphase nucleus.
• During the cell division (mitosis and meiosis)
chromatin fibres become thick ribbon-like
structures which are known as the chromosomes.

51.  Chemically, chromatin consists of DNA and proteins. Small quantity of RNA
may also be present but the RNA rarely accounts for more than about 5 per
cent of the total chromatin present.
 Most of the protein of chromatin is histone, but “nonhistone” proteins are
also present.
 The protein : DNA weight ratio averages about 1:1.
 Histones are constituents of the chromatin of all eukaryotes except fungi,
which, therefore, resemble prokaryotes in this respect.

52. DNA Packaging in Eukaryotes
 Chromosomes each consist of a linear DNA
molecule, employ a different type of packing
strategy to fit their DNA inside the nucleus.
 At the most basic level, DNA is wrapped around
proteins known as histones to form structures
called nucleosomes.
 This nucleosome is linked to the next one with
the help of a linker DNA. This is also known as
the “beads on a string” structure.
 This is further compacted into a 30 nm fiber,
which is the diameter of the structure.
 At the metaphase stage, the chromosomes are
at their most compact, are approximately 700
nm in width, and are found in association with
scaffold proteins.

53. The fibres of the chromatin are twisted, finely anastomosed and uniformly
distributed in the nucleoplasm.
Two types of chromatin material have been recognised,
e.g., heterochromatin and euchromatin.
A. Heterochromatin. The darkly stained, condensed region of
the chromatin is known as heterochromatin. The condensed portions of
the nucleus are known as chromocenters or karyosomes or false nucleoli.
The heterochromatin occurs around the nucleolus and at the periphery. It
is supposed to be metabolically and genetically inert because it contains
comparatively small amout of the DNA and large amount of the RNA.
B. Euchromatin. The light stained and diffused region of the
chromatin is known as the euchromatin. The euchromatin contains
comparatively large amount of DNA.

54. • Snustad, D. P., & Simmons, M. J. (2015). Principles of genetics.
John Wiley & Sons.
 Cooper, G. M., & Hausman, R. E. (2004). The cell: Molecular
approach. Medicinska naklada.
 P.S. Verma & V.K. Agarwal (2005). Cell
biology, genetics, molecular biology, evolution and ecology.
REFERENCE

55. This Photo by Unknown author is licensed under CC
BY-NC-ND.