The nucleus is a membrane-bound organelle that houses the cell's genome and controls gene expression. It contains DNA, which is replicated, transcribed into RNA, and processed. The nuclear envelope, made of two membranes, regulates transport of molecules into and out of the nucleus through nuclear pores. Inside the nucleus, DNA is organized into chromatin and packaged into chromosomes. The nucleolus forms around DNA that encodes ribosomal RNA and is the site of ribosome biogenesis.

1. NUCLEUS
• Nucleus is membrane enclosed organelle
found in eukaryotic cell.
• Eukaryotes usually have a single nucleus, but a
few cell types, such as mammalian RBC have
no nuclei, and few others including osteoclasts
have many.
• By housing the cell genome, nucleus serve
both as the repository of genetic information
and as the cell control center.

2. 
• DNA replication, transcription , rna processing
all take place within the nucleus, with only
final stage of gene expression(translation)
localized to cytoplasm.
• The presence of nucleus thus allows gene
expression to be regulated by post-
transcriptional mechanism, such as alternative
splicing.

3. NUCLEAR ENVELOPE
An electron micrograph of a nucleus.
The inner and outer nuclear
membrane are joined at nuclear pore
complexs(arrows).

4. Illustrate the continuity of
outer nuclaer membrane
with endoplasmic reticulum

5. Schematic of the nuclear envelope.
the inner nuclear membrane is lined by the nuclear lamina, which serves
as attachment site for chromatin

6. The electron micrograph show nuclear pore: many nuclear pore are visible in this freeze
Preparation of the nuclear envelope

7. Nuclear lamina is a meshwork of filaments underlaying the inner nuclear membrane
And extending into the interior of the nucleus

9. Model of lamin assembly
The lamin polypetide form dimer in which the central alpha helical region of two polypetide
chain are wound around each other.
Further assembly may involve the head-to-tail association of dimer to form linear polyper and
the side by side association of polyper to form higher order structure

10. The inner nuclear membrane contain several integral proteins,such as EMERIN and the
LAMINA B Receptor(LBR) that interact with nuclear lamins.
The lamins also interact with Chromatin.

11. Nuclear pore complex(NPC)
• Transport of proteins from the cytoplasm into
the nucleus &movement of macromolecules,
including mRNA, tRNA, and ribosomal subunit
out of the nucleus occur through NPC.
• Mass of pore structure is 125 million Da which
is about 30 times larger than a ribosome
• NPC is made up of multiple copies of 30
different proteins called nucleoporin.

12. 
• It is roughly octagonal, membrane embedded
structure from which approx.100nm long
filament extend into the nucleoplasm. The
distal end of these filament are joined by the
terminal ring, forming a structure called
nuclear basket.
• The membrane embedded portion of NPC is
also attached directly to nuclear lamins

14. NPC; The complex consist of an assembly of eight spoke attached to the rings on the
Cytoplasmic and nuclear side of the nuclear envelope. The spoke-ring assembly surround a
Central channel. Cytoplasmic filaments extended from the cytoplasmic ring and filaments
Forming the nuclear basket extend from the nuclear ring.

15. Molecular traffic through
NPC
Small molecules and some
proteins with molecular mass
less than approx.20-40 Kd
pass freely through pore in
either direction.
In contrast, macromolecules
(proteins and RNAs of large
size)are transported by a
selective, energy dependent
mechanism.

16. Selective transport of proteins to &
from the nucleus
• Several millions macromolecules selectively
pass b/w the nucleus &the cytoplasm every
minute.
• Importin : from cytoplasm to nucleus
• Import-in transport protein (histones,
transcription factor, DNA pol. ,RNA pol. ,
splicing factors & many others) containing
Nuclear localization signal (NLS) into the
nucleus.

17. 
• NLS that are recognized by transport receptor
& direct the transport of protein through NPC.
• NLS first discovered in simian virus 40 (SV40)
T-antigen, a virus encoded protein that
initiates viral DNA replication in infected cell.
• NLS rich in seven basic AA that are Pro-Lys-Lys-
Lys-Arg-Lys-val

18. 
• Often, however, the AA that form the NLS are
close together but not immediately adjacent
to each other i.e. Nucleoplasmin (a protein
involved in chromatin assembly) consist of two
parts: a Lys-Arg pair followed by four Lys
located ten AA farther downstream(BIPARTITE

19. NLS are recognized by proteins that function as
nuclear transport receptor
Most are members of the karyopherin protein family & function either as Importins or
exportins

20. Movement of molecule through the nuclear
pore is regulated by a protein called Ran
• Ran is small GTP binding protein whose
conformation & activity are regulated by GTP
binding & hydrolysis.
• For Ran, enzyme GAP ( GTPase activating protein)
are present in Cytosol that hydrolyise the GTP to
GDP and enzyme GEF( Guanine nucleotide
exchange factor) that stimulate the exchange of
GDP for GTP are present at nuclear side.

22. 
• Like import proteins have NLS , Export protein
(from nucleus to cytosol) have NES ( nuclear
export signal).
• The NLS of importein protein interact with
importein which guide it through NPC.
• Impotein is heterodimer of alpha & beta subunit.
• Alpha- interact with cargo protein & beta-
interact with protein in the nuclear pore to
shuttle the cargo protein through it.
• Protein in pore is called FG Nucleoprotein which
line the channel of NPC,basket, cyto. Filaments.

23. Protein import through NPC

25. EXPORT
• In export :- proteins, tRNA, & ribosomal
subunit from the nucleus to the cytoplasm.
• NES( Nuclear export signal) that stimulate
protein to be export from the nucleus to
cytoplasm.
• Three diff. classes of NES:- 1) a leucine rich
sequence found in PKI ( inhibitor of protein
kinase A) & in protein HIV.
As well as two diff. sequence identified in two
diff. Heterogenous ribonucleoprotein particle

26. 
• Exportin or nuclear export receptor in the
nucleus called Exportin-1
• Expotin-1 first form complex with Ran-GTP causes
a conformational change in it that increase its
affinity for NES , so that a trimolecular cargo
complex is formed
• Like importin, exportin interact transiently with
FG-repeats in FG nucleoporin & diffuse through
NPC

28. Complex b/w cargo protein bearing NES, Exportin, & Ran-GTP form in nucleus. Following
Transport through NPC, Ran-GAP stimulate the hydrolysis of the cargo protein & exprtin into
The cytoplasm. Exportin is then transported back to nucleus.

29. Regulation of nuclear protein import

30. Transport of SnRNAs b/w nucleus & cytoplasm
Transport receptor protein that bind to the 5’cap of SnRNA appear to be involved in the
Export of the snRNA to the cytoplasm. In contrast, sequence present on snRNP protein are
Responsible for the transport of snRNPs from the cytoplasm to the nucleus.
Function: splicing of other RNA

31. Internal organization of the nucleus
• Nucleus has an internal structure that organizes the
genetic material & localize nuclear function.
• Lamins serve as sites of chromatin attachment &
organize other proteins into functional nuclear
bodies.
• Chromatin organized into large loops of DNA &
specific region of these loops are bound to lamin
matrix by lamin binding protein in the chromatin.
• Many other protein have lamin dependent
complexes that has role in dna repair, gene
regulation, chromatin organization & signal
transduction.

32. Chromosomes & high order chromatin structure
• Chromatin bcm highly condensed during mitosis
to form the compact metaphase chromosome
that are distributed to daughter nuclei.
• During interphase :-
• Heterochromatin: highly condensed &
transcriptionally inactive
• Euchromatin: decondensed & active
•

33. Heterochromatin
Constitutive
• Same region in dna
sequence of all cell that are
generally not transcribed
such as satellite sequence
present at centromere ---
1,9,16,19 & Y Chromosome
facultative
Different region in dna
sequence of different cell
that are not transcribed.
i.e. x-chromosome in
female, in one cell one is
inactive & other cell is
another type

34. Chromatin distribution
• Each chromosome occupies a different
territory, with centromere & telomere
attached to opposite sides of the nuclear
envelope.
Organization of drosophila chromosomes
Five chromosome arm in different color

35. A model of chromosome organization
The chromosome occupy discrete territories
Separated by interchromosomal domain in
Which RNA processing and transport are
Thought to occur

36. Clustered site of DNA replication
The discrete site of dna was visualized within cell nuclei by labelling cell with
Bromodeoxyuridine, an analog of thymidine that is incorporated into dna and then detected
By staining with fluorescent Ab
DNA replication appears to take place in large structure that contain multiple replication
Complexes organized into distinct functional bodies , which have been called replication
Factories.

37. Localization of splicing components
While actively transcribed genes appear to be
Distributed throughout the nucleus, components of
The mRNA splicing machinery are concentrated in
Discrete nuclear bodies termed nuclear speckles.

38. Nuclei contain PML & cajal bodies
• PML (nuclear dots or nuclear bodies or promyelocytic
leukemia protein) are known to interact with chromatin
& are the site of accumulation of transcription factors &
chromatin modifying proteins (such as histone
deacetylases).
• Cajal bodies contain protein coilin & contain high
concentration of splicing small nuclear-
ribonucleoprotein(snRNP) possibly indicating that they
function to modify RNA after it has been transcribed
from DNA.

39. PML Bodies
Cajal bodies

40. Nucleolus & rRNA processing
• Nucleolus: is the largest structure in the nucleus
of eukaryotic cellls
• It is the site of rRNA transcription & processing as
well as aspects of ribosome assembly
• Also participate in formation of signal recognition
particle (SRP) & play role in the cell response to
stress.
• It is a ribosome production factory, designed to
fulfill the need for regulated & efficient
production of rRNA & assembly of ribosomal
subunit.

42. Ribosomal RNA gene
• The nucleolus, which is not surrounded by
membrane is associated with the chromosomal
regions that contain the genes for the 5.8s,18s,&
28s rRNA. These are transcribe by RNA pol. 1,
yielding a 45S ribosomal precursor rna.
• The 45S pre-rRNA is processed to the 18s of the
40s of small & to the 5.8s, 28s of 60s large
ribosomal subunit.
• Transcription of 5s rRNA, which is also found in
the 60s, takes place outside the nucleolus in
higher eukaryotes & is catalyzed by RNA pol.3

43. The genes for 5.8S, 18S , & 28S rRNA are clustered in tandem arrays on five different
Chromosomes 13,14,15,21,&22.
the 5s rRNA genes are present in single tandem array on Chromosome 1.
The rRNA genes are organized in tandem arrays, separated by nontranscribed
Spacer DNA

44. Transcription & processing of rRNA
The 45S pre-rRNA transcript contains external transcribed spacer(ETS) at both end and
Internal transcribed spacer(ITS) b/w the sequence of 18s,5.8s,28s rRNAs. These pre-rRNA is
Processed via a series of cleavage to yield the mature rRNA species.

45. 
• The processing of pre-rRNA requires the
action of both proteins and RNA that are
localized to the nucleolus.
• The small nucleolar RNAs(snoRNAs) function
in pre-rRNA processing.
• snoRNA+Protein=snoRNP(Small nucleolar
ribonucleoprotein) function as:
• 1)Form splicesome complex for splicing.
• 2)methylation of specific ribose residue.
• 3)formation of pseudouridines by converting
uridine

46. Role of snoRNAs in base modification

47. RNA assembly

48. Ribosome assembly
• Ribosome protein are imported to the nucleolus
from the cytoplasm & begin to assemble on
pre-rRNA prior to its cleavage. As the pre-rRNA is
processed, additional ribosomal proteins & the 5S
rRNA(which is synthesized elsewhere in the
nucleus) assemble to form preribosomal particles.
• The final step of maturation follow the export of
preribosomal particles to the cytoplasm, yielding
the 40S & 60S ribosomal subunits.

49. Thankyou
By Sukhmeet
Kang