Nuclear envelope, NPC, Nucleolus

1. (Nuclear envelope ,
nuclear pore complex
and nucleolus)
-by BSc(h) Zoology
Ist Year

2. * Nucleus is a biochemical compartment that
houses the genetic material and serves as site
of transcription and RNA processing in
eukaryotic cells.

3.  History: first discovered & named by Robert Brown(1833).
 Term Nucleolus : Bowman(1840).
 Term chromatin : W. Flemming(1879).
 Term cytoplasm & nucleoplasm : Strasburger (1882).
Occurrence-found in all eukaryotic cells of plants &
animals(exception- mature sieve tubes &erythrocytes).
*Prokaryotes lack true nucleus.
Number- mononucleate , binucleate (paramecium ,hepatocytes ,
chondrocytes),polynucleate cells (3-100).

4. The nucleus consists of various parts. It is bounded by a thin but clearly defined
covering, the nuclear envelop or karyotheca. Within the envelope is a clear fluid
substance called nucleoplasm or nuclear sap or karyolymph is present in which
the solutes of the nucleus are dissolved. Suspended in the nucleoplasm are
network of protein-containing fibrils called nuclear matrix; fine intermingled
nucleoprotein filaments collectively referred to as the chromatin; and one or
more spherical bodies known as nucleoli (singular, nucleolus). There are no
membranes or microtubules inside the nucleus.
Chemical Composition: The nucleus is composed of about 9-
12% DNA, 5% RNA, 3% lipids, 15% simple basic proteins such as
histone or protamines, enzymes such as polymerases for the synthesis
of DNA and RNA, organic phosphates and inorganic salts or ions such
as Mg++, Ca++ and Fe++ .

5. The nuclear envelope has a complex structure, consisting of
two nuclear membranes, an underlying nuclear lamina, and
nuclear pore complexes .The nucleus is surrounded by a system
of two concentric membranes, called the inner(5-10 nm thick)
and outer nuclear membranes.The outer nuclear membrane is
continuous with the endoplasmic reticulum. In contrast, the inner
nuclear membrane carries unique proteins that are specific to the
nucleus.

6. Underlying the inner nuclear membrane is the nuclear lamina, a fibrous
meshwork that provides structural support to the nucleus .The nuclear
lamina is composed of one or more related proteins called lamins. Most
mammalian cells, for example, contain four different lamins,
designated A, B1, B2, and C.
The lamins associate with each other to form filaments .

7. Nuclear Pores: The nuclear envelope is generally perforated by minute
apertures, the nuclear pores that control the passage of some molecules and
particles. The pores are formed by fusion of the inner and outer membranes
of the nuclear envelope. There may be 1000 to 10,000 pores per nucleus.
Each nuclear pore is fitted with
an apparatus called the pore complex
which fills considerable part of the pore.

8. STRUCTURE OF NPC
The pore complex is nearly cylindrical, projects into both cytoplasm and
nucleoplasm, and projects beyond the rim of the pore over the nuclear
envelope. The pore complex consists of two rings, the annuli, one located at the
cytoplasmic rim of the pore and the other at the nucleoplasmic rim. Each
annulus comprises eight symmetrically arranged subunits, and sends a spoke
into the pore. The spoke encloses a channel about 100 to 200 Å wide. Ions and
small molecules of the size of monosaccharide, disaccharides or amino acids
pass freely between the
nucleus and cytoplasm. The
pore complexes do control the
passage of larger molecules,
such as RNA and proteins, and
of ribosomal subunits. The
pore complexes also act as
a barrier to some molecules
such as DNA of chromosomes.

9. FUNCTION OF NPC
1. It maintains the shape of the nucleus.
2. It keeps the nuclear contents in place and distinct from cytoplasm.
3. It regulates the flow of materials into and out of the nucleus by
active transport and out pocketing.
4. Its pores allow the exit of ribosomal subunits formed in the
nucleolus and tRNA and mRNA synthesized on the chromosomes.
2 mechanisms of transport:
1.PassiveTransport- Small molecules with
MM< 20-40 kd diffuse passively through
open aqueous channels ( dia.- 9 nm).
2.ActiveTransport- Macromolecules
pass actively through central pore of
NPC(dia.- 10-40 nm).

10. *Transparent , semi-solid , granular & slightly acidophilic material.
*Also called nuclear sap or karyolymph.
*Composed of:
1.Nucleic Acids (DNA & RNA)
2.Proteins
a.Basic proteins – takes basic stains. Eg- Nucleoprotamines-MW=4kD
usually bound with DNA.
b.Non histones/Acidic proteins – eg-phosphoproteins.
3.Enzymes- necessary for DNA & RNA synthesis. Eg- polymerases,
NAD synthetase, adenosine deaminase, aldolase etc.
Also contains cofactors & coenzymes like ATP & Acetyl CoA.
4.Lipids
5.Minerals( P,K,Na,Ca,Mg etc.)

11. Functions of nucleoplasm :-
1.It is the site for the synthesis of DNA, RNAs, ribosomal subunits, ATP and
NAD.
2.It supports the nuclear matrix, chromatin material and nucleoli.
3. It provides turgidity to the nucleus.
Nuclear Matrix
The nuclear matrix is a network of thin, criss-crossing, protein-
containing fibrils that are connected at their ends to the nuclear
envelope. It forms a sort of nuclear skeleton.
Functions:
1. It maintains the shape of the nucleus.
2. Chromatin fibers are anchored to nuclear matrix.
3.The machinery for various nuclear activities, such as transcription and
replication, is associated with the matrix.
4. It has also been implicated in the processing of newly formed RNA
molecules and their transport through the nucleus.

12. The term chromatin was first coined by Flemming in 1879.The
chromatin occurs in an interphase (non-dividing) nucleus as fine
filaments, the chromatin fibers.The chromatin occupies most of
the nucleus.The chromatin fibers are simply extremely extended
chromosomes. During cell division, the chromatin fibers, by
condensing and tight coiling, form short, thick, rod like bodies
known as chromosomes.
2 types :- a) Euchromatin b) Heterochromatin
Heterochromatin appears as small, darkly staining, irregular particles
scattered throughout the nucleus or accumulated adjacent to the nuclear
envelope.
Euchromatin is dispersed and not readily stainable. Euchromatin is prevalent
in cells that are active in the transcription of many of their genes while
heterochromatin is most abundant in cells that are less active or not active.

13.  Prominent spherical colloidal acidophilic
body.
 Bacteria & yeasts lack nucleolus.
 Number – one,two or many depending on no.of chromosomes.
 Position – Eccentric.
 The nucleoli disappear during cell division, and are reformed at
specific sites, the nucleolar organizers or nucleolar organizer regions
(NORs), of certain chromosomes, the nucleolar chromosomes, at
the end of cell division before the chromosomes become diffuse.
Chemical composition :
*Not bounded by any limiting membrane.
*Contains NO DNA, 4 types of rRNAs, 70 types of ribosomal proteins,
RNA binding proteins ( eg-Nucleolin) & RNA splicing
Nucleoproteins.
*Contains phospholipids,calcium ions etc.

14.  They are sites for biogenesis of ribosomal subunits (40s &
60s).
 Here 3 types of rRNA are transcribed ( 18s,5.8s,28s) from
precursor transcript which undergo processing with the
help of proteins.
 All the components of
ribosomes are synthesi-
sed and assembled into
two types of ribosomal
subunits and are
transported back to
cytoplasm .

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

16. . References