The nucleus is a double-membrane bound structure in eukaryotic cells that contains the genetic material DNA. During interphase, the nucleus contains a network of chromatin fibers in the nucleoplasm. The chromatin fibers are made up of nucleosomes, which consist of DNA wound around histone proteins. The nucleolus and endosomes are also found in the nucleoplasm. The nuclear membrane separates the nucleus from the cytoplasm, and contains nuclear pores that allow for transport between the two compartments.

1. Structural Organization of
the Interphase Nucleus of
Animal Cell

2. Interphase
Interphase is the phase of the cell cycle in which a
typical cell spends most of its life. Interphase is the 'daily living'
or metabolic phase of the cell, in which the cell obtains nutrients
and metabolizes them, grows, reads its DNA, and conducts other
"normal" cell functions. The majority of eukaryotic cells spend
most of their time in interphase. This phase was formerly called
the resting phase. However, interphase does not describe a cell
that is merely resting; rather, the cell is actively living, and
preparing for later cell division.

3. The Nucleus
• Nucleus is the double membrane
bound structure of an eukaryotic cell
containing chromosome.
• It is the most important part of the
cell situated in the cytoplasm.
• All the cellular activities are
controlled by it. Nucleus is a
directing and organizing unit
without which the cell could not
exist.
• It contains chromosomes where the
genetic material DNA is found.

4. Ultra structure of Nucleus
The nucleus consists of the following main
parts –
1. Nucleolemma or nuclear membrane
(karyotheca)
2. Nuclear sap or karyolymph or
nucleoplasm
3. Chromatin network or fibers
4. Nucleolus
5. Endosomes

5. Nuclear membrane
• The nucleus is separated from the cytoplasm by a limiting membrane called as karyotheca
or nuclear membrane. It consists of two “unit membranes”.
• Each membrane ranges from 75 to 90A0
in thickness and its structure resembles the
structure of a plasma membrane.
• The two membranes of the nuclear envelope fuse at a number of places to form minute
pores called the nuclear pores. Through these pores movement of RNA and protein takes
place between the nucleus and the cytoplasm.
• The outer and inner membrane remain separated by a space of 10 to 50 A0
width. This
space is known as perinuclear space.

6. Nucleoplasm
• The space between the nucleolus and the nuclear membrane of nucleus is
filled with a transparent, semi-solid, granular and homogeneous matrix during
interphase called nucleoplasm or nuclear sap or karyolymph.
• Primarily, it is composed of proteinous material and is the main site for
enzyme activity.
• Nucleus contains chromatin reticulum and nucleolus.
• It is chemically formed of waters, sugars, minerals, enzymes, mRNA and
tRNA molecules.

7. Chromatin reticulum
• It is a much coiled elongated filamentous structure of chromatin which forms a
network in the nucleoplasm. Such network is observed in interphase nucleus only.
• It appears as a viscous, gelatinous substance.
• During cell division, it becomes fragmented, thickened, shortened and condensed
to form thread like structure called chromosome.
• Chromatin reticulum is chemically composed of nucleoprotein called chromatin.
It consists of DNA and histone protein.

8. Nucleosome:
 The chromatin reticulum contains single DNA molecule.
 In recent years, it has been shown that chromatin reticulum consists of
repeatedly occurring units called nucleosomes. These are also known as
nu-bodies.
 Nucleosomes consists of a core made up of four histone proteins, namely
H2A, H2B, H3 and H4 (2 molecules of each kind) around which the DNA
is wound to form a super helix.
 The DNA strand makes two turns around the core of histone protein. The
two ends of the DNA strands around the core of histone protein remain
joined by another histone protein called H1.
 The two adjacent nucleosomes are joined by a piece of DNA strand called
linker DNA or internucleosomal DNA.

10. The chromatin fibers in interphase nucleus is differentiated into two types :-
1. Heterochromatin: It is the part of a chromatin fiber that takes a darker
stain. It contains large amount of RNA and small amount of DNA. It is
genetically inert due to lack of hereditary information.
2. Euchromatin: It is the lightly stained portion of a chromatin reticulum.
The euchromatin contains comparatively large amount of DNA. It plays
an active role in heredity.

11. Nucleolus
• It is a dark, spherical dense granular
mass of the resting nucleus.
• It is generally acentric in position and is
associated with nucleolar organizer
region of the nucleolar chromosome.
• Its size varies from cell to cell and is
usually related with the synthetic
activity of the cell. The number also
varies.

12. Components of nucleolus:
During cell division the nucleolus generally disappears during the first stage
or prophase stage, but it reappears in the daughter cells. Morphologically,
two components may be defined in a nucleolus:
• Pars amorpha: This is a component of nucleolus which first disappears but
reappears at the end of division. This is the amorphous part which begins to
disappear just prior to the breakdown of nuclear membrane during cell
division and reappears in daughter nuclei as division completes.
• Nucleolonema: This is the second and permanent component which does
not disappear but remains persistent throughout the cell cycle. It is a
filamentous structure having 80A0 fibrils with which 150A0 particles of
ribonucleoprotein are attached.

13. Endosomes
• An endosome is a membrane-bounded compartment
inside eukaryotic cells.
• It is a compartment of the endocytic membrane transport pathway
from the plasma membrane to the lysosome.
• These are rather smaller chromatin bodies present in the
nucleoplasm of nucleus.
• They are like nucleolus but smaller in size, showing changeable
structure.

14. Thank You.