Structural Organization of the Interphase Nucleus of Animal Cell

InterphaseInterphase 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.

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.

Ultra structure of NucleusThe 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

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.

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.

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.

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.

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.

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.

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.

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.

Thank You.