The nucleus is a double-membrane organelle found in eukaryotic cells that contains most of the cell's genetic material. It has a spherical shape but can be other shapes depending on the cell. The nucleus contains chromatin with DNA, nucleolus, and is surrounded by a nuclear envelope. It acts as the control center of the cell by transmitting genetic information for protein synthesis, cell division, growth and differentiation.

1. Nucleus
PRAKASH DHAKAL
M. Sc. Public Health Microbiology
Tribhuvan University, Nepal

2. Nucleus
 Nucleus is the first organelle to be discovered. It was discovered by Anton
von Leeuwenhoek .
 It is a double membrane enclosed organelle found only in eukaryotic cells.
 Eukaryotes usually have a single nucleus but a few cell types, such as
mammalian red blood cells have no nucleus but others may have many.
 Nucleus contains most of the genetic material; DNA with a large variety of
proteins such as histones to form chromosomes.
 Nucleus is the control centre of the cell and is the most prominent
organelle in cell.
 Nucleus can be easily observed under light microscope.

4. Structure
 Generally the nucleus is spherical but it may be fusiform, ellipsoidal,
flattened depending upon cell shape and function.
 The size of the nucleus is variable. In mammalian cells, it occupies about
10% of the total cell volume.
 The nucleus consists of four components:
1) Nuclear membrane or nuclear envelope
2) Nucleoplasm or Nuclear sap
3) Nucleolus
4) Chromatin fiber or chromatin network

5. 1) Nuclear envelope or Nuclear membrane:
 It consists of two membranes; an inner and an outer membrane.
 The distance between two membrane is about 10 to 50 nm.
 The nuclear membrane completely encloses the nucleus and separates the
cell’s genetic material from the surrounding cytoplasm.
 The outer membrane is continuous with the membrane of the rough
endoplasmic reticulum (RER) and contains ribosomes.
 The space between two membrane is called the perinuclear space.
 Like the cell membrane, the nuclear envelope consists of phospholipids that
form a lipid bilayer.
 The envelope helps to maintain the shape of the nucleus and assists in
regulating the flow of molecules in and out of the nucleus through nuclear
pore.

6. Nuclear pores:
 Nuclear pores are composed of several proteins. The pores are about 125
millions Dalton in weight.
 The pores are about 100 nm in total diameter but 9 nm is available to
diffuse molecules freely.
 The small space selectively allows the small water soluble molecules while
preventing larger molecules (e.g. DNA, RNA and proteins) from entering
or exiting the nucleus.
 The larger molecules needs separate transport mechanisms.
 Generally a mammalian cell contains about 3000-4000 pores throughout
its envelope.

7. Fibrous lamina or Nuclear lamina:
 The nuclear membrane of many cells contains an additional layer called
fibrous or nuclear lamina.
 It is inner towards the inner membrane facing the interior of the nucleus.
 It is composed of fine filaments made up of proteins. They provide
mechanical support to nuclear membranes.
 It is supposed that it influences the exchange of materials between
nucleus and cytoplasm.
2) Nucleoplasm or Nuclear sap:
 Nucleoplasm is the viscous liquid that fills inside of the nucleus.
 It is similar in composition to the cytoplasm found outside the nucleus.

8. 3) Nucleolus :
 The nucleus contains a large, spherical and acidophilic dense granule
known as the nucleolus. It is membrane less.
 It looks large dark spot within the nucleus when observed through
microscope.
 A nucleus may contains up to four nucleoli, but within each species the
number of nucleoli is fixed. During cell division, the nucleolus disappears.
 Some studies suggests that the nucleolus are involved in manufacturing
ribosome , cell ageing and senescence.

9. 4) Chromatin fiber or chromatin network:
 The cell nucleus contains the cell’s genetic material in the form of DNA
molecules organized into structures called chromosomes.
 Each human cell contains roughly six feet of the DNA. Chromosomes are
organized in a DNA-protein complex known as chromatin.
 The chromatin appears as thread like, coiled and elongated structure.
 These are stained with basic dyes like Feulgen’s stain, Acetocarmine etc,
that is why termed as chromatin fibers or chromatin substance (Greek,
Chrome: color).
 These are visible during interphase stage of cell division.

10. Functions
 Nucleus contains all the genetic information in its chromatin.
 Nucleus takes part in transmission of genetic information from parent cell
to its daughter cells and from one generations to the next.
 Nucleus forms ribosomes on its nucleolus .
 It is responsible for protein synthesis, cell division, growth and
differentiation.