Nucleus

The nucleus plays an important role in the cell. It contains the genetic material of the cell and has a direct influence on the metabolic activities of the cell. It is an essential part of the cell and if removed experimentally then protein synthesis ceases and the cell soon dies.
 

neuron cell


The exchange of material occurs between the nucleus and the cytoplasm.

It is present in all of the body cells except the erythrocytes which are incapable of protein synthesis and have limited metabolic activity.

Size of the nucleus

Its diameter usually ranges from 3 to 10 micrometers.

Shape

it is found generally in a spherical shape but in some cells, it may be oval, fusiform, lobulated or irregular in shape.

Position of the nucleus

The position generally depends upon the shape of the cell. In spherical and polygonal cells, it is normally centrally located but may be eccentric.
 
In cuboidal and squamous cells the nucleus occupies the central position.
 
In columnar cells, the nucleus usually lies in the basal region of the cell.

Numbers of nucleus

Usually, only one nucleus in each cell but some cells are
Binucleated contains 2 nuclei
Multinucleated  contain many nuclei

Structure of the nucleus

It is surrounded by a double membrane know is a nuclear envelope and also contains a darkly staining bodies called nucleoli.
 
Space which is not occupied by the chromatin or nucleoli is filled with a semifluid material known is nuclear sap.

Nuclear Envelope

In an ordinary light microscope, the nuclear envelope is visible as a dark blue or purple line. An electron microscope shows that the nuclear envelope consists of two membranes separated by a narrow space known is perinuclear cisternal space.

 
The nuclear envelope is 40 nm thick.

 
At many points in the nuclear envelops there are small circular pores which are known is Nuclear Pores. Nuclear Pores are usually 70 nm in diameter and through which exchange of materials occurs between the cytoplasm and nucleus.

 
Ions and small molecules pass through the nuclear pores by simple diffusion without consuming energy.

 
Large molecules such as messenger RNA and histones pass through the nuclear pores by receptor-mediated transport.

Chromotain

These are coarse granules distributed irregularly in the nucleus. It consists of deoxyribonucleic acid and histone proteins.
 
DNA exists in the form of a flexible rod-like structure known is chromosomes.
 
Chromosomes may be folded and form dense masses which are known as heterochromatin while the straight and uncoiled part of the chromosomes from euchromatin.
 
An electron microscope shows that the structural unit of the chromatin are nucleosomes. which consist of a core of 8 histone proteins wrapped by two complete turns of DNA molecule.
 
Nucleosomes are joined by the linker DNA.
 
The number of chromosomes is constant. for example, the somatic cells of the body contain 46 chromosomes.

Nucleolus

The nucleolus is a dense roughly spherical intranuclear structure which is not surrounded by the membrane.
 
Nucleolus can be seen only in a resting cell while in dividing cells it disappeared.
 
Ribosomal RNA is synthesized and assembled in the nucleolus. It is well developed in the cells which are involved in active protein synthesis.
 
In the electron microscope, the nucleolus appears as a sponge-like network of electron-dense material and has a granular appearance and hence is called pars granulosa.
 
The other part of the electron-dense material consists of densely packed filaments and is called pars fibrosa.

Other nuclear bodies

In addition to the nucleolus, other nuclear bodies are also present in the nucleus. Other nuclear bodies that are present in the cell nucleus are,
1. Cajal bodies
2.  Polymorphic interphase karyosomal association
3.  PromyelocyticParaspeckles.
4.   Paraspeckles
5.   Splicing speckles

Cajal bodies

There are 1 to 10/compact structures present in the cell nucleus known as Cajal bodies or coiled bodies. The diameter of Cajal bodies ranges from 0.2 micrometers to 2 micrometers that depend on cell type. Cajal bodies are involved in RNA processing and I’m messenger RNA modification.

Polymorphic interphase karyosomal associations

These structures were first discovered in 1991. The functions of these structures are unclear however some scientists thought that these structures promote the transcription of smaller nuclear RNA.

Polymorphic Leukemia bodies

These are spherical bodies found throughout the nucleoplasm. PML bodies show an association with Cajal bodies and cleavage bodies. These structures mean PML bodies regulate many nuclear functions such as DNA replication and transcription.

Splicing speckles

These are subnuclear structures located in the interchromatin Region of the nucleoplasm. These structures appeared as clusters when seen with an electron microscope.

Paraspeckles

These are irregularly shaped structures that are present in the interchromatin space of the nucleus. These structures are transcription-dependent. paraspeckles are altered by changes in cellular metabolic activity.

Nuclei per cell

There are many types of eukaryotic cells and usually, they have a single nucleus but some cells have no nuclei, while other eukaryotic cells have several nuclei. On the basis of nuclei, cells are divided into anucleated and multinucleated cells.

Anucleated cells

These are the cells that contain no nucleus. Due to the absence of a nucleus, these are incapable of dividing to produce daughter cells.

Example
Red blood cells or erythrocyte that also lacks mitochondria. Red blood cells mature in the bone marrow where they lose their nuclei.

Multinucleated cells

These are the cells that contain multiple nuclei.

For example
In human skeletal muscles, cells and syncytium are multinucleated cells.

Functions of the nucleus

1.the control center of the cell
2.influence metabolic activity of the cells
3.Ribosomal RNA synthesis
4.if it is removed the cell dies.
5.contain chromosomes
6. Nuclear pores are present which allow an exchange of material
7. Small molecules or ion can enter through the nucleus without consuming energy.
8. Large molecules or ions can enter the nucleus by receptor-mediated transport.

Summary

The nucleus is the control center of the cell and plays a very important role in the cell. The nucleus contains genetic material. The nucleus is surrounded by a double membrane. inside the nucleus, there is a nucleolus that is not surrounded by a membrane. There are also other nuclear bodies present in the nucleus such as Cajal bodies etc.  The number of nuclei is different in different cells. Red blood cells lack nuclei.