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Nucleus

Nucleus the most prominent organelle of the cell. The number of nuclei may vary, they may be uni-nucleate (single nucleus), bi-nucleate (two nuclei) or even multi-nucleate. 

Nucleus is present in all eukaryotic cells, they may be absent in few cells like the mammalian RBCs. 

The shape of the nucleus is mostly round, it may be oval, disc shaped depending on the type of cell.




History

Nucleus was the first cell organelle to be discovered. Antonie von Leeuvenhoek (1632 - 1723) observed lumen (nucleus) in the red blood cells of salmon.Nucleus was also described  by Franz Bauer in 1804 and by Robert Brown in 1831.

 Robert Brown in 1833 named and discovered nucleus in plant cells.

 

What is a Nucleus ?

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The word nucleus is derived from a Latin word nucleus or nuculeus which means 'kernel'. Nucleus a double-membrane bound cell organelle present in eukaryotic cells. The nucleus constitutes most of the genetic material of the cell - the DNA.

The nucleus maintains the integrity of the genes which regulate the gene expression, in-turn regulating the activities of the cell. Therefore, the nucleus is known as the control center of the cell.


Nucleus Structure

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The nucleus is the largest organelle of the cell. The nucleus appears to be dense, spherical organelle.  It occupies about 10% of the total volume of the cell.

In mammalian cells the average diameter of the nucleus is approximately 6 micrometers. A semi-fluid matrix nucleoplasm is seen inside the nucleus which is a viscous fluid and is similar to the composition of the cytoplasm.

Nuclear Envelope

The nuclear envelope is also known as the nuclear membrane. 

It is made up of two membranes the outer membrane and the inner membrane. 

The outer membrane of the nucleus is continuous with the membrane of the rough endoplasmic reticulum. 

The space between these layers is known as the perinuclear space. 

 The nuclear envelope encloses the nucleus and separates the genetic material of the cell from the cytoplasm of the cell. 

It also serves as a barrier to prevent passage of macro-molecules freely between the nucleoplasm and the cytoplasm.





Nuclear Pore

The nuclear envelope is perforated with numerous pores called nuclear pores. 

The nuclear pores are composed of many proteins known as nucleoproteins.

The nuclear pores regulate the passage of the molecules between the nucleus and cytoplasm. 

The pores allow the passage of molecules of only about 9nm wide. The larger molecules are transferred through active transport. 

Molecules like of DNA and RNA are allowed into the nucleus. But energy molecules (ATP), water and ions are permitted freely.

Chromosomes

The nucleus of the cell contains majority of the cells genetic material in the form of multiple linear DNA molecules. 

These DNA molecules are organized into structures called chromosomes.
 
The DNA molecules are in complex with a large variety of proteins (histones) which form the chromosome. 

In the cell they are organized in a DNA-protein complex known as chromatin. 

During cell-division the chromatin forms well-defined chromosomes. 

The genes within the chromosomes consists of the cells nuclear genome.

Mitochondria of the cell also contains a small fraction of genes. 

Human cells has nearly 6 feet of DNA, which is divided into 46 individual molecules.



Nucleolus

The nucleolus is not surrounded by a membrane, it is a densely stained structure found in the nucleus. 

The nucleoli are formed around the nuclear organizer regions.
 
It synthesizes and assembles ribosomes and r RNA. 

The number of nucleoli is different from species to species but within a species the number is fixed. 

During cell division, the nucleolus disappears. 

Studies suggest that nucleolus may be involved in cellular aging and senescence.

Functions of the Nucleus

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Below you could see the functions of the nucleus

It controls the heredity characteristics of an organism.

It is responsible for protein synthesis, cell division, growth and differentiation. 

Stores heredity material in the form of deoxy-ribonucleic acid (DNA) strands.
Also stores proteins and ribonucleic acid (RNA) in the nucleolus. 

It is a site for transcription process in which messenger RNA (m RNA) are produced for protein synthesis.

Aids in exchange of DNA and RNA (heredity materials) between the nucleus and the rest of the cell.

Nucleolus produces ribosomes and are known as protein factories.

It also regulates the integrity of genes and gene expression.

Animal Cell Nucleus

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Animal cell nucleus is a membrane bound organelle. It is surrounded by double membrane. The nucleus communicates with the surrounding cell cytoplasm through the nuclear pores. The DNA in the nucleus is responsible for the hereditary characteristics and protein synthesis.  The active genes on the DNA are similar, but some genes may be turned on or off depending on the specific cell type.  This is the reason why a muscle cell is different from a liver cell. Nucleolus is a prominent structure in the nucleus. This aids in ribosomes production and protein synthesis.

Animal Cell Nucleus

Plant Cell Nucleus

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Plant cell nucleus is a double-membrane bound organelle. It controls the activities of the cell and is known as the master mind or the control center of the cell.

The plant cell wall has two layers - the outer membrane and the inner membrane, which encloses a tiny space known as perinuclear space.

The nucleus communicates to the cell cytoplasm through the nuclear pores present in the nuclear membrane. The nuclear membrane is continuous with the endoplasmic reticulum. The DNA is responsible for cell division, growth and protein synthesis.
Plant Cell Nucleus

Bacterial Cell Nucleus

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The bacterial cell does not contain any nucleus. The bacterial chromosome is not enclosed in a membrane bound nucleus.The bacterial chromosome is circular and located in the cytoplasm.

Structure of Bacterial Cell
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