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Discovery of Nucleus:
The nucleus was discovered by Robert Brown in 1831. Robert Brown was a Scottish botanist.

Definition of Nucleus:
The nucleus is a double membrane-bounded structure present in all eukaryotic cells having genetic materials of the cell. It is the most visible and important part of the cell.

Interesting Facts about Nucleus:
1. The nucleus is a double membrane-bounded structure.
2. In an animal cell nucleus is present at the center of the cell while in a plant cell it lies at the periphery.
3. The eukaryotic nucleus is surrounded by a double membrane while the prokaryotic nucleus has no membrane.
4. The nucleus takes approximately 10 percent of the cell's volume.


Number of Nuclei:
The number of nuclei is varied from cell to cell.
√ The most cell has only one nucleus, the cell is called uninucleate.
√ A cell having two nuclei is called binucleate
√ A cell having many nuclei is known as polynucleate.

Size of Nucleus:
They are typically about 10 micrometers in diameter.

Structure and Components of Nucleus:
The nucleus consists of the following components:
1) Nuclear Envelope
2) Nuclear Pores
3) Nucleoplasm
4) Nucleolus
5) Chromosomes


1) Nuclear Envelope:
The nucleus is covered by a double membrane called the nuclear envelope.
In a eukaryotic cell nuclear membrane separate the nucleus from the cytoplasm.
The outer membrane is continuous with the endoplasmic reticulum, while the inner membrane encloses the nuclear contents.

2) Nuclear Pores:
The double membrane nuclear envelope fused at some interval which forms pores known as nuclear pores.
The nuclear pores are composed of transport proteins called nucleoporins. The nuclear pores involve in the exchange of materials between cytoplasm and nucleoplasm.

For example, when RNA is synthesized from DNA inside the nucleus, the RNA will transport to the cytoplasm through these nuclear pores.
Then RNA will combine with ribosomes in the cytoplasm and the process of protein synthesis will be initiated.
Similarly many enzymes (protein) are required for certain processes in the nucleus. As enzymes are prepared in the cytoplasm they will be transported to the nucleoplasm through nuclear pores.

3) Nucleoplasm:
The granular matrix and aqueous part inside the nucleus are called Nucleoplasm.
It is composed of a mixture of proteins like histone and non-histone protein, enzymes like DNA and RNA polymerase, free nucleotides, metal ions, etc. Here chromatin materials are suspended and nucleolus is present.

4) Nucleolus:
It is a non-membranous structure present inside the nucleus and helps in the production of ribosomes. It has two regions. Outer or Peripheral granular region which contains precursor for ribosomal subunits Central Fibril region which contains rRNA and rDNA.

5) Chromosomes:
Chromosomes are thread-like, color structures formed from chromatin materials, clearly appearing at the time of cell division.
The chromosomes are more visible in the living cell.
In non-dividing cells, chromosomes are in the form of a network of fine threads known as chromatin materials.
When the cell start division, chromatin material is condensed and chromosomes structure is formed.
Chromosomes are made up of DNA and a special type of protein known as histone proteins.
A visible chromosome consists of two thread-like structures called chromatids.
The chromatids are connected by a small rounded body called centromere or primary constriction.

Chromosome numbers are always specific for a particular species. For example, in the human cell, it is 46, in reddish 18 and onion 16.
The prokaryotic cells do not contain a well-defined nucleus. Rather, their chromosomes are made of DNA only and are submerged in the cytoplasm.
Diploid and Haploid Number of Chromosomes:
The full number of chromosomes in a normal body cell is diploid (2n) whereas haploid is the half number of chromosomes present in a germ or gamete cell.
Functions of Nucleus:
1. The nucleus control all activities that take place in the cell and act as a control center just like the brain in a human.
2. It contains heredity material (DNA) which transfers information from parents to offspring.
3. Nucleus help in direct protein synthesis. DNA in the nucleus transcript to mRNA. The mRNA transfers to the cytoplasm and combines with ribosomes which start the formation of protein as mRNA informed by the nucleus.
4. The nucleolus (a part of the nucleus) also helps in the synthesis of ribosomes.
5. It controls cell growth and cell division.

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